Non-Methane Hydrocarbon (NMHC) at Pico Mountain, Azores
1. Oxidation Chemistry in the North-Atlantic Region
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D. Helmig1*, D. M. Tanner1, R.C. Owen2, R. E. Honrath2 and D.D. Parrish3
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Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, CO
80309, USA
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Department of Civil and Environmental Engineering, Michigan Technological University,
Houghton MI 49931, Michigan, USA
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CSD/Earth System Research Laboratory, National Oceanic and Atmospheric Administration,
Boulder, CO 80303, USA
*corresponding author: Detlev.Helmig@colorado.edu
Manuscript submitted to Journal of Geophysical Research
Revised Version
October 26, 2007
Abstract
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Data from one year of continuous measurements (August 2004- August 2005, in part overlapping
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with the field campaign of the International Consortium on Atmospheric Research on Transport
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and Training, (ICARTT) study) of non-methane hydrocarbons (NMHC) at the Pico Mountain
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observatory (2225 m asl) on Pico Island, Azores were used to investigate seasonal variations in
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NMHC mixing ratios, their oxidation chemistry and transport patterns in the central North Atlan-
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tic Region. NMHC mixing ratios at Pico in general were higher than data reported from a simi-
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lar data set from Mauna Loa Observatory, Hawaii. Substantially enhanced NMHC levels during
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the summer of 2004 were attributed to the impact of long-range transport of biomass burning
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plumes resulting from Northern Canada and Alaskan wildfires which further indicates the impact
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of biomass burning plumes on atmospheric composition many days downwind of these emission
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sources. During summer, NMHC absolute levels and ratios reflect the higher degree of photo-
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chemical processing than during other seasons. Analyses of NMHC ratios point towards changes
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in source region seasonal NMHC emission ratios. NMHC observations show lower influence
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from chlorine chemistry than data reported from within the marine boundary layer. Ozone in
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excess of 35 ppbv was measured at Pico Mountain throughout all seasons. Enhanced ozone
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levels were observed in air that had relatively ‘fresh’ photochemical signatures (e.g. ln [pro-
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pane]/[ethane] > -2.5). In more processed air (‘older’ air with ln [propane]/[ethane] < -2.5)
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ozone was generally a lower levels (< 40 ppbv). These findings indicate that the lower tropo-
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sphere over the mid--North Atlantic is dominated by photochemical ozone destruction in contrast
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to the mid-North Pacific where other studies have reported that the photochemistry is much more
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nearly ozone neutral.
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1. Introduction
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Atmospheric non-methane hydrocarbons (NMHC) show considerable variations on spatial and
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temporal scales, their concentrations being determined by the strength of emission sources and
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atmospheric removal processes, which are mostly due to reaction with the OH radical. Reaction
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rate constants increase significantly with the molecular size within a given class of NMHC,
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causing lighter, saturated NMHC exhibiting slower atmospheric decay and longer lifetimes.
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Atmospheric concentrations decline at slow enough rates that they remain high enough to be
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measured after several days of transport to remote downwind locations. Since many individual
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NMHC have common emission sources and since their emission ratios vary comparatively little,
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changes in absolute concentrations and NMHC ratios can be used as tools to decipher atmospher-
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ic transport and oxidation chemistry. Several researchers have investigated this utility and have
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presented a framework for the interpretation, in particular using observations of atmospheric C2-
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C6 NMHC.
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The possibilities for using NMHC data for interpretations of atmospheric oxidation processes are
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particularly promising in situations where observations can be obtained in air that has traveled
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for extended periods of time without influence from recent emissions or surface processes.
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Hence, remote islands that are high enough to probe free tropospheric air offer ideal locations for
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this research. These considerations motivated the measurement of NMHC at the mountaintop
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observatory site on Pico Island, Azores. Several other previous studies have shown the strong
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influence of outflow from the North American continent on atmospheric observations made in
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the Azores and how measurements there can provide valuable insight in North American emis-
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sions and their processing during transport. Measurements described in this article commenced
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in the summer of 2004, in overlap with the International Consortium for Atmospheric Research
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on Transport and Transformation (ICARTT) campaign in the North Atlantic Region (Fehsenfeld
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et al., 2006), and have been continuous for most times when the station was on power. These
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data provide one of the very few continuous annual records of NMHC from a lower free-
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troposphere measurement site. Here we present interpretations of the first year of data for a
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characterization of the oxidation chemistry in the North Atlantic region. In the companion man-
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uscript (Helmig et al., 2007, submitted for publication) NMHC in conjunction with FLEXPART
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model result are used for research on the seasonal transport.
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2. Methods
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2.1. Pico Mountain Station
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The Pico Mountain observatory is located in the summit caldera of the inactive Pico Mountain
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volcano (38.47N, 28.40W), the highest mountain on Pico Island, and in the Azores, Portugal. At
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2225 m asl, lower, free tropospheric air is sampled at the station during most times. Buoyant
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upslope flow affects the Pico Mountain station much less than some other marine mountain
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observatories, as a result of the latitude, size, and topography of Pico Island. Intensive meteoro-
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logical measurements for analysis of flow conditions were presented by Kleissl et al. (2007).
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Chemical measurements of nitrogen oxides, carbon monoxide and isoprene at the observatory
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showed very little influence from island emission sources even during uplifting events (or
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upslope flow), indicating that during most periods the station is negligibly impacted by inland
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emissions (Kleissl et al., 2007). Further site descriptions, data and interpretations from other
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research, including studies of oxidized nitrogen species, ozone, carbon monoxide and of aerosol
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properties at Pico Mountain have been presented previously (Honrath and Fialho, 2001; Honrath
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et al., 2004; Lapina et al., 2006) and in other contributions to the special ICARTT issue (Owen et
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al., 2006; Val Martin et al., 2006).
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2.2 NMHC Measurements
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The remoteness of the Pico Mountain site and the limitations for power and for supply of cryo-
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gen and consumable gases determined the design of an analytical system that was tailored to-
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wards this unique situation. All consumable gases and blank air were prepared at the site with
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low-power gas generators. The instrument was designed to follow automated startup and shut-
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down procedures and could be remotely controlled from our Boulder, CO, offices. Ozone was
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removed by flowing the sample air through an ozone scrubber prepared from sodium-thiosulfate-
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impregnated glass wool. After sample drying and NMHC focusing on a peltier-cooled multi-
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stage solid adsorbent trap, NMHC were analyzed by thermal desorption with gas chromatog-
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raphy (GC) separation and flame ionization detection (FID). Quantified NMHC included ethane,
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propane, n-butane, i-butane, i-pentane, n-pentane and isoprene (the ethane record doesn’t begin
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until in fall 2004 when some modifications in the focusing procedure allowed its quantitative
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analysis). Sample volumes of 600 ml (10 min collection time) and 3000 ml (50 min collection
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time) were collected semi-continuously (every few hours) and sample volumes were alternated
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for quantification of ethane (in the 600 ml sample) and NMHC > C2 (in the 3000 ml sample),
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respectively. Typically, a total of 12 ambient air samples, one standard and one blank sample
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were analyzed per day. Data were electronically transferred daily to our laboratory for instant
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quality control and analysis. More instrumental details have been provided elsewhere (Tanner et
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al., 2006).
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NMHC in ambient air samples were quantified using compound-specific FID response factors.
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The instrument was calibrated by regular injections of a compressed ambient air sample (breath-
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ing grade air, Airgas, Boulder, CO0 that was quantified prior to shipment using numerous grav-
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imetrically prepared hydrocarbon standards in the NOAA Earth System Research Laboratory.
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The NOAA calibration scale has previously been found to be on average within 5% agreement
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with that of several other laboratories in the U.S., Canada and Europe including the 60-
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component NMNC standard that was used in the round-robin analysis within the Nonmethane
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Hydrocarbon Intercomaprison Experiment (Apel et al., 1994). The quantifications in the refer-
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ence gas were also compared against our own laboratory NMHC calibration scale (with was
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developed from a series of other gravimetrical or cross-referenced NMHC gas standards) and
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deviations of all quantified NMHC were < 10%
A second, remote, ambient air reference gas
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(collected at Niwot Ridge, Colorado, and quantified in the same way by NOAA) was injected
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every 3-4 days for quality control. The primary calibration reference gas was returned to Boul-
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der in spring 2006 and quantified again against the NOAA ESRL gravimetric hydrocarbon
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standard scale. That analysis resulted in mixing ratios that were within -4.2 to 2.6 % for the C2-
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C5 NMHC reported in this study in comparison to the values that were determined two years
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earlier, prior to the shipment to Pico. From these analysis, the stated ± 5% accuracy of the
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NOAA calibration, and assuming linearity over the whole measurement range, the accuracy error
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of the Pico measurements was estimated to be within the range of -6.5 to 5.6 %. Analytical
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precision was estimated from 16 measurements of the breathing air standard over a 21-day peri-
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od in April 2005. These measurements resulted in relative standard deviations of 0.7 – 4.2 % at
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the mixing ratios in this reference gas. From these measurements the overall uncertainty, com-
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bining analytical accuracy and precision was estimated to be within the range of equal or less of
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± 7.7 %, although it should be noted that this value is expected to increase for data closer to the
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detection limit. Detection limits were determined monthly as 3 x of the integrated noise level at
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the peak retention times or at 2 x the standard deviation of the blank signal (in cases where peaks
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could be detected in the blank). From these repeated measurements, median detection limits
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were calculated as 17, 6, 2-4, and 1 pptv for C2, C3, C4, and C5 NMHC respectively; during
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summer ’05 the C3 detection limit improved to ~3 pptv). Ethene, propene, benzene, and toluene,
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while captured with this system, where excluded from the analysis because of higher and incon-
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sistent blanks, which made their quantification at low pptv levels not feasible.
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3. Results and Discussion
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3.1 NMHC Mixing Ratios
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Plots with the individual sample data (representing a total of 1958 analyzed air samples) for
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ethane, propane, and n-butane from Aug. 2004–Sept. 2005 were presented by Tanner et al.
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(2006). For a better illustration of the seasonal changes of NMHC here we combined these data
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to monthly whisker plots that show the minimum, 5, 25, 50, 75, and 95 percentile, and the max-
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imum values of measured mixing ratios during each month of available measurements (Fig. 1).
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As a first approximation the seasonal cycle of NMHC background mixing ratios can be approxi-
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mated with sinusoidal best fit curves (Rudolph, 1995), however higher resolution data have also
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shown that with decreasing lifetime observed seasonal cycles deviate increasingly from this
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behavior, where the winter maxima become increasingly narrow and the summer minima in-
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creasingly broad (Goldstein et al. 1995). The Pico data do not quite have the temporal resolution
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and high number of data points to clearly demonstrate this behavior. A further constraint is that
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with increasing molecule size an increasing fraction of the data (in particular of summer values)
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fall below the detection limit. Therefore we only applied a best fit sinusodidal regression func-
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tion, defined by y = A + B sin (day+C) (calculated by performing a least-square fit regression
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analysis, from the diurnal mean data) to the ethane and propane data using all available meas-
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urements without attempting to further filter the data to identify a background subset. These
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regression functions are our best estimate for the description of the seasonal behavior of NMHC
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at the observatory during all conditions. The A-values in the regression equation, calculated at
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985 pptv for ethane and 185 pptv for propane are thus our best estimates for the annual mean
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mixing ratios of these two NMHC at the station.
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These data show a distinct seasonal cycle of NMHC with lower mixing ratios in the summer and
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maximum values in late winter. This behavior to a large extent is driven by the annual concen-
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tration changes of the OH radical, which are closely linked to the latitudinal solar radiation cycle.
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High variability in NMHC mixing ratios was observed at any given time of year. It is notewor-
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thy that all of these features show relations and dependencies upon the individual NMHC reac-
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tivity with OH and the resulting NMHC lifetime. The longest-lived NMHC, ethane, shows the
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relatively smallest amplitude between the mean winter and summer mixing ratios and the small-
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est relative variability on short (e.g. weeks) time scales. All of these features increase with
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increasing molecule size (respectively shorter OH lifetime). The seasonal maximum and mini-
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mum of ethane occurs the latest of all compounds (early March and early September, respective-
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ly, determined from the timing of the minimum and maxima of the best fit curve), as due to its
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slower OH reaction, ambient levels respond with a longer delay to the seasonal OH cycle. Heav-
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ier NMHC were found to maximize as early as mid January and minimize as early as mid July.
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These features in the Pico NMHC data are in agreement with results from a number of other
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sites, which have been presented along with their seasonal cycles, and discussed in detail in the
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literature (e.g. Jobson et al., 1994; Goldstein et al., 1995; Gautrois et al., 2003).
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A number of other NMHC records have been presented in the literature. Here we selected two
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particular data sets for a comparison and to highlight the most prominent features in the NMHC
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data from Pico Mountain. The data included in Figure 1 are measurements made from September
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1991 to August 1992 during the Mauna Loa Observatory Photochemical Experiment-2
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(MLOPEX-2, at 19oN, 155oW) (Greenberg et al., 1996) and from April 1990 to October 1992 at
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the continental, remote boreal site in Fraserdale, Ontario (50oN, 82oW) (Jobson et al., 1994).
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The MLOPEX-2 data are of particular interest as they allow a comparison of the conditions in
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the mid-Pacific with the Atlantic Pico site. Similar to the Pico Mountain observatory, Mauna
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Loa (MLO) is a remote mountaintop island location, where, during downslope conditions, free
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tropospheric air is sampled that has traveled over the ocean for several days. MLO has a more
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prominent diurnal upslope-downslope cycle and data presented by Greenberg et al. were divided
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into the occurrences of these two flow regimes. Included in Figure 1 are the 25/50/75 percentiles
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for the time periods spanned by the width of the boxes during downslope (i.e. free tropospheric
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air) conditions. Upslope data for MLO typically were higher, with relative enhancements in-
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creasing with decreasing molecule liftetime. MLOPEX-2 data are consistently lower for all
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NMHC and during all seasons. The differences between Pico and MLO mixing ratios increase
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with decreasing NMHC lifetime, e.g. while ethane mixing ratios agree within ~20%, n-butane
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values at MLO are more than 5 times lower than at Pico. In contrast to MLO, Fraserdale, a
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remote, low elevation continental forest site, experiences overall higher NMHC values than Pico.
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Again, differences in these two data sets become more pronounced with molecular weight, but in
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this case with the Pico data becoming increasingly lower.
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Lower NMHC concentrations at Pico than at Fraserdale and still lower concentrations at MLO
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are likely due to several reasons. Probably of greatest importance is the distance to adjacent
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continents, which is about two times greater for MLO than Pico. The longer transport and pho-
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tochemical processing times result in more depleted NMHC concentrations at MLO compared to
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Pico. Secondly, aircraft profiles have shown that NMHC mixing ratios generally decline with
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height within the free troposphere (e.g. Blake et al., 1997). Pico is higher than Fraserdale, and
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MLO is about 1200 m higher in elevation than the Pico Mountain station; consequently NMHC
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mixing ratios are expected to be highest at Fraserdale, followed by Pico and MLO. This is simp-
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ly a manifestation of atmospheric oxidation, since the NMHC sources are primarily at the sur-
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face. Thirdly, NMHC mixing ratios in the lower troposphere decrease towards lower latitude
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(Rudolph, 1995). Again, this dependency implies higher NMHC levels at Fraserdale (50oN),
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followed by Pico (38oN) and MLO (19oN). This spatial distribution also reflects chemical oxida-
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tion since OH has a latitudinal gradient. Further comparisons of the Pico data with several other
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NMHC data sets from higher northern latitudes in Canada, the Atlantic Region and Europe (as
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summarized by Gautrois et al., 2003) show that Pico NMHC levels are without exception lower,
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both during the winter and the summer compared to the further northern locations that were
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considered in this data comparison. One other point to consider is that possible temporal trends
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in NMHC may bias this site comparison as both the MLO and Fraserdale data are 12-15 years
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older than our Pico measurements. Unfortunately, reports of NMHC trends at remote back-
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ground sites are scarce and do not allow a conclusive evaluation of this question. Measurements
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made in Finland have shown decreasing levels of shorter-lived compounds and increasing trends
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of longer-lived NMHC (Hakola et al., 2006). In source regions in Europe and the U.S. NMHC
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emissions and resulting ambient air mixing ratios have generally been decreasing over the past
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decade (EPA, 2003; Stemmler et al., 2005; Plass-Duelmer and Berresheim, 2006).
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Figure 2 compares the cumulative distributions of NMHC during fall 2004 (September 22 to
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December 20), winter 2004-2005 (December 21 to March 19), spring 2005 (March 20 to June
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20) and summer 2005 (June 21 to September 21). In these analyses the median value is located
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at the center of the y-axis. From here the y-axis scale is stretched such that data within one
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standard deviation of the log-normally distributed data is distributed within half the distance
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from the median as data within two standard deviations and so forth (i.e. the y-axis scale in
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essence is a linear scale of the standard deviation). Distributions that do not extend to lower
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percentage ranges result from respective fractions of these data being below the instrument
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detection limit (for instance, i- and n-pentane were below the detection limit in ~4% of the
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measurements during fall 2004, whereas during summer 2005, ~85% and 60% of chromatograms
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did not have i- and n-pentane peaks that were large enough to quantify). The regression line
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slopes through these distributions indicate the variability of the atmospheric concentrations of a
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given compound. Linear behavior indicates a Gaussian distribution of the data, deviations from
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linearity are indicative of higher mode influencing the distribution, which may imply different
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behavior of NMHC data in air sampled from different sources or at different times. Steeper
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slopes are observed for longer-lived NMHC (e.g. ethane) as these compounds have higher at-
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mospheric background concentrations, which reduces the relative variability caused by emission
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and aging influences. It is noteworthy that regression line slopes are lower for the summer,
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which can be attributed to the shorter seasonal atmospheric lifetime and resulting lower absolute
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concentrations and a higher relative variability driven by a larger range in the degree of photo-
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chemical aging.
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Isoprene was not detected (< 1 pptv) in either winter or nighttime samples. During spring,
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isoprene was occasionally observed during the day. Occurrences and mixing ratios of isoprene
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increased during late summer; e.g. during August 2005, isoprene was detected on 60% of all
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afternoons with a maximum mixing ratio of 26 pptv observed on 1 Aug., 2005 (see figure 8 in
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Kleissl et al. 2007). Since alkanes and alkenes dropped to their lowest seasonal levels during the
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summer, under such upslope conditions, isoprene can become the second most abundant (after
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ethane) NMHC in air sampled at the observatory. Given the much faster OH reaction of iso-
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prene than for other identified NMHC, isoprene, even at these relatively low levels, makes a
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major contribution to the overall OH reactivity from NMHC. For the two days with the highest
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isoprene mixing ratios during 2005, considering all C2-C5 NMHC quantified in our measure-
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ments, and using upper estimates of 3 pptv for ethene and 2 pptv for propene, we calculated that
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the OH reactivity from isoprene contributed e.g. 94 % (DOY 213) and 84 % (DOY 222) to the
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overall OH reactivity from NMHC at their mixing ratios measured on those days.
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There is very little vegetation growing on the upper ~700 m of the slopes of Pico Mountain and
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the most plausible explanation for isoprene observations at the observatory is the upslope
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transport of air from lower island elevations. The increases in observed isoprene during the
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summer months were attributed to the expected summertime increases in isoprene emission rates
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from vegetation on Pico Island, and by seasonal changes of frequency of buoyant uplift flow that
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transports air from lower parts of Pico to the observatory (Kleissl et al., 2007). A correlation
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analysis between NMHC and isoprene in identified upslope events was used to investigate for
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possible anthropogenic signatures in upslope air. N-butane was chosen as an anthropogenic
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tracer as butane is abundantly used on the island for domestic cooking and heating and there are
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no known biogenic sources. This analysis was done by comparing isoprene and n-butane data in
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two subsets of samples. On days when isoprene was detected at the station, the mean mixing
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ratio during the 12-14 hours window (which was the time when maximum daily values were
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observed) was 4.0 ± 5.7 ppbv. On these same days during 22–6 hours isoprene was not detected
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in a single sample (< 1 ppbv). In the same subset of samples, n-butane was 17.1 ± 21.1 ppbv
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during 12-14 hours and 17.4 ± 21.6 ppbv during 22-6 hours. Since no increase in n-butane was
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evident in the elevated isoprene samples, it was concluded that the identified upslope air did not
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have any anthropogenic signature. Most likely, upslope air originated from elevations several
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hundred meters below the observatory but not from the populated areas of the island, which are
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at much lower elevations along the coastline. Consequently, emissions of other NMHC from
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island sources were considered a negligible influence on the NMHC composition in air sampled
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at the station. As, except for isoprene, no systematic enhances of NMHC were seen in air that
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was identified as upslope flow versus air that was clearly attributed to free tropospheric origin,
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NMHC data were not further filtered or broken up dependant on flow conditions.
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3.2 Biomass Burning Events
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During the summer of 2004 the NMHC system was still going through some modifications and
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optimization steps. The data record for this period is not as complete and precision, accuracy
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and detection limits were on the order of 50% worse than after November of 2004. Also, ethane
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was not quantitatively retained under the experimental conditions used. The available data was
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used to investigate NMHC occurrences in the summer 2004 biomass burning plumes that were
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encountered at Pico. As discussed in detail in other contributions to the ICARTT issue, the
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summer of 2004 was characterized by an unusually high occurrence of boreal wild fires in
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Northern Canada and Alaska, outflow of which was frequently observed at Pico (Val Martin et
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al., 2006). Comparison of the NMHC > C2 for summer 2004 with data from the corresponding
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period during 2005 shows a higher variability as well as overall higher mixing ratios during
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2004. Peak mixing ratios for propane in fire plumes at times increased significantly above their
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seasonal background levels. The data for propane and n-butane within fire plumes 3-5, as de-
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fined in Val Martin et al. (2006) in comparison with data during Day of Year (DOY) 209-227,
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which were from outside the fire plumes, and comparison with the data from 2005 (which was
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not effected by fire events) are shown in Table 1. (Please note that during 2004, a few samples
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within and outside of fire events had anomalously high n-butane peaks, which could not be
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explained by any transport analyses and most likely were due to a measurement artifact. Points
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with a n-butane/propane ratio greater than 2 were removed from the analysis, which resulted in
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removing 9% of non-fire event points and 4% of fire event points.) Enhancements seen for both
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propane and for n-butane in the plumes were significant: mixing ratios for both compounds were
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on the order of 2-3 times higher than during non fire events in 2004, and all of the 2005 data
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from the same time period. Average propane levels increased by a factor of 3.2 (190 pptv during
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fire events compared to 60 ppt outside of fire events), and average n-butane levels increased by a
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factor of 2.8 (22 pptv during fire events compared to 8 ppt outside of fire events). Larger en-
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hancement in the propane levels would be expected because of two effects: emission factors for
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propane in biomass burning emissions are about 3 to 5 times higher than for n-butane (Andreae
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and Merlet, 2001), and the propane lifetime is about two times longer than for n-butane, allowing
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a higher fraction of the propane biomass emissions to remain in these plumes after transport to
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Pico. The NMHC data for the 2004 summer, albeit limited, underscore conclusions derived from
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observations of CO, NOx and black carbon (Val Martin et al., 2006), in that biomass plumes
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from the Alaskan wildfires continued to affect atmospheric composition and oxidation chemistry
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after 6-15 days of transport to the Azores region.
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3.3 NMHC Variability
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The relationship between the variability of NMHC and their lifetimes can be used to characterize
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the importance of local emissions on the air composition at a given site. In this analysis the
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variability of NMHC (expresses as lnx, the standard deviation of the natural logarithm of all
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measurements) is plotted against the estimated atmospheric liftetime (in a double-logarithmic
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plot such (Jobson et al., 1998, 1999). This data distribution can be approximated by lnx = A -b.
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The A and b coefficients from the regression line analysis through these data have been used to
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provide further characterization of the influence of emission sources on the data distribution.
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Using one other atmospheric component with known atmospheric lifetime, best fit analysis
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through the combined data has also yielded estimates for mean OH radical fields during transport
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of air to the measurement site (Ehhalt et al., 1998; Williams et al., 2000, 2001).
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While previous studies have applied this relationship to characterize data sets and sampling sites
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from mostly shorter campaigns, the Pico data offers an opportunity to test for this dependency
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and possible seasonal variations using a full year of data. The variability of NMHC during each
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of the four seasons is reflected by the slopes of regression lines through the data in Figure 2
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where the calculated slope values are inversely related to the variability of NMHC mixing ratios
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within a given season, with the inverse of the slope of each regression line being lnx. An esti-
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mate of the seasonal lifetime, of each NMHC was obtained from its OH reaction constant and
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the seasonal OH radical concentration. [OH] was estimated at a 1-day resolution according to
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(Goldstein et al., 1995):
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[OH] = A [1-B cos(2 t/365)],
(1)
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where A=1.6*106 and B=0.80. This relationship was derived from monthly average OH values
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(Spivakovsky et al., 2000) for 800 hPa, 36.0oN, 27.5oW. Here, daily OH concentrations from
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this equation were averaged to seasonal OH values within the defined time periods. Reaction
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rate constants are adjusted to the temperatures measured at the Pico Mountain station during the
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respective season. This local lifetime represents an estimate for the conditions at the receptor
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site; the actual lifetime may have deviated from this estimate dependent on the conditions actual-
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ly encountered during transport to Pico. Please note that this analysis is not very sensitive to the
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assumed [OH], but more so towards the relative reactivity differences between individual com-
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pounds. Consequently, errors in the estimated [OH] will have little effect on the results for the
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regression coefficients (Jobson et al., 1999).
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The data distribution with the regression results of this analysis including solutions for lnx = A
374
-b for each seasonal data set are shown in Figure 3.
375
estimates are well correlated with the seasonal lifetime estimates (R2 values range from 0.93-
376
0.99). The calculated A- and b-values for the five subsets of seasonal data range from 1.4-2.3
377
and 0.43-0.60, respectively; the later four subsets, which all have the full set of all C2-C5 NMHC,
378
give an even narrower range of 1.4-1.7 for A, and 0.43-0.44 for b. These finding suggests a high
379
similarity of the NMHC behavior between the seasons and that the absence of detectable differ-
In each subset of seasonal data the lnx
12
380
ences in influences on the NMHC variability between the seasons, which can be taken further to
381
assumer similarity in source regions and transport pathways for the differentiated seasons. The
382
best fit linear regression analysis through the data for all seasons yielded lnx = 1.60 -0.44. The
383
exponent b in this equation has been taken to describe the importance of sink terms in the re-
384
gional variability budget. Interpretation of observed values of b from different sites has shown
385
that b approaches 0 near urban areas, where the variability is strongly influenced by differences
386
in the strength of local emission sources. b-Values close to 1 were found in stratospheric data,
387
where the variability is low and dominated by chemical loss alone (Jobson et al., 1998; 1999).
388
Interestingly, the mean Pico value of 0.44 ± 0.03 is close to results from three other data sets
389
from marine environments, even though those data resulted from shorter observations. B-values
390
from the Arctic Boundary Layer (ABLE3A), the equatorial Atlantic (TRACE-A), and the west-
391
ern Pacific (PEM-West B) experiment all ranged from 0.46–0.53 (Jobson et al., 1999). This
392
comparison illustrates a rather high similarity between the continuous, seasonal Pico data and the
393
results from the comparatively short aircraft campaigns. It also underscores that the NMHC data
394
from the Pico Mountain observatory behave similar as the data sets from the marine troposphere.
395
396
3.4 Ratios of NMHC
397
398
Ratios of NMHC were investigated for the seasonal behavior of observations, and chemical
399
processing and oxidation chemistry during transport. Correlation plots of all saturated C2-C5
400
NMHC combinations as well as of these NMHC with CO are provided in the Appendix section
401
to this paper. Results for the linear regression analyses of correlation plots are given in Table 2.
402
A common feature in these data is that regression line slopes of NMHCA/NMHCB (with carbon
403
number NMHCA < NMHCB) increase monotonically with increasing carbon number of NMHCB.
404
This behavior is expected as the atmospheric mixing ratios (and lifetimes) of NMHC generally
405
decrease with increasing carbon number. For an individual pair of NMHC, the regression line
406
slopes become larger towards the summer, as the longer-chain NMHC are removed from the
407
atmosphere faster than the more stable, shorter-chain NMHC. Regression coefficients generally
408
decrease towards the summer, as shorter liftetimes, lower concentrations, and higher relative
409
variability cause the correlation between individual compounds to become weaker. However,
410
the correlation between CO and ethane (which have very similar lifetimes) is notably weaker,
13
411
likely because of their differences in primary and secondary sources. Although fossil and biofuel
412
combustion is a common source of both gases, natural gas production and distribution is a major
413
source of ethane, but not of CO, and CO is also a degradation product of hydrocarbons in the
414
atmosphere.
415
416
Ratios of NMHC can serve as a tool for quality control in NMHC measurements (Parrish et al.,
417
1998). Deviations of NMHC data from expected depletion by OH have been used to infer con-
418
centrations of secondary reactions, such as of the chlorine (Finlayson-Pitts, 1993; Rudolph et al.,
419
1996, 1997; Aresene et al., 2007; Pszenny et al., 2007) and the nitrate radical (Penkett et al.,
420
1993). These analyses rely on the assumption that variations of emission ratios of NMHC pairs
421
are relatively small in source regions. A good body of NMNC data from urban areas supports
422
this assumption, however there have been a few reports that point towards seasonal changes of
423
NMHC emission ratios in source regions (Greenberg et al., 1996; Swanson et al., 2003; Lee et
424
al., 2006). The OH reaction rate constants of the isomeric pairs i-butane and n-butane, and of i-
425
pentane and n-pentane are very similar; consequently, the ratios of these two compound pairs are
426
expected to change comparatively little during transport from OH oxidation. The i-butane/n-
427
butane correlation n the data from Pico, differentiated by the time of year, shown in Figure 4
428
(upper left panel). The upper right panel investigates the behavior of the [i-butane]/[n-butane]
429
ratio as a function of total concentration (of n-butane). A tight relationship between the concen-
430
trations of two isomers is obvious. Other than for a few outliers and a slight increase in scatter at
431
lower mixing ratios (which likely is due to a loss of precision at lower mixing ratios), the two
432
butane isomers show no obvious change in their correlation throughout the course of the year.
433
Subsets of the data for the four seasons showed no statistically significant difference in the re-
434
gression line slope describing the i-butane versus n-butane distribution. The regression line
435
slope for all data was calculated to be 0.51 ± 0.01 (R2 = 0.96), and the geometric mean ratio was
436
calculated to be 0.46 ± 0.12. Similar values (range 0.37–0.55) have been reported in data from a
437
multitude of other sites in both continental and marine environments (e.g. Bottenheim and Shep-
438
herd, 1995; Bottenheim et al., 1997; Greenberg et al., 1996; Parrish et al., 1998). When the data
439
was grouped into 10-percentile bins the means within each bin did not show any differences in
440
the i-buane/n-butane ratio either, indicating a constant behavior over the dynamic range of mix-
441
ing ratios observed at the station (Figure 4, upper right panel). The Pico data show a much
14
442
different behavior than recently reported observations on the island of Crete (Arsene et al.,
443
2007). These areas divided their data in three classifications (marine, rural-summer, rural-
444
winter), but given the proximity of both sites to coast and their low elevation all three groups of
445
data are expected to have a strong marine signature. All three subsets showed increases in the [i-
446
butane]/[n-butane] ratio with decreasing [n-butane], as well as with increasing degree of photo-
447
chemical processing. This deviation was explained by the influence of chlorine atom influence,
448
as the ~50% faster reaction of the chlorine atom with the n-isomer will shift the [i-butane]/[n-
449
butane] ratio to higher values during transport. Other marine, polar data showed similarly in-
450
creasing ratios of this isomer pair in air that had been subjected to prolonged processing in the
451
marine boundary layer (Hopkins et al., 2002; Read et al., 2006). Since direct measurements of
452
chlorine at ambient levels are extreme difficult and have only been accomplished in a few situa-
453
tions (Spicer et al., 1998; 2002; Finley and Saltzman, 2006), several researchers have attempted
454
to use the behavior of NMHC ratios for deducing estimates of average Cl atom levels and much
455
of our current understanding of the ambient Cl atom distribution stems from NMHC observa-
456
tions (Wingenter et al., 1996; Rudolph et al., 1997; Arsene et al., 2007; Pszenny et al., 2007).
457
The fact that the [i-butane]/[n-butane] does not show the features seen in the marine data (i.e.
458
increases at lower mixing ratio and higher degree of processing) points towards a much lower
459
influence of chlorine chemistry at this site. This finding is consistent with our previous conclu-
460
sion, that air sampled at the station has low marine boundary layer influence. It also further
461
underscores the belief that chlorine atom concentrations decline rapidly with altitude, and that
462
chlorine chemistry is of most significance in marine boundary layer air (Singh et al., 1996; Spic-
463
er et al., 1998) but of minor importance in the free troposphere.
464
465
The results of the same analysis for the two pentane isomers are shown in the two lower panels
466
of Fig. 4. The regression analysis through the data in Fig. 4c yielded a slope of 0.69 ± 0.08 (R2 =
467
0.93); a geometric mean ratio of 0.78 ± 0.31 was calculated for all of the included data. Upon
468
closer inspection, it appears that the pentanes behave somewhat different than the butane iso-
469
mers. Increasingly higher [n-penane]/[i-pentane] ratios are observed moving from the higher
470
mixing ratio percentiles (mostly winter data) towards the lower percentiles of the data (mostly
471
spring and summer data). The mean value of the [n-pentane]/[i-pentane] ratio increased from
472
0.64 ± 0.11 in the upper 10 percentile of the data to a value of 1.15 ± 0.51 in the lowest 10 per-
15
473
centile of the data.
A notable change in the seasonal behavior of the pentane isomers is also
474
apparent in the cumulative distribution plots (Fig. 2). The n-pentane isomers are the only pair of
475
compounds that switch places during the course of the year. While the i-pentane distribution
476
shows the higher values in the fall and winter samples, the summer distribution is reversed, with
477
i-pentane being found at lower mixing ratios. Unfortunately, the interpretation of the pentane
478
data is somewhat limited by the fact that the measurement sensitivity was not sufficient to detect
479
either one or both pentane isomers in a significant fraction of the summer samples. For n-
480
pentane, only 41%, and for i-pentane, an even small fraction (14%) of the summer measurements
481
were above the detection limit. Only 11% of the samples had both i- and n-pentane. So, any
482
conclusions about these data are from the highest 11% of concentrations observed during the
483
summer. Consequently, interpretations are based upon a small fraction of the overall observa-
484
tions. During the summer there were a considerable number of samples that had relatively high
485
n-pentane (8-45 pptv), but with i-pentane below the detection limit. A loss of correlation of
486
pentanes with other NMHC and between the two pentane isomers is also evident in the low R2
487
values for the summer data in Table 2. For samples had have both i- and n-pentane data (n=34)
488
above the detection limit, the i-pentane mixing ratio and standard deviation was 3.7 ± 2.0 pptv
489
and the n-pentane was 3.0 ± 1.5 pptv.
490
491
However, the available pentane data from Pico i point towards a similar behavior as observations
492
from other studies. Similarly enhanced [n-pentane]/[i-pentane] ratios during summer months and
493
in low-concentration (well aged) samples were also evident in the Fraserdale data (Jobson et al.,
494
1994) as well as during ICARTT in the WP-3D data set (D. Parrish, unpublished data). This
495
behavior points to either different source region emission ratios, to different oxidation chemistry
496
during the summer months.
497
498
The [n-pentane]/[i-pentane] ratios in the Pico as well as the other remote data sets can be con-
499
trasted to overall seem to the pentane ratios seen in data from source regions. For instance [n-
500
pentane]/[i-pentane] ratios in the extensive data sets (2005) from Hohenpeissenberg, Germany,
501
2005 are 0.58 ± 0.15 (median ± standard deviation) for January and 0.37 ± 0.10 for July (Plass-
502
Duelmer and Berresheim, 2006; and 2007, personal communication). Results from measure-
503
ments in the Southeastern United States were similar, with a ratio of 0.60 ± 0.05 for winter, and
16
504
0.43 ± 0.13 for summer (mean ± standard deviation of median seasonal data from four sites,
505
Hagerman et al., 1997). This trend is further manifested by extensive NMHC data from Harvard
506
forest, that show a 60% decrease in [n-pentane]/[i-pentane] during summer. There are two strik-
507
ing features in this comparison. Firstly, the [n-pentane]/[i-pentane] in the Pico data overall is
508
higher than in the source region data, and secondly the seasonal tendency is reversed, with the
509
source region [n-pentane]/[i-pentane] data generally decreasing towards the summer, while an
510
increasing trend is seen at Pico (and similarly at the other above-mentioned remote sites).
511
512
The behavior in the Pico (and other aforementioned remote) data is contrary to expected OH
513
kinetics. A relative decrease of [n-pentane]/[i-pentane] would be expected during summer and
514
with increasing transport time, due to the slightly higher n-pentane OH reaction rate constant (4.0
515
x 10-12 cm3 molecule-1s-1 at 298 K) compared to i-pentane (3.7 x 10-12 cm3 molecule-1s-1) (Atkin-
516
son, 1997). The deviation from this expected behavior could conceivably point towards another
517
reaction pathway with a different seasonality than OH, or an increasing influence of different
518
NMHC sources with different emission ratios during the summer. Since chlorine favors the n-
519
isomer (as does OH), chlorine reaction would contribute to a further reduction of [n-pentane]/[i-
520
pentane] during photochemical processing and can therefore be ruled out as a possible explana-
521
tion. Reaction with NO3 would shift the isomer ratio in favor to the n-isomer (as reaction of the
522
iso-pentane is about two times faster), but a first estimate of the [NO3] needed to account for the
523
observed shift in [n-pentane]/[i-pentane] resulted in unrealistically high NO3 levels. A possible
524
explanation, raised by one of the reviewers of this manuscript, would be that a stronger influence
525
of biomass burning activities during the summer shifts the pentane isomer ratio more towards the
526
n-isomer, and that the influence of biomass burning would be more significant at the Pico loca-
527
tion compared to the other more northern latitude sites used in our data comparisons. Indeed,
528
biomass burning emission factors compiled by Andreae and Merlet (2001) give a range of 0.4 –
529
2.1 for [n-pentane]/[i-pentane], with a median value of 1.4 in six different biofuel types.
530
531
As mentioned earlier, unfortunately, only 14% of the summer measurements had both pentanes
532
with signals above the detection limit, so this analysis can only be based on a small subset of the
533
measurements. Another point that needs to be considered is a potential bias in the analysis. For
534
instance, it may be possible that at lower mixing ratios (e.g. < 5 pptv) analytical artifacts (e.g. an
17
535
unidentified coelution of a small peak with n-pentane) could cause an increasingly important bias
536
in these calculations. New measurements, including higher sample volume collections and more
537
analytical tests, were conducted at Pico in 2006 and are planned for 2008-2009. With the higher
538
sensitivity expected in these new data a stronger data set is anticipated which together with
539
inclusion of transport studies will be applied for a re-evaluation of this analysis.
540
541
By calculating the ratio of pairs of hydrocarbons the influence of mixing on the composition of a
542
given air parcel can be eliminated. The analysis of [NMHC] against [NMHC a]/[NMHCb] there-
543
fore allows a more sensitive investigation of the seasonal NMHC oxidation than using concentra-
544
tions alone. For the analyses Figure 5 [propane] and [n-butane] were plotted against [pro-
545
pane]/[ethane] and [n-butane]/[ethane]. Since with this selection of pairs [NMHCa]/[NMHCb]
546
decrease with photochemical processing lower values in these ratios, along with overall lower
547
absolute mixing ratios are seen in the summer (lower left portion of the four graphs) and higher
548
values in the winter (towards the upper right part of these graphs). Another interesting point is
549
that the seasonal distribution is such that at a given [NMHCa]/[NMHCb] lower absolute mixing
550
ratios are observed during the earlier part of the year and during the later part. This hysteresis
551
behavior is not surprising. If the seasonal cycle of considered NMHC pairs would be in phase
552
than data in this plot would be expected to fall on top of each other during the earlier and later
553
part of the year when the same [NMHCa]/[NMHCb] is observed. The fact that the data does not
554
show this behavior further exemplifies that the shift in the seasonal maxima and minima for
555
individual NMHC (moving toward earlier in the year with decreasing atmospheric lifetime).
556
557
The distribution of NMHC in a double natural logarithm plot of [n-butane]/[ethane] versus [pro-
558
pane]/[ethane] can be used to investigate photochemical processing that occurs during atmos-
559
pheric transport (Rudolph and Johnen, 1990; McKeen and Liu, 1993). The ratios of selected
560
NMHC pairs, with the less reactive NMHC in the denominator progressively decrease as aging,
561
i.e. OH oxidation, proceeds. It is helpful to view two theoretical limits indicated by the solid
562
lines in Figure 6. The steeper “kinetic” line indicates the evolution of isolated air parcels with
563
the assumed common emission ratios of the compounds (indicated by triangles in Figure 5) when
564
subjected to oxidation by OH radicals. Applied rate constants were k = 0.18 x 10-12, 0.89 x 10-12
565
and 2.05 x 10-12 molecules-1 cm3s-1 for ethane, propane and n-butane, respectively (Atkinson and
18
566
Arey (2003) with T = 273 K). The less steep “dilution” line indicates the effect of dilution of an
567
air parcel starting at the assumed molar emission ratio with “background” air that has aged to the
568
point that there are negligible concentrations of the more-reactive NMHC (propane, n-butane) in
569
the numerators of the ratios. The measured Pico data, collected in air parcels subjected to both
570
aging and dilution, fall between the two lines. Both lines start from a common point that is
571
determined by the molar emission ratios in the source region. For the representation of the two
572
lines in Figure 5 we chose the mean of the extensive data sets from Goldstein et al. (1995), and
573
Swanson et al. (2003) (0.63 for [propane]/[ethane] and 0.35 for [n-butane]/[ethane]. This data
574
pair, indicated by the black diamond as well as the individual data from these two references are
575
included in Figure 6. Their mean value falls right on the line of the regression line through all of
576
the Pico observations and therefore can be considered as a good representation for source region
577
emissions for the year-round data. The seasonal differences in the degree of NMHC oxidation
578
are clearly visible in these plots. During winter, most data have larger ratios, fall near the as-
579
sumed emission ratios, and are less variable, indicative of less photochemical processing. In
580
contrast, spring and summer data are more scattered, and the lower [NMHC]/[ethane] ratios are
581
indicative of the higher degree of NMHC oxidation that occurred during transport. The two-
582
sided regression through all data yields a slope of 1.60 (±0.04) (black, staggered line in Figure
583
5), which is within the range of slopes (1.44- 1.78) in the 11 data sets summarized by Parrish et
584
al., (2007).
585
586
A number of available additional source region data pairs were added to this figure to investigate
587
the variability and fit of the Pico data to these other observations. It is striking that while there is
588
close agreement with the convergence of the Pico data and the same NMHC ratios seen in other
589
data sets (Goldstein et al., 1995; Swanson et al., 2003; Pollmann et al., unpublished data) several
590
other data sets seem to deviate from this behavior (Seila et al., 1989; Warnecke et al., 2007). In
591
the later study a plethora of new NMHC observations off the coast of New England were nor-
592
malized to observed levels of acetylene and carbon monoxide and source region emission factors
593
were calculated by plotting these ratios against estimated photochemical age of the air mass and
594
extrapolating this dependency to a zero processing time. A closer investigation of the seasonal
595
behavior of the Pico data provides some possible insight into this discrepancy. The regression
596
analysis of the four seasonal subsets of these data resulted in regression line slopes of 1.58 ±
19
597
0.15, 1.58 ± 0.07, 1.61 ± 0.11, and 1.40 ± 0.18 for the fall, winter, spring, and summer, respec-
598
tively. These lines, added in color in Fig. 6 show that of all seasonal data the Pico summer
599
measurements provide the closest match with the emission ratios from Warnecke et al., (2007),
600
which were based on mid-summer observations. The slope values of the other seasonal data
601
subsets are too high and miss the New England emission ratios by a much larger margin. The
602
much better agreement between the Pico summer data and the Warnecke et al. emission rates
603
suggest a seasonally changing emission ratios in the source region, where either [n-
604
butane]/[ethane] is lower or [propane]/[ethane] is higher during the summer than during other
605
times of the year.
606
supported by the recent analysis of Lee et al. (2006), who similarly observed reduced butane
607
values in their data from Harward forest and attributed these changes to the mandated reduction
608
of gasoline composition (volatility) during the summer, which was implemented in the early
609
1990s as a measure to curb summertime ozone production in the Eastern U.S.
The first assumption (lower summertime [n-butane]/[ethane] emissions) is
610
611
612
3.5 NMHC Processing and Ozone
613
In general, understanding the temporal variability of tropospheric ozone at any particular loca-
614
tion is complex because several processes can have significant impacts, and these impacts vary
615
strongly on different time scales. In-situ photochemical production and destruction proceed at
616
rates that vary with the ambient levels of ozone precursors, and variables such as sunlight and
617
water vapor concentrations. Surface deposition and destruction by reaction with local emissions
618
of NO or reactive NMHC can drastically reduce near-surface ozone concentrations at rates that
619
vary with the flux of local emissions and the structure and evolution of the planetary boundary
620
layer. Transport of ozone from the stratosphere or from upwind regions of strong photochemical
621
production can greatly increase ozone concentrations. NMHC ratios and concurrent measure-
622
ments of ozone allow interpretations on the oxidation chemistry of the atmosphere as the relative
623
change of ozone with observed NMHC ratio evolution is an indication of ozone production or
624
loss during long-range transport (Parrish et al., 2004).
625
626
The Pico Mountain site is a good location to isolate the effects of the regional photochemical
627
production and destruction in the central North Atlantic. Kleissl et al. (2007) showed that air
20
628
sampled at the site is characteristic of the lower free troposphere with no significant effects from
629
surface deposition, destruction by reaction with local emissions, or photochemical production
630
from locally emitted precursors. The varying influence of the transport of stratospheric ozone
631
often dominates the variability of ozone in free tropospheric data sets. Since ozone from the
632
stratosphere has a steep, negative correlation with CO (see e.g., Danielsen et al., 1987)) the
633
influence of stratospheric ozone transport can be evaluated from the correlation of ozone with
634
CO. Honrath et al. (2004) discuss the strong, positive ozone-CO correlations observed at Pico
635
Mountain; their Figure 7, which presents these correlations, shows only a few scattered points
636
with relatively high ozone at low CO that may be due to stratospheric influence. Thus, con-
637
sistent with other Pico analyses (Lapina et al., 2006; Owen et al., 2006; and Val Martin et al.,
638
2006), Honrath et al. (2004) concluded that the ozone variability is dominated by the influence of
639
the regional photochemical production and destruction in the central North Atlantic.
640
641
Parrish et al. (1992, 2004) demonstrate that NMHC aging correlates with increasing ozone when
642
photochemical ozone production dominates, and correlates with decreasing ozone when ozone
643
destruction processes dominate. These previous studies focused upon marine boundary layer
644
observations to avoid the confounding influence of stratospheric ozone, which can disrupt the
645
ozone-NMHC relationships. This same analysis can be directly applied to the Pico data present-
646
ed here. Figure 7 shows the dependence of ozone concentrations on the natural logarithm of
647
[propane]/[ethane] as the indicator of the photochemical processing in each season. During fall
648
and winter ozone concentrations are relatively constant with no dependence on the NMHC ratios.
649
In spring and summer ozone has higher variability, both in the higher and lower concentration
650
ranges. The regression lines that are included in the spring and summer graphs were calculated
651
by using a linear, least-squares weighted regression algorithm that allows for uncertainties in
652
both the x and y variables. A weighting for each variable was applied by multiplication with
653
1/σ2, where σ is the estimated relative? uncertainty in each variable. This analysis indicates an
654
increasing correlation between the degree of hydrocarbon processing (as indicated by the hydro-
655
carbon ratio) and the levels of ozone that were observed at Pico during the spring-summer
656
months. It is also apparent that higher ozone levels were consistently observed in air that had
657
relatively ‘fresh’ photochemical signatures (i.e. ln [propane]/[ethane] > -2.5), and lower ozone
658
levels were seen in more processed air (i.e. ln [propane]/[ethane] < -2.5). These relationships
21
659
also indicate that in spring and summer the highest ozone concentrations were observed when air
660
masses most recently transported from continental source regions impacted the site, and lower
661
concentrations were observed in air masses that had been processed for longer times. Evidently
662
the photochemical environment experienced by aged air masses in the central North Atlantic is
663
characterized by net photochemical destruction of ozone in spring and even more strongly in
664
summer. Note that the same analysis was conducted on a subset of these data that were filtered
665
of periods with suspected upslope conditions according to the criteria given in Kleissl et al.,
666
(2007). This analysis yielded regression line slopes for the ozone-ln [propane]/[ethane] relation-
667
shipes that were within 5% of those in Fig. 6 and not statistically different. This finding further
668
confirms the negligible influence of island effects on the Pico Mountain ozone observations.
669
670
During the summer of 2004 the NASA DC-8 aircraft conducted flights over the western North
671
Atlantic Ocean as part of the INTEX-A field study (Singh et al., 2006). Data collected over the
672
North Atlantic during those flights have isolated the vertical distribution of ozone chemical
673
sources and sinks, and were used to conduct box model calculations initialized with observed
674
concentrations of measured species (J.R. Olson and J.W. Crawford, private communication,
675
2007). These calculations found that net ozone destruction dominated in the lower troposphere,
676
while net production characterizes the upper troposphere. Thus, the INTEX aircraft analyses are
677
consistent with the present study based on the lower troposphere monitoring at the Pico Moun-
678
tain station.
679
680
Table 3 compares the springtime and summertime slope of the ozone - ln [propane]/[ethane]
681
relationship found at Pico Mountain (data for fall and winter were not considered because there
682
was no correlation in those data) with those reported from the north temperate Pacific marine
683
boundary layer. Based on the earlier data included in Table 3, Parrish et al. (2004) argued that
684
the recent Pacific studies (ITCT-2K2, TRACE-P) find evidence for only weak net ozone destruc-
685
tion (small positive slopes) in the more remote Pacific marine boundary layer, or no evidence for
686
net ozone destruction (PHOBEA). This weak photochemical destruction is in sharp contrast with
687
the much stronger photochemical destruction indicated by a study at Point Arena from nearly
688
two decades earlier. The exception to this picture is the strong photochemical production (large
689
negative slope) in the PEM West-B study, which focused on the region of strong outflow of
22
690
ozone precursor emissions from Asia to the western North Pacific. Comparison of these selected
691
results from the Pacific region to those from Pico suggest that, at the present time, spring- and
692
summertime photochemistry more effectively destroys ozone in lower free troposphere over the
693
central North Atlantic than in the springtime marine boundary layer of the central North Pacific.
694
695
696
4. Summary and Conclusions
697
698
Air sampled at Pico shows high variability in NMHC and their ratios during all times of the year.
699
This observation is indicative of the atmospheric transport conditions that bring air with variable
700
flow, origin and photochemical history to the station. Overall, concentrations of NMHC at Pico
701
Mountain are lower than at remote, higher northern latitude sites, but higher than at MLO. The
702
observed NMHC levels at Pico reflect the increased influence of the adjacent continents on air
703
composition in the central Atlantic region in comparison to the Northern Mid-Pacific (MLO), as
704
well as the station’s latitude and elevation above sea level.
705
706
Short-chain NMHC remain elevated in air masses that have been influenced by either anthropo-
707
genic injections or biomass burning after time scales of 1-2 weeks during their transport to Pico.
708
Isoprene data sensitively identify summertime (mostly buoyant) upslope flow occurrences, and
709
were found to be the best of all chemical tracers for identifying upsloping air (results reported by
710
Kleissl et al., 2007). Even though isoprene levels remain low, isoprene can contribute signifi-
711
cantly to OH reactivity under such conditions.
712
713
Subsets of seasonal data showed similar behavior in NMHC variability as a function of OH
714
lifetimes. Regression analysis of this relationship provided evidence for the remote character of
715
the Pico site and the lack of major local influences on NMHC levels. Furthermore, this analysis
716
suggests similar transport regimes and source regions throughout the year. Ratios of butane
717
isomers behave as expected from OH chemistry. This implies a lower influence of chlorine
718
reactions in the Pico data compared to a number of reports from marine boundary layer observa-
719
tions.
720
23
721
Spring- and summertime ozone and [propane]/[ethane] ratios show higher variability, indicating,
722
as expected, more extensive photochemical processing than during wintertime, paralleling the
723
solar radiation cycle. During spring and summer, ozone is observed to decrease with increased
724
photochemical processing as measured by the [propane]/[ethane] ratio. This relationship indi-
725
cates that this processing is characterized by net ozone destruction in air masses reaching the
726
Pico site. This behavior is in contrast to the springtime North Pacific, where the photochemical
727
processing is much more nearly ozone neutral, i.e. it neither produces nor destroys ozone.
728
729
730
Acknowledgments
731
732
We thank P. Goldan, NOAA Earth System Research Laboratory, Boulder, CO for the reference
733
analysis of the primary NMHC standard prior and after its use at Pico, M. Dziobak and M. Val
734
Martin, Michigan Technological University, for GC instrument maintenance tasks at Pico, and T.
735
Jobson, Washington State University, for the Fraserdale data. This research was funded by a
736
grant from the NOAA Office of Global Programs (award # NA03OAR4310072). REH and RCO
737
acknowledge support from NOAA grant NA03OAR4310002 and National Science Foundation
738
grant ATM-0535486.
739
740
741
742
743
744
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28
5000
4000
3000
___Pico
___Mauna Loa
___Fraserdale
ethane
5000
4000
3000
2000
___Pico
___Mauna Loa
___Fraserdale
propane
1000
400
400
300
200
100
40
30
20
300
10
200
4
3
2
100
1000
400
300
200
mixing ratio (pptv)
mixing ratio (pptv)
1000
jul
aug sep oct nov dec jan feb mar apr may jun
___Pico
___Mauna Loa
___Fraserdale
jul
i-butane
1000
400
300
200
100
40
30
20
10
jul
aug sep oct nov dec jan feb mar apr may jun
___Pico
___Mauna Loa
___Fraserdale
jul
aug
n-butane
100
40
30
20
10
4
3
2
4
3
2
1
1
aug
mixing ratio (pptv)
mixing ratio (pptv)
2000
jul
aug sep oct nov dec jan feb mar apr may jun
jul
aug
1
jul
aug sep oct nov dec jan feb mar apr may jun
jul aug
29
mixing ratio (pptv)
400
300
200
___Pico
___Mauna Loa
___Fraserdale
i-pentane
1000
400
300
200
mixing ratio (pptv)
1000
100
40
30
20
10
n-pentane
100
40
30
20
10
4
3
2
4
3
2
1
___Pico
___Mauna Loa
___Fraserdale
jul
aug sep oct nov dec jan feb mar apr may jun
1
jul
aug
jul
aug sep oct nov dec jan feb mar apr may jun
jul
aug
Figure 1
Whisker plots of monthly data for ethane, propane, i-butane, n-butane, i-pentane and n-pentane. The 5, 25, 50, 75, 95 percentiles are
indicated by the horizontal lines of each box, the vertical lines extend to the minimum and maximum observed values. The width of
the box indicates the time period over which data was acquired for a given month. The vertical dotted lines show the windows that
were applied in the seasonal (winter, spring, summer, fall) analysis. Data from a remote boreal forest site in Canada (Jobson et al.,
1994) and 25, 50, and 75 percentile data from Mauna Loa Observatory (Greenberg et al., 1996) were added for comparison. Best
sinusoidal fit curves were included for ethane and propane, but not for the higher alkanes as their annual cycle increasingly deviates
from this behavior.
30
Fall '04
i-pentane
i-butane
n-butane
propane
y = 0.89x - 2.2 y = 1.3x - 4.4 y = 1.6x - 7.0 y = 1.8x - 9.7
R2 = 0.99
R2 = 0.99
R2 = 0.99
R2 = 0.99
Winter '05
ethane
y = 3.5x - 24
R2 = 0.99
n-pentane
y = 1.2x - 3.4
R2 = 0.99
99
99
95
90
95
90
Percent of distribution
Percent of distribution
n-pentane
y = 0.94x - 2.1
R2 = 0.98
80
70
50
30
20
10
5
1
ethane
i-butane
n-butane
propane
y = 1.7x - 6.9 y = 1.6x - 7.2 y = 2.5x - 12 y = 4.5x - 32
R2 = 0.99
R2 = 0.99
R2 = 0.99
R2 = 0.99
80
70
50
30
20
10
5
1
1
10
100
Mixing ratio (ppt)
1
1000
n-pentane
y = 0.93x - 1.2
R2 = 0.98
10
100
Mixing ratio (ppt)
1000
Summer '05
Spring '05
ethane
n-pentane
n-butane
propane
i-butane
y = 0.97x - 1.7 y = 1.4x - 4.7 y = 2.7x - 17
y
=
0.96x
+
0.02
y
=
0.80x
+
0.13
y = 0.77x + 0.76
R2 = 0.99
R2 = 0.99
R2 = 0.99
R2 = 0.99
R2 = 0.96
R2 = 0.95
i-pentane
ethane
i-butane
n-butane
propane
y = 3.2x - 32
y = 0.82x - 1.0 y = 0.84x - 1.9 y = 1.4x - 3.5 y = 1.2x - 6.0
R2 = 0.98
R2 = 0.96
R2 = 0.99
R2 = 0.95
R2 = 0.98
i-pentane
99
99
95
90
95
90
Percent of distribution
Percent of distribution
i-pentane
y = 1.2x - 3.9
R2 = 0.98
80
70
50
30
20
10
5
1
80
70
50
30
20
10
5
1
1
10
100
Mixing ratio (ppt)
1000
1
10
100
1000
Mixing ratio (ppt)
Figure 2
Cumulative distributions of NMHC at the Pico Mountain station during the four measurement seasons.
31
2
Summer '04 y=2.3x-0.60 (R2=0.93)
Fall '04
y=1.7x-0.43 (R2=0.92)
Winter '05
y=1.6x-0.43 (R2=0.99)
Spring '05
y=1.4x-0.44 (R2=0.93)
Summer '05 y=1.4x-0.43 (R2=0.99)
lnx
1
ethane
propane
i-butane
n-butane
i-pentane
n-pentane
0.6
0.5
y = 1.60x-0.44
R2 = 0.91
0.4
0.3
0.2
1
2
3
4 5 6
10
20
30 40 5060
100
200
400
OH lifetime (d)
Figure 3
The standard deviation of the natural logarithm of individual NMHC mixing ratios during the four seasons at their calcualted seasonal
OH lifetime. In the upper right corner results for the regression analysis on seasonal subsets of the data are given in the same colors
are the data points in the graph. Please note that the summer 2004 analysis was done without consideration of ethane, which largely
increases the uncertainty in the calculation. Regression analysis through all data results in the regression line and equation given in
black.
32
200
Jan
2
Jan
Dec
Dec
100
1
Oct
Sep
Sep
Aug
Jul
10
Nov
Oct
Jun
May
i-butane/n-butane
i-butane (pptv)
Nov
Aug
Jul
Jun
May
0.1
Apr
Apr
Mar
Mar
Feb
1
3
4 5
10
100
400
Feb
Jan
0.03
n-butane (pptv)
3
10
100
400
100
Jan
4
Jan
Dec
3
Dec
Nov
Nov
2
n-pentane (pptv)
Sep
Aug
10
Jul
Jun
May
Apr
n-pentane/i-pentane
Oct
Oct
Sep
Aug
1
Jul
Jun
May
0.5
Apr
0.4
Mar
Feb
1
1
10
i-pentane (pptv)
Jan
n-butane (pptv)
100
200
Jan
Mar
0.3
Feb
0.2
1
10
i-pentane (pptv)
100
Figure 4
Mixing ratio of i-butane versus n-butane (left) and ratio of i-butane/n-butane versus n-butane
(right) in the upper graphs and of n-pentane versus i-pentane (left) and ratio of n-pentane/ipentane versus i-pentane (right) in the lower graphs The circles with the error bars in the right
graph show the mean and standard deviation of the data within 10-percentile bins of the data
distribution.
33
200
Jan
400
800
J
Jan
D
Dec
N
Nov
100
O
Oct
[propane] (pptv)
100
Aug
Jul
Jun
[n-butane] (pptv)
S
Sep
A
J
J
10
M
May
A
Apr
10
M
Mar
F
Feb
4
-5
-4
-3
-2
ln([propane]/[ethane])
-1
1
-5
Jan
0
800
-4
-3
-2
ln([propane]/[ethane])
-1
0
400
Jan
J
Dec
D
Nov
N
100
Oct
O
Aug
Jul
Jun
10
4
-7
-6
-5
-4
-3
ln([n-butane]/[ethane])
-2
-1
S
[n-butane] (pptv)
[propane] (pptv)
Sep
100
J
A
J
J
10
May
M
Apr
A
Mar
M
Feb
F
Jan
1
-7
-6
-5
-4
-3
ln([n-butane]/[ethane])
-2
Figure 5
Relationship of the propane and n-butane as a function of the ln ([propane]/[ethane]) (upper
graphs) and of propane and butane as a function of the ln ([n-butane]/[ethane]) with the color
coding representing the time of the measurement. The ln [NMHCa]/[NMHCb], with the faster
reacting NMHC in the denominator reflects a scale of photochemical processing, where hihger ln
[NMHCa]/[NMHCb] values represent a low degree of photochemical oxidation and lower, more
negative values a higher degree of oxidation.
34
-1
J
Fall 2004 to Summer 2005
Jan
0
-1
ln([n-butane]/[ethane])
-2
-3
-4
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
all data
fall 2004
winter 2005
spring 2005
summer 2005
Dec
Nov
Oct
Sep
Aug
Jul
Jun
May
-5
Apr
Mar
-6
Feb
-7
-5
-4.5
-4
-3.5
-3
-2.5
-2
-1.5
ln([propane]/[ethane])
-1
-0.5
0
0.5
Jan
Figure 6
Relationship between the natural logarithms of [n-buante]/[ethane] versus [propane]/[ethane] for
all Pico NMHC data with the seasonal dependency illustrated by the color bar. The two-sided
linear regression through all data yielded the black staggered line (slope = 1.60 ± 0.04; R2 =
0.88). The data are bound by two lines respresenting the behavior of NMHC pairs assuming a
sole dependency on OH oxidation (kinetic slope) and sole dependency on mixing, where background mixing ratios were assumed negligible for propane and n-butane. The filled, black
diamond marks the point of assumed source emission ratio for the calculation of the two filled
line. Also added are source emission ratios from other studies, with (1) Boston, New York City,
ratioed to acetylene, July 5-August 12, 2004, Warneke et al. ( 2007); (2) Boston, New York City,
ratioed to acetylene, xxx-xxx, 2002, Warneke et al. ( 2007), (3) Boston, New York City, ratioed
to CO, July 5-August 12, 2004, Warneke et al. (2007), (4) Los Angeles, xxx-xxx, 2002, Warneke
et al. (2007), (5) Boston, New York City, xxx-xxx, 2006, Baker et al. (xxxx)/Warneke et al.,
(2007), average of 39 U.S. Cities, xxxx-xxxx, Seila et al. (1989)/Warnecke et al. (2007), (6)
Harward Forest, XX, summer xxxx-xxxx, Goldstein et al. (xxxx); (8)Harward Forest, XX, winter, xxxx-xxxx, Goldstein et al., (xxxx); (9) Summit, Greenland, xxxx-xxxx, Swanson et al.
(xxxx), (10) global mean ratio seen in Jan. 2005 flask samples from the NOAA Cooperate Flask
Network for samples collected at 30-60oN (Pollmann et al., unpublished results).
35
Fall 2004 to Summer 2005
100
90
80
70
Jan
Spring 2005
Summer 2005
Dec
Nov
60
Oct
50
Ozone (ppbv)
Sep
40
Aug
Jul
30
Jun
May
20
Apr
Mar
Feb
10
-5
-4
-3
-2
ln([propane]/[ethane])
-1
0
Jan
Figure 7
Ozone in relation to the natural logarithm of [propane]/[ethane]. The two lines indicate the linear
least-squares fit regression lines to the log-transformed data for spring and summer, with the end
points of both lines spanning the range of observed x-values. The slopes of these regression
lines with their 95% confidence limits and correlation coefficients are 0.72 +/- 0.07, R2=0.43, for
spring, and 1.02+/-0.12, R2=0.22 for summer, respectively.
36
Table 1
Comparison of propane and n-butane mixing ratios (in pptv) seen outside and within fire events, as well as during the corresponding
time period in 2005, and total number of samples considered in each group of data.
Propane
(DOY 209-227)
2004
2005
not fire fire events all data
event
Percentiles
5
25
50
75
95
n:
n-Butane
(DOY 209-227
2004
2005
not fire fire events all data
event
8
25
51
83
151
59
67
112
143
691
15
20
33
48
86
<2
<2
<2
11
31
<2
<2
23
40
55
<2
2
5
8
14
100
23
105
91
22
105
37
Table 2
Correlation between NMHC with regression line slopes (m), intercept (b) and regression coefficient (r2) during fall (F), winter (W),
spring (SP) and summer (S).
CO
F
CO
n
m
b
R2
ethane
n
m
b
R2
propane n
m
b
R2
i-butane n
m
b
R2
n-butane n
m
b
R2
i-pentane n
m
b
R2
n-pentane n
m
b
R2
95
1
0
1
95
0.03
72.0
0.43
95
0.06
82.6
0.53
94
0.24
88.4
0.61
95
0.13
87.2
0.57
94
0.33
91.3
0.66
93
0.51
91.4
0.64
W
249
1
0
1
249
0.05
48.6
0.61
249
0.09
89.8
0.36
248
0.42
93.0
0.44
249
0.19
95.7
0.41
249
0.46
101
0.41
248
0.62
103
0.34
ethane
SP
SU
F
W
316 253
95 249
1
1 16.0 11.3
0
0 -524
-60
1
1 0.43 0.61
316 253 107 251
0.03 0.04
1
1
83.6 67.2
0
0
0.62 0.31
1
1
299 245 107 251
0.12 0.37 1.62 1.77
107 74.1 588 726
0.65 0.58 0.64 0.72
284
96 106 250
0.72 3.08 5.10 7.85
114 85.7 822 819
0.59 0.28 0.44 0.79
316 169 107 251
0.33 0.98 3.14 3.73
114 83.4 752 859
0.52 0.25 0.55 0.76
221
43 106 251
1.07 -1.10 7.27 8.59
120 106 878 976
0.54 0.02 0.52 0.71
214
98 103 250
1.47 -0.54 11.4 11.8
119 96.8 883 1004
0.52 0.03 0.52 0.61
propane
SP
SU
F
W
SP
SU
316 253
95 249 299 245
18.3 7.84 9.07 4.10 5.57 1.57
-1063 -129 -601 -178 -537 -98.2
0.62 0.31 0.53 0.36 0.65 0.58
332 256 107 251 315 248
1
1 0.40 0.41 0.29 0.11
0
0 -116 -214 -206 -19.6
1
1 0.64 0.72 0.88 0.48
315 248 107 251 315 248
2.98 4.51
1
1
1
1
771 391
0
0
0
0
0.88 0.48
1
1
1
1
300
99 106 250 300
99
18.0 37.5 3.40 4.10 6.39 10.9
975 541 131 71.4 59.6 20.0
0.79 0.24 0.81 0.94 0.95 0.70
329 172 107 251 312 164
8.61 -1.15 2.00 2.03 2.89 3.18
919 614 95.2 83.7 56.4 22.2
0.67 0.00 0.92 0.97 0.88 0.48
236
43 106 251 236
43
26.2 -24.6 4.43 4.60 9.37 -3.09
1130 763 182 149 112 82.1
0.67 0.05 0.79 0.88 0.80 0.02
224
99 103 250 224
99
33.8 0.77 7.17 6.56 12.9 0.42
1106 607 179 160 97.3 50.8
0.58 0.00 0.84 0.82 0.79 0.00
i-butane
F
W
94 248
2.57 1.07
-205 -68.4
0.61 0.44
106 250
0.09 0.10
-39.0 -70.2
0.44 0.79
106 250
0.24 0.23
-19.7 -13.0
0.81 0.94
106 250
1
1
0
0
1
1
106 250
0.52 0.48
-2.61 4.67
0.91 0.98
105 250
1.28 1.12
16.3 19.1
0.96 0.95
102 250
2.04 1.58
15.1 21.9
0.94 0.86
SP
284
0.82
-86.3
0.59
300
0.04
-39.1
0.79
300
0.15
-8.01
0.95
300
1
0
1
297
0.44
0.42
0.91
236
1.55
6.99
0.88
224
2.13
4.44
0.85
SU
96
0.09
-5.12
0.28
99
0.01
-0.76
0.24
99
0.06
-0.18
0.70
99
1
0
1
97
0.35
0.42
0.64
38
0.61
3.55
0.11
46
0.23
4.13
0.03
n-butane
i-pentane
F
W
SP
SU
F
W
95 249 316 169
94 249
4.48 2.12 1.58 0.25 2.01 0.88
-343 -140 -162 -14.0 -173 -70.3
0.57 0.41 0.52 0.25 0.66 0.41
107 251 329 172 106 251
0.18 0.20 0.08 0.00 0.07 0.08
-81.0 -149 -59.2 10.31 -46.9 -71.2
0.55 0.76 0.67 0.00 0.52 0.71
107 251 312 164 106 251
0.46 0.48 0.30 0.15 0.18 0.19
-33.8 -37.2 -12.4 1.08 -25.7 -24.9
0.92 0.97 0.88 0.48 0.79 0.88
106 250 297
97 105 250
1.74 2.04 2.08 1.84 0.75 0.85
14.5 -7.17 2.68 2.56 -11.0 -14.6
0.91 0.98 0.91 0.64 0.96 0.95
107 251 329 172 106 251
1
1
1
1 0.40 0.41
0
0
0
0 -13.6 -10.8
1
1
1
1 0.91 0.94
106 251 233
42 106 251
2.27 2.31 3.35 1.25
1
1
41.6 31.1 15.5 9.47
0
0
0.91 0.94 0.83 0.11
1
1
103 250 224
71 102 250
3.66 3.30 4.74 -0.02 1.59 1.42
39.8 36.2 8.97 12.2 -0.94 2.25
0.94 0.88 0.86 0.00 0.97 0.92
n-pentane
SP
SU
F
W
SP
SU
221
43
93 248 214
98
0.51 -0.02 1.26 0.54 0.35 -0.05
-56.7 5.48 -108 -41.7 -37.8 8.58
0.54 0.02 0.64 0.34 0.52 0.03
236
43 103 250 224
99
0.03 0.00 0.05 0.05 0.02 0.00
-25.9 4.74 -29.5 -43.6 -15.3 3.53
0.67 0.05 0.52 0.61 0.58 0.00
236
43 103 250 224
99
0.09 -0.01 0.12 0.13 0.06 0.01
-7.77 3.88 -17.8 -16.3 -4.25 3.30
0.80 0.02 0.84 0.82 0.79 0.00
236
38 102 250 224
46
0.57 0.19 0.46 0.54 0.40 0.12
-2.80 1.92 -5.67 -8.89 -0.55 2.68
0.88 0.11 0.94 0.86 0.85 0.03
233
42 103 250 224
71
0.25 0.09 0.26 0.27 0.18 -0.01
-2.18 2.16 -9.05 -6.92 -0.40 4.39
0.83 0.11 0.94 0.88 0.86 0.00
236
43 102 250 218
33
1
1 0.61 0.65 0.68 0.10
0
0 1.27 0.32 1.85 2.58
1
1 0.97 0.92 0.89 0.02
218
33 103 250 224
99
1.32 0.18
1
1
1
1
-1.38 3.19
0
0
0
0
0.89 0.02
1
1
1
1
38
Table 3
Ozone relationships to ln[propane]/[ethane] in the marine troposphere in spring at northern temperate latitudes.
Location
Point Arena, Californiaa
PEM West-B – Asian outflow in Western N. Pacifica
ITCT-2K2 – Eastern N. Pacifica
PHOBEA – Eastern N. Pacifica
TRACE-P – West to central N. Pacifica
Pico-Mountainb
Dates
24 April to 9 May 1984
8 February to 14 March 1994
22 April to 19 May 2002
March – May 1997-1999;2001-2002
26 February to 10 April 2001
20 March to 20 June 2005
21 June to 21 September 2005
a
Includes marine boundary layer measurements only; Parrish et al. (2004)
b
Lower free troposphere measurements; this work
Slope
0.86 ± 0.10
-0.39 ± 0.11
0.19 ± 0.06
-0.03 ± 0.08
0.19 ± 0.04
0.72 ± 0.07
1.02 ± 0.12
39
Non-Methane Hydrocarbon (NMHC) at Pico Mountain, Azores
2. Air Transport in the North-Atlantic Region
D. Helmig1*, D. M. Tanner1, R.C. Owen2, R. E. Honrath2 and D.D. Parrish3
1
Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, CO
80309, USA
2
Department of Civil and Environmental Engineering, Michigan Technological University,
Houghton MI 49931, Michigan, USA
3
CSD/Earth System Research Laboratory, National Oceanic and Atmospheric Administration,
Boulder, CO 80303, USA
*corresponding author: Detlev.Helmig@colorado.edu
Manuscript submitted to Journal of Geophysical Research
Revised Version
October 15, 2007
Abstract
NMHC concentrations and their relative ratios were valuable in identifying transport situations
where anthropogenically influenced air from the mid and western U.S. was transported to Pico in
5-8 days. Interpretations of NMHC ratios as a scale for photochemical processing (‘photochemical clock’) was shown to yield results that were in agreement with interpretations derived from
the particle dispersion model FLEXPART.
Introduction
Selected NMHC can be used as indicators or tracers for specific emission sources or events. For
instance, isoprene being an abundant biogenic volatile organic compound emission can serve as
an indictor for emission influences from vegetation (Fehsenfeld et al., 1992), while methyl chloride and acetonitrile are suitable tracers of biomass burning plumes (Lobert et al., 1999; De
Gouw et al., 2003). Ratios of NMHC, e.g. acetylene and benzene to light n-alkanes are potential
indicators of influence from biomass burning. These emission ratios are relatively high in bio-
40
mass burning compared to anthropogenic combustion processes, and their relatively long atmospheric lifetime and low natural background allow using them as tracers after many days of
transport (Andreae and Merlet, 2001; DeGouw et al., 2004). Light, saturated and unsaturated
NMHC have been used to identify influences from urban energy use and petrochemical industries (Blake and Rowland, 1995; Jobson et al., 2004). Diurnal concentration changes of the
unsaturated NMHC ethene and propene allowed the identification of occurrences of upslope and
downslope flow conditions at Mauna Loa Observatory (Greenberg et al., 1996). The variability
of NMHC concentrations can provide information on the impact of, or the distance from pollution sources.
Experimental
2.1. Pico Mountain Station
The Pico Mountain observatory is located in the summit caldera of the inactive Pico Mountain
volcano (38.47N, 28.40W), the highest mountain on Pico Island, and in the Azores, Portugal. At
2225 m asl, lower, free tropospheric air is sampled at the station during most times. Buoyant
upslope flow affects the Pico Mountain station much less than some other marine mountain
observatories, as a result of the latitude, size, and topography of Pico Island. Intensive meteorological measurements during summer 2004 indicated possible daytime buoyant upslope flow
during portions of ~39% of the days studied, accounting for ~16% of the measurement hours.
However, impacts on nitrogen oxides, ozone, and carbon monoxide (CO) were small, implying
that boundary layer air did not reach the summit on most of these days. Outside the MaySeptember period, buoyant upslope flow is rare, but wind-driven mechanical lifting has the
potential to carry marine boundary layer air to station altitude. This mechanical upslope flow has
a strong seasonal cycle that results from higher winds and higher marine boundary layer heights
during winter. As a result, the potential frequency of marine boundary layer lofting to the station
altitude (which may be greater than the actual frequency) reaches 37 to 59% during October to
April, but drops to <18% during the remaining months (Kleissl et al., 2007). Chemical measurements of nitrogen oxides, carbon monoxide and isoprene at the observatory showed very little
41
influence from island emission sources even during uplifting events (or upslope flow), indicating
that during most periods the station is negligibly impacted by inland emissions (Kleissl et al.,
2007). Further site descriptions, data and interpretations from other research, including studies
of oxidized nitrogen species, ozone, carbon monoxide and of aerosol properties at Pico Mountain
have been presented previously (Honrath and Fialho, 2001; Honrath et al., 2004; Lapina et al.,
2006) and in other contributions to this special issue (Owen et al., 2006; Val Martin et al., 2006).
2.3 FLEXPART Simulations
The FLEXPART particle dispersion model (version 6.2, Stohl et al., 1998, 2005; Stohl and
Thomson, 1999) was used to investigate NMHC source regions and transport times.
FLEXPART was driven with data from the European Centre for Medium Range Weather Forecasts (ECMWF) (ECMWF, 2005) with a 1 degree by 1 degree horizontal resolution, 60 vertical
levels and a temporal resolution of 3 hours, using meteorological analyses at 0000, 0600, 1200,
and 1800 UTC, and ECMWF 3-hr forecasts at intermediate times (3, 9, 15, 21 UTC).
FLEXPART was run in its backward mode to create “retroplumes,” similar to backward trajectories. Retroplumes are simulated from the release at the receptor of thousands of particles that are
advected backwards in time. Retroplumes are superior to trajectories in that they allow for an
assessment of the deformation of an air mass as it travels and they can be used for determining
source regions and contributions for observed enhancements (Seibert and Frank, 2004). Retroplumes were initiated every three hours with 4,000 particles released over a one hour time interval into a 1 degree x 1 degree grid box centered on the Pico Mountain station, over an altitude
range of 2000-2500 m asl. Particles were followed backward in time for 20 days. In order to
account for differences in air density between the release cell and upwind sources, the residence
times of the particles were normalized by the air density in each cell to yield the specific
weighted volume residence time (SWVRT). The SWVRT in the footprint layer (0-300 m) was
then folded with emissions according to the technique described by Stohl et al. (2003) and
Seibert and Frank (2004) to calculate a time series of age classes (in ppbv) at the station from
North American, European, and Asian emissions. All three sources had significant impacts,
though North American emissions were dominant. Emissions were based on the EDGAR 3.2
42
Fast Track 2000 dataset (Olivier et al., 2001) for anthropogenic sources only with a 1 degree
resolution.
Results
1. Seasonal Behavior of Hydrocarbon Ratios
2. Case Studies of Illustrative Transport Events
Here three events with contrasting NMHC levels and ratios are selected and the transport patterns responsible for each are investigated. Figure 7a shows seven weeks of data for four NMHC
during spring 2005 (note the logarithmic concentration scale). NMHC concentrations are highly
variable (close to 10-fold changes occurred several times during this period) and highly correlated. The relative variations are highest for the shorter-lived compounds. The springtime decline
in the NMHC mixing ratios can be discerned beneath the shorter-term variations. Figure 7b
gives the ln [propane]/[ethane] and ln [n-butane]/[ethane] time series for the same data. Again,
high variability is apparent with high ln [NMHCi]/[ethane] ratios (indicating ‘fresh’, i.e. little
processed air) coinciding with periods of enhanced absolute NMHC mixing ratios and low ln
[NMHCi]/[ethane] ratios (indicating ‘old’, i.e. well processed air) coinciding with periods of low
absolute NMHC mixing ratios.
Three periods were selected for closer investigation. These periods were separated by two air
mass changes when air switched from a ‘fresh’ signature to an ‘old’ signature and then back to
‘fresh’. These three 1-2.5 day intervals are indicated by the colored circles in Figure 7b. The
corresponding data points are marked by the same colors and compared with all data during this
April-May period in the ln [n-butane]/[ethane] versus ln [propane]/[ethane] plot in Figure 8a.
Here data from these three periods cluster in different regions in this plot, which indicates distinct differences in photochemical history of these air masses.
43
The FLEXPART retroplume results for contributions of enhanced CO at Pico from Northern
Hemisphere anthropogenic emissions are shown in Figure 7c. Please note that Figure 7c shows
the CO age distributions for emissions from all regions of the Northern Hemisphere. The discussion below reports ages for the subsets of North American and Asian emissions, which are not
explicitly shown in this figure but account for most of the simulated CO enhancements. This
analysis shows changes in simulated CO occurring very close in timing to the NMHC (and ln
[NMHC]/[ethane]) changes over the three selected periods. The first episode with increasing
NMHC ratios is paralleled by a ~25 ppbv increase in CO.
The 4/19-4/20 episode with low
NMHC mixing ratio data shows correspondingly low CO enhancements. The third period, with
the overall highest NMHC and ln [NMHC]/[ethane] ratios, coincides with similarly the overall
highest CO concentrations. The corresponding times of the FLEXPART events are shown in
Figure 8b in the same color code as the corresponding data periods indicated in Figure 7b. Upon
closer inspection of the time series data a slight time shifts between the FLEXPART results and
the measured data are notable. The offsets in the timing of the FLEXPART versus the observed
events probably indicates that the meteorological data that drive the FLEXPART program do not
exactly capture the times of frontal passage at Pico, even though FLEXPART well describes the
transport and mixing within the air masses on either sides of the frontal boundary.
FLEXPART retroplumes for the three episodes marked in Figure 7b and 8 are shown in Figure 9.
The left hand column shows horizontal pathways taken by the plume, derived by vertically
summing the specific volume-weighted residence time (SVWRT, see section 2.3) of the plumes.
The right hand column shows the source contribution, which is the result of folding the SVWRT
in the footprint layer (0-300 m) with emissions.
The source contribution map for the first episode (Figure 9b) indicates that the bulk of the North
American emissions originated over the central western US, primarily in Colorado; emissions
from this region reached Pico approximately 6-11 days later (mean age 9.1 days). However,
during portions of the event even larger contributions resulted from sources further upwind in
southeastern China, with FLEXPART ages ranging from 10-20 days (mean 15 days). (Note that
FLEXPART calculations were carried out only to a maximum age of 20 days, so emissions
earlier than this are not included in these mean age values.) The transport from Asia to North
44
America was complex, with multiple pathways (Figure 9a), resulting in the wide range of ages
for Asian emissions. Transport from North America to the Azores, however, was well organized
and, from 4 days prior to arrival onward, quite rapid. This portion of the retroplume pathway is
consistent with warm conveyor belt transport associated with a cold front located over this region
of North America on April 13-15, and reached an altitude of 6 km prior to descending to Pico.
In contrast, FLEXPART results show that air sampled during 4/19-20 originated over the southeastern Northern Pacific and traveled over Mexico and the Gulf of Mexico, before spending 1012 days within the mid-Atlantic region, circulating around the Azores High and slowly descending from about 10 km altitude to the Pico Mountain station (Figure 9c). The CO time series
(Figure 7c) shows no CO present at the station less than 10 days old, with most of the CO enhancement falling into the 15-20 day-old range. The weak emission signal from southwestern
Mexico (Figure 9d) occurred in the 15-20 day window, while the weaker signals over the eastern
U.S. and the Caribbean occurred in the 10-15 day window. The high ages and small simulated
CO enhancements indicate little input of emissions into relatively clean marine air. The average
age of the small modeled CO enhancement ranges from 13-17 days (Figure 7c).
Finally, air sampled during the 4/21-23 episode passed over large portions of North America
before arriving at the Pico (Figure 9e). The height of the plume during transit over the U.S. was
relatively low, generally less than 2 km, and transport over the Atlantic to the station occurred
between 3 and 5 km. Low-level transport of this type is a mechanism by which North American
emissions frequently impact the Pico Mountain station (Owen et al., 2006). The source contribution map indicates that sources across the eastern U.S. and Canada were responsible for the
enhancements observed at this time (Figure 9f). The average age of the CO enhancements for
this episode ranged from 7.5-11 days, with contributions from emissions 4-15 days old (Figure
7c). This wide distribution of ages and the geographical range of the emissions indicate the
mixing of fresh emissions from many North American source regions and little mixing with
cleaner marine air.
In summary, the FLEXPART results are consistent with the photochemical age trends indicated
by the NMHC ratios during these three periods. They indicate that the periods identified with
45
‘photochemically fresh’ air coincided with air transport over populated regions of North America
and Asia, with the injection of fresh anthropogenic emissions. In contrast, the period that was
identified as ‘photochemically old’ was attributed to conditions where air had resided over the
Atlantic Ocean for an extended period of time (> 10 days) with little injection of emissions for
20 days prior to arrival at the station.
3.7 Relationship between CO and NMHC ages in the Transport Events
The FLEXPART results presented in Figure 7 give an indication of the average age of the CO
emitted in the past 20 days and transported to the Pico Mountain site. The photochemical processing, as indicated by the changes in the NMHC ratios can provide another, independent estimate of the age of the NMHC transported in the same air parcels. However, there is an important fundamental difference between these two age estimates. The former is based upon a
model calculation that extends only 20 days into the past; the FLEXPART simulations do not
account for CO emitted at earlier times. Thus, the average age of these CO enhancements is
really a lower limit on the average age of the total CO concentration. In contrast the latter estimate is based on measured total NMHC concentrations, so it represents the age of the total
NMHC concentrations. Here the relationship between these two different age estimates is discussed.
The derived average CO enhancement ages shown on the right side y-axis of Figure 7c are 1213, 14-17, and 7.5-11 days for the three case studies discussed in the preceding section. The
maximum enhancement in Figure 7c is ~65 ppbv. Measured CO at the station during 4/21-4/23
was 145-185 ppbv. Between the two transport events, when air more representative of the seasonal CO background was encountered, CO was 115-120 ppbv. Consequently, the ~65 ppbv
enhancement predicted by FLEXPART is in reasonable agreement with the CO observations at
the station.
Parrish et al. (2007) show that approximate NMHC ratios can be calculated for a sampled air
parcel from the corresponding FLEXPART age spectrum. This calculation requires an estimated
46
average [OH], average ratios of NMHC to CO in emissions, and reaction rate constants. Molar
emission ratios were estimated as 0.0114 for ethane/CO (Parrish et al., 2007), and the same
propane/ethane and n-butane/ethane ratios and reaction rate constants as those used for Fig. 5
were applied. The 24-hour [OH] was estimated here as 1.0 x 106 molecules cm-3 to approximately reproduce the dynamic range of the NMHC ratios in Figure 8a (smaller values of [OH] fail to
reproduce the small observed ratios and larger values of [OH] produce ratios smaller than those
observed). The estimated value is the same as that selected by Parrish et al. (2007) for springtime at similar latitudes in the Pacific, and is in close accord with the value of 1.2 x 106 molecules cm-3 obtained from Equation (1) above for the dates of these episodes. For the calculation
of NMHC ratios, each age spectrum is extrapolated to times earlier than the 20 days covered by
the FLEXPART calculations in the manner described by Parrish et al. (2007).
The NMHC
ratios calculated for all age spectra in Figure 7c are shown in Figure 8b with the points from the
time periods indicated in Figure 7b similarly marked here.
Equation 3 of Parrish et al. (2007),
ta OH   
 [A] [A] 
1
ln  ln 0 ,
kA  kB  [B]  [B]0 
using the above parameters, allows the assignment of an approximate average photochemical
 sampled air parcel. <ka and kb> are OH-weighted average rate constants of
age, ta, to each
NMHCS with the ratio [A]/[B] and molar emission ratio [A] 0/[B]0. [OH[ is the constant, average
OH concentration over time ta. It should be noted that different values for ta are obtained from
the propane/ethane and n-butane/ethane ratios; this is necessarily so because the data in Figure 8a
define a linear relationship with a slope considerably smaller than that expected from kinetic
behavior alone. This difference in derived photochemical ages from the different ratios is the
result of mixing of emissions of different ages; in a mixture of emissions, a slower reacting
NMHC will always have an older average age than a faster reacting NMHC. Parrish et al.
(2007) show that the photochemical age calculated from the n-butane/ethane ratio is a good
approximation for the average age of the propane in that air parcel. The diamonds in Figure 8b
give the ages that correspond to the n-butane/ethane ratio scale on the ordinate of that graph. For
47
the three episodes marked in Figure 8b the FLEXPART calculated n-butane/ethane ratios yield
photochemical ages of 15-18, 27-31, and 9-14 days, respectively.
The estimated photochemical ages derived from the FLEXPART calculated NMHC ratios are
systematically older than the corresponding mean CO ages that were derived from FLEXPART.
This difference is expected because the photochemical ages represent the average age of all of
the NMHC in the sampled air parcel, not just the fraction emitted in the past 20 days. For the
third episode, the agreement between the CO ages (7.4-10.4 days) and the photochemical ages
(9-14 days) derived from FLEXPART is closest because this period is dominated by relatively
fresh emissions from North America. For the other two episodes the CO enhancements are
relatively small, and thus account for a smaller fraction of the total CO. The CO ages, which
apply only to these enhancements, must considerably underestimate the average age of all emitted CO in these sampled air parcels, and are thus smaller than the NMHC photochemical ages,
which approximate the average age of propane in these parcels.
Photochemical ages can also be calculated from the corresponding experimentally measured
ratios in Figure 8a substituted into the above equation. This yields 12-16, 24-27, and 10-14 days,
respectively, for the three periods. The agreement between the former ages, derived from the
FLEXPART calculations, and these latter ages, derived from the experimental NMHC measurements, provides evidence that FLEXPART age spectra yield a good description of the emission
sources, transport, mixing and aging of the light saturated NMHC over North America and the
North Atlantic region.
Another point of consideration is seasonal changes in synoptic transport patterns, and hence
possible changes in source regions and length of processing time during transport. Transport to
the Azores is strongly affected by the Azores/Bermuda high pressure and the zonal flow present
on average throughout the midlatitudes. A stronger high during summer and faster zonal flow
during winter results in moderate seasonal variations in transport pathways to Pico Mountain.
Based on a clustering analysis of backward trajectories to the Pico Mountain station (J. M. Strane
et al., manuscript in preparation), we estimate that transport dominated by the Azores/Bermuda
high (i.e., trajectories averaging to locations entirely within the Atlantic basin) occurs approximately 40 to 50% of the time during spring through fall, but less often (approximately 30% of
48
the time) during winter. Conversely, rapid flow that transits the North American continent in
less than 10 days occurs only about 10-15% of the time during summer and fall, but about 25%
of the time during winter and spring. Since transport within the North Atlantic (around the
Azores/Bermuda high) brings highly aged air to the station, seasonal variation in flow patterns is
expected to contribute to the lower mean hydrocarbon levels and increased photochemical ages
observed during summer.
Conclusions
Three episodes of contrasting NMHC levels were selected from a one-week period of April 2005
for intensive investigation. FLEXPART analysis clearly distinguished the emissions and their
transport to Pico, and accounted for the contrasts in these data. A simple model based upon the
FLEXPART age spectra quantitatively reproduced the observed relationships between NMHC
ratios, both for the entire spring season, and the contrasts between these three selected episodes.
Acknowledgments
We thank D. Henriques, Institute of Meteorology, Ponta Delgada, Portugal for providing the
ECMWF data used in this work, Andreas Stohl, Norsk Institutt for Luftforskning (NILU),
Kjeller, Norway for providing and assisting in running the FLEXPART model, the Data Support
Section of NCAR’s Scientific Computing Division for making the NCEP FNL analyses available
for download. This research was funded by a grant from the NOAA Office of Global Programs
(award # NA03OAR4310072).
REH and RCO acknowledge support from NOAA grant
NA03OAR4310002 and National Science Foundation grant ATM-0535486.
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2
20
1
15
0
10
-1
5
-2
0
-3
-5
-4
-10
-5
-6
Oct-04
-15
ln(propane/ethane)
ln(n-butane/ethane)
Flexpart mean CO age
-20
Nov-04
Dec-04
Jan-05
Feb-05
Mar-05
Apr-05
May-05
Jun-05
Jul-05
Aug-05
Figure 1
Mean CO age derived from FLEXPART and ratios of [propane]/[ethane] and [n-butane]/[ethane]
between October 2004 – September 2005.
54
0
Jan
-0.5
Dec
Nov
-1
Oct
ln([propane]/[ethane])
-1.5
Sep
-2
Aug
-2.5
Jul
Jun
-3
May
-3.5
Apr
-4
Mar
-4.5
-5
Feb
6
8
10
12
14
16
Flexpart Mean CO age (days)
18
20
Jan
Figure 2
Relationship between the ln ([propane]/[ethane]) and the mean FLEXPART-calculated CO age.
55
ethane
propane
n-butane
n-pentane
10000
100
10
1
Mar-27
Apr-3
Apr-10
Apr-17
Apr-24
May-1
May-8
May-15
A
0
4/17 16:00 to
4/19 1:00
4/21 5:00 to
4/23 15:30
ln(propane/ethane)
ln(n-butane/ethane)
-1
4/19 17:00 to
4/20 15:30
-2
ln([HC]/[ethane])
mixing ratio (pptv)
1000
-3
-4
-5
-6
Mar-27
Apr-3
Apr-10
Apr-17
Apr-24
May-1
May-8
May-15
B
56
120
20
CO (ppbv)
7
10
15
20
mean age
100
15
80
10
60
5
40
0
20
-5
0
Mar-27
Apr-3
Apr-10
Apr-17
Apr-24
May-1
May-8
Mean CO age (days)
4
-10
May-15
C
Figure 3
Mixing ratios of four NMHC (A) and the natural logarithms of [propane]/[ethane] and [nbutane]/[ethane] for 21 days in spring 2005 (B). Panel (C) shows for the same time window the
FLEXPART calculations for the enhancement of CO at Pico from contributions of emissions of
CO over North America. Contributions to the overall enhancement are broken up in age groups
of integrated 0-4, 0-7, 0-10, 0-15 and 0-20 days transport classes. The derived average CO
enhancement transport time is shown as the dotted, black data series on the secondary y-axis.
57
0
3/27 - 5/16
slope = 1.85 ± 0.03
r2 = 0.82
-1
0
3/15 - 5/19
slope = 1.99 ± 0.03
-1
4/21 15:00 to
4/24 00:00
-2
ln(n-butane/ethane)
ln(butane/ethane)
-2
-3
-4
-5
-6
kinetic slope = 2.61
0
2
r = 0.96
10
-3
4/17 18:00 to
4/18 18:00
-4
20
-5
4/19 00:00 to
4/20 15:00
30
-6
Kinetic slope = 2.61
-7
-5
-4
-3
-2
ln(propane/ethane)
-1
0
-7
-5
-4
-3
-2
-1
0
ln(propane/ethane)
Figure 4a and b
Distribution of the marked data points in Fig. 7B in the ln-ln photochemical age plot in comparison to all spring 2005 data (left hand
graph) B) ln-ln photochemical age plot calculated from the FLEXPART age spectra plotted in Figure 7C. The colored points indicate
approximately the same time periods indicated in Figure 7B, and the numbers in diamonds indicate the approximate days of photochemical processing required to reach the observed NMHC ratios from the emission ratios (triangle).
58
Figure 5
Results for retroplumes with release times centered on 00 UT on April 18 (A&B), April 20
(C&D) and April 22 (E&F). The left column shows the total column (0-15 km) SVWRT. White
numerals indicate the location of maximum column-integrated SVWRT at each integral day
upwind. (Note that at large upwind times, the plume is spread over a very large region, with
maximum SVWRTs below the minimum value plotted.) The right column shows contribution of
sources at each location to CO at Pico. Colors are logarithmically scaled (100-1%) according to a
maximum value for each plot type (1.4x104 seconds*m3/kg for the SVWRT, 1.75 ppbv of CO for
source contributions), as shown by the scale at the right. The plotted area for the first event was
expanded to illustrate the significant contribution from East Asia.
59
Appendix to Part A Manuscript
3000
Fall '04
Winter '05
Spring '05
Summer '05
2500
ethane (pptv)
2000
1500
1000
500
0
0
20
40
60
80
100
120
140
160
180
200
120
140
160
180
200
CO (ppbv)
800
Fall '04
Winter '05
Spring '05
Summer '05
700
propane (pptv)
600
500
400
300
200
100
0
0
20
40
60
80
100
CO (ppbv)
60
180
Fall '04
Winter '05
Spring '05
Summer '05
160
140
i-butane (pptv)
120
100
80
60
40
20
0
0
20
40
60
80
100
120
140
160
180
200
120
140
160
180
200
CO (ppbv)
400
Fall '04
Winter '05
Spring '05
Summer '05
350
n-butane (pptv)
300
250
200
150
100
50
0
0
20
40
60
80
100
CO (ppbv)
61
160
Fall '04
Winter '05
Spring '05
Summer '05
140
i-pentane (pptv)
120
100
80
60
40
20
0
0
20
40
60
80
100
120
140
160
180
200
120
140
160
180
200
CO (ppbv)
100
Fall '04
Winter '05
Spring '05
Summer '05
90
80
n-pentane (pptv)
70
60
50
40
30
20
10
0
0
20
40
60
80
100
CO (ppbv)
62
800
Fall '04
Winter '05
Spring '05
Summer '05
700
propane (pptv)
600
500
400
300
200
100
0
0
500
1000
1500
2000
2500
3000
2000
2500
3000
ethane (pptv)
180
Fall '04
Winter '05
Spring '05
Summer '05
160
140
i-butane (pptv)
120
100
80
60
40
20
0
0
500
1000
1500
ethane (pptv)
63
400
Fall '04
Winter '05
Spring '05
Summer '05
350
n-butane (pptv)
300
250
200
150
100
50
0
0
500
1000
1500
2000
2500
3000
2000
2500
3000
ethane (pptv)
160
Fall '04
Winter '05
Spring '05
Summer '05
140
i-pentane (pptv)
120
100
80
60
40
20
0
0
500
1000
1500
ethane (pptv)
64
100
Fall '04
Winter '05
Spring '05
Summer '05
90
80
n-pentane (pptv)
70
60
50
40
30
20
10
0
0
500
1000
1500
2000
2500
3000
ethane (pptv)
180
Fall '04
Winter '05
Spring '05
Summer '05
160
140
i-butane (pptv)
120
100
80
60
40
20
0
0.0
100.0
200.0
300.0
400.0
500.0
600.0
700.0
800.0
propane (pptv)
65
400
Fall '04
Winter '05
Spring '05
Summer '05
350
n-butane (pptv)
300
250
200
150
100
50
0
0.0
100.0
200.0
300.0
400.0
500.0
600.0
700.0
800.0
500.0
600.0
700.0
800.0
propane (pptv)
160
Fall '04
Winter '05
Spring '05
Summer '05
140
i-pentane (pptv)
120
100
80
60
40
20
0
0.0
100.0
200.0
300.0
400.0
propane (pptv)
66
100
Fall '04
Winter '05
Spring '05
Summer '05
90
80
n-pentane (pptv)
70
60
50
40
30
20
10
0
0.0
100.0
200.0
300.0
400.0
500.0
600.0
700.0
800.0
propane (pptv)
400
Fall '04
Winter '05
Spring '05
Summer '05
350
n-butane (pptv)
300
250
200
150
100
50
0
0.0
20.0
40.0
60.0
80.0
100.0
120.0
140.0
160.0
180.0
i-butane (pptv)
67
160
Fall '04
Winter '05
Spring '05
Summer '05
140
i-pentane (pptv)
120
100
80
60
40
20
0
0.0
20.0
40.0
60.0
80.0
100.0
120.0
140.0
160.0
180.0
120.0
140.0
160.0
180.0
i-butane (pptv)
100
Fall '04
Winter '05
Spring '05
Summer '05
90
80
n-pentane (pptv)
70
60
50
40
30
20
10
0
0.0
20.0
40.0
60.0
80.0
100.0
i-butane (pptv)
68
160
Fall '04
Winter '05
Spring '05
Summer '05
140
i-pentane (pptv)
120
100
80
60
40
20
0
0.0
50.0
100.0
150.0
200.0
250.0
300.0
350.0
400.0
250.0
300.0
350.0
400.0
n-butane (pptv)
100
Fall '04
Winter '05
Spring '05
Summer '05
90
80
n-pentane (pptv)
70
60
50
40
30
20
10
0
0.0
50.0
100.0
150.0
200.0
n-butane (pptv)
69
100
Fall '04
Winter '05
Spring '05
Summer '05
90
80
n-pentane (pptv)
70
60
50
40
30
20
10
0
0.0
20.0
40.0
60.0
80.0
100.0
120.0
140.0
160.0
i-pentane (pptv)
70