Non-Methane Hydrocarbons (NMHC) at Pico Mountain, Azores
1. Oxidation Chemistry in the North-Atlantic Region
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D. Helmig1*, D.M. Tanner1, R.E. Honrath2, R.C. Owen2, 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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Chemical Sciences Division/Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO 80303, USA
*corresponding author: Detlev.Helmig@colorado.edu, Tel. 001 303 492-2509
Manuscript submitted to Journal of Geophysical Research
Revised Version
January 9, 2008
Abstract
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Measurements of non-methane hydrocarbons (NMHC) at the Pico Mountain observatory at 2225
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m asl on Pico Island, Azores, Portugal, from 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 Transformation (ICARTT) study) were used to investigate NMHC sources and seasonal
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oxidation chemistry in the central North Atlantic Region. NMHC levels were low compared to
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continental sites at higher northern latitudes, and NMHC behavior showed characteristics of the
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remote free troposphere. Nonetheless, NMHC mixing ratios at Pico in general were higher than
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data reported from a similarly located Pacific mountain site at Mauna Loa Observatory, Hawaii,
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which is indicative of a greater influence of the adjacent continents on air composition at Pico.
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Substantially enhanced NMHC levels during the summers of 2004 and 2005 were attributed to
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long-range transport of biomass burning plumes originating from fires in Northern Canada,
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Alaska, and Siberia. This finding further shows the continuing impact of biomass burning
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plumes on atmospheric composition and chemistry many days downwind of these emission
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sources. Seasonal cycles with lower NMHC concentrations and lower ratios of more reactive to
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less reactive NMHC during summer reflect the higher degree of photochemical processing oc-
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curring during transport. The NMHC concentrations indicate no significant role of chlorine atom
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oxidation on NMHC. Ozone above 35 ppbv was measured at Pico Mountain throughout all
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seasons. Enhanced ozone levels were observed in air that had relatively ‘fresh’ photochemical
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signatures (e.g. ln [propane]/[ethane] > -2.5). During spring-summer air that was more pro-
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cessed (‘older’ air with ln [propane]/[ethane] < -2.5) on average had lower ozone levels (down to
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< 20 ppbv). This relationship indicates that conditions in the lower troposphere over the mid-
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North Atlantic during these seasons lead to photochemical ozone destruction on the time scale of
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the transport to Pico. This behavior contrasts to that in the mid-North Pacific where other recent
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studies have found that the photochemistry is more 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 are determined by the strength of emission sources and
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their atmospheric removal processes, which are mostly due to reaction with the OH radical.
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Reaction rate constants increase with the molecular size within a given class of NMHC, causing
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lighter, saturated NMHC to exhibit slower atmospheric decay and longer lifetimes. Their atmos-
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pheric concentrations decline at slow enough rates that they can be measured after several days
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of transport to remote downwind locations. Many individual NMHC have common emission
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sources and their emission ratios vary comparatively little. This allows changes in absolute
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concentrations and NMHC ratios to be used as tools to decipher atmospheric transport and oxida-
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tion chemistry. Several researchers have investigated this utility and have presented a frame-
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work for the interpretation of NMHC concentrations, particularly for observations of the light
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C2-C5 alkanes.
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The possibilities for using NMHC data for investigation 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 influence
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of outflow from the North American continent on atmospheric observations made in the Azores
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and how measurements there can provide valuable insight in North American emissions and their
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processing during transport. NMHC measurements described in this article commenced in the
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summer of 2004, as a contribution to the International Consortium for Atmospheric Research on
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Transport and Transformation (ICARTT) campaign in the North Atlantic Region (Fehsenfeld et
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al., 2006), and have continued through most times when the station was on power. These data
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provide one of the few annual records of NMHC from a lower free-troposphere measurement
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site.
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The first year of data was analyzed with the objective to better characterize the potential influ-
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ence of local sources on air composition and chemistry at the site and to investigate the degree of
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local (Pico) and neighboring island influences, and long-range transport of NMHC on air compo-
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sition at the station. Other questions address the degree and frequency of air transport with
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anthropogenic emissions from the source regions bordering the North Atlantic and of biomass
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burning plumes. Furthermore, NMHC ratio analysis and relationships between NMHC and
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ozone were utilized to gain further insight into the seasonal oxidation chemistry occurring during
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atmospheric transport across the North Atlantic region. In the companion manuscript Hornath et
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al., 2008 (manuscript submitted for publication) evaluate how application of the FLEXPART
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transport model, assumptions of NMHC/CO emission ratios in upwind source regions, and simu-
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lated NMHC destruction by OH chemistry during transport can provide a description of NMHC
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behavior at Pico and how these results compare with the actual observations at the station.
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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 at 2225 m asl in the summit caldera of the inactive
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Pico Mountain volcano (38.47 N, 28.40 W), the highest mountain on Pico Island, and in the
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Azores, Portugal. Intensive meteorological measurements (Kleissl et al., 2007) have led to the
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conclusion that buoyant and wind-driven upslope flow affects the Pico Mountain station much
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less than some other marine mountain observatories, as a result of the latitude, size, and topogra-
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phy of Pico Island. Chemical measurements at the observatory showed very little influence from
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island emission sources even during uplifting events (or upslope flow), indicating that atmos-
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pheric processes at the station have negligible impact from island emissions (Kleissl et al., 2007).
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Further site descriptions, data and interpretations from other research, including studies of oxi-
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dized nitrogen species, ozone, carbon monoxide and of aerosol properties at Pico Mountain have
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been presented previously (Honrath and Fialho, 2001; Honrath et al., 2004; Lapina et al., 2006)
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and in other contributions to the special ICARTT issue (Owen et al., 2006; Val Martin et al.,
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2006).
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2.2 Chemical 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 gas chromatography system
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that was tailored towards this unique situation. All consumable gases and blank air were pre-
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pared at the site with low-power gas generators. The instrument followed automated startup and
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shutdown procedures and could be remotely controlled from our Boulder, CO, offices. Outside
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air was continuously drawn to the instrument from a heated inlet 5 m above ground. 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 subsequent gas
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chromatography (GC) separation and flame ionization detection (FID). Quantified NMHC
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included ethane, propane, i-butane, n-butane, i-pentane, n-pentane and isoprene (the ethane
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record doesn’t begin until in fall 2004 when some modifications in the focusing procedure al-
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lowed its quantitative analysis). Sample volumes of 600 ml (10 min collection time) and 3000
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ml (50 min collection time) were collected semi-continuously (every few hours). These sam-
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pling volumes were alternated for quantification of ethane (in the 600 ml sample) and NMHC >
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C2 (in the 3000 ml sample), respectively. Typically, a total of 12 ambient air samples, one
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standard and one blank sample were analyzed per day. Data were electronically transferred to
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our laboratory for immediate quality control and analysis. More instrument details have been
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provided by Tanner et 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, CO) that was quantified prior to shipment against numerous
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gravimetrically prepared hydrocarbon standards in the NOAA Earth System Research Laborato-
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ry, Boulder, CO. The NOAA calibration scale has previously been found to be on average with-
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in 5% agreement with that of several other laboratories in the U.S., Canada and Europe. This
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includes results obtained for the 60-component NMNC standard that was used in the round-robin
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analysis within the Nonmethane Hydrocarbon Intercomaprison Experiment (Apel et al., 1994).
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The quantifications in the reference gas were also compared against our own laboratory NMHC
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calibration scale (with was developed from a series of other gravimetrical or cross-referenced
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NMHC gas standards) and deviations of all quantified NMHC were < 10% A second remote
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ambient air reference gas (collected at Niwot Ridge, Colorado, and quantified in the same way
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by NOAA) was injected every 3-4 days for quality control. The primary calibration reference
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gas was returned to Boulder in spring 2006 and quantified again against the NOAA ESRL grav-
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imetric hydrocarbon standard scale. That analysis resulted in mixing ratios for the C2-C5 NMHC
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reported in this study that agreed within -4.2 to 2.6 % with the values that were determined two
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years earlier, prior to the shipment to Pico. From these analyses, the stated ± 5% accuracy of the
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NOAA calibration scale, and assuming linearity over the whole measurement range, the accuracy
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error 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 reference gas over a 21-day
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period in April 2005. These measurements resulted in relative standard deviations of 0.7 – 4.2 %
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at the mixing ratios in this reference gas. From these measurements the overall uncertainty,
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combining analytical accuracy and precision was estimated to be equal to or less than ± 7.7 % for
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all reported compounds, although it should be noted that this value is expected to increase for
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data approaching the detection limit. Detection limits were determined monthly as 3 times the
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integrated noise level at the peak retention times or as 2 times the standard deviation of the blank
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signal (in cases where peaks could be detected in the blank). From these repeated determina-
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tions, median detection limits were calculated as 17, 6, 2-4, and 1 pptv for C2, C3, C4, and C5
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NMHC, respectively; during summer 2005 the C3 detection limit improved to ~3 pptv). Ethene,
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propene, benzene, and toluene, while captured with this system, were excluded from the analysis
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because of higher and inconsistent blanks, which made their quantification at low pptv levels not
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feasible.
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Observations of isoprene were used for investigating the influence of emissions from Pico Island
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on the NMHC distribution at the site. Isoprene was not detected (< 1 pptv) in either winter or
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nighttime samples. During spring, isoprene was occasionally observed during the day. Occur-
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rences and mixing ratios of isoprene increased during late summer; e.g. during August 2005,
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isoprene was detected on 60% of all afternoons, with a maximum mixing ratio of 26 pptv ob-
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served on 1 Aug., 2005 (see figure 8 in Kleissl et al. 2007). There is little vegetation growing on
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the upper ~700 m of the slopes of Pico Mountain and the most plausible explanation for isoprene
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observations at the observatory is the upslope transport of air from lower island elevations. A
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correlation analysis between NMHC and isoprene in identified upslope events was used to inves-
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tigate possible anthropogenic signatures in upslope air. N-butane was chosen as an anthropogen-
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ic tracer as butane is abundantly used on the island for domestic cooking and heating and there
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are no known biogenic butane sources. This analysis was done by comparing isoprene and n-
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butane data in two subsets of samples. On days when isoprene was detected at the station, the
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mean isoprene mixing ratio (with standard deviation) during the 12-14 hours (local time) win-
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dow (which was the time when maximum daily values were observed) was 4.0 ± 5.7 pptv. On
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these same days during 22–6 hours isoprene was not detected in a single sample (< 1 pptv). In
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the same subsets of samples, n-butane was 17.1 ± 21.1 pptv during 12-14 hours, and 17.4 ± 21.6
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pptv during 22-6 hours. Since no increase in n-butane was evident in the elevated isoprene
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samples, it was concluded that the identified upslope air did not have any anthropogenic signa-
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ture. Most likely, upslope air originated from elevations several hundred meters below the ob-
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servatory but not from the populated areas of the island, which are at much lower elevation along
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the coastline. As no systematic enhancements of NMHC other than isoprene were seen in air
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that was identified as upslope flow versus air that was clearly attributed to free tropospheric
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origin, NMHC data were not further selected according to flow conditions.
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Since the alkanes and alkenes dropped to their lowest seasonal levels during the summer, during
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mid-day to early afternoon isoprene at times became the second most abundant (after ethane)
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NMHC in air sampled at the observatory. Given the much faster OH reaction with isoprene than
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with other identified NMHC, isoprene, even at these relatively low levels, makes a major contri-
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bution to the overall OH reactivity from NMHC. For the two days with the highest isoprene
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mixing ratios during 2005, considering all C2-C5 NMHC quantified in our measurements, and
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using upper estimates of 3 pptv for ethene and 2 pptv for propene, we calculated that the OH
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reactivity from isoprene contributed e.g. 94 % (day of year (DOY) 213) and 84 % (DOY 222) to
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the overall OH reactivity from NMHC at their mixing ratios measured on those days. However,
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given the short atmospheric lifetime it is clear that the episodic occurrences of isoprene are
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solely due to small scale local effects and do not have an impact on the interpretations of the
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long-lived NMHC, whose oxidation is predominantly determined by their chemistry during long-
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range transport.
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Ozone was determined using a commercial ultraviolet absorption instrument (Thermo Environ-
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mental Instruments, Inc., Franklin Massachusetts, Model 49C), and CO was determined using a
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commercial instrument modified by the addition of a zeroing system (Thermo Environmental,
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Inc., Model 48C-TL). The absence of O3 loss in the inlet line was verified once per day; CO
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instrument calibration checks were performed at the same time. More details on these chemical
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measurements are provided by Honrath, et al., 2004, Owen et al., 2006, and Lapina et al. (2008,
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submitted manuscript).
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3. Results and Discussion
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3.1 NMHC Mixing Ratios and Comparison to Other Data Sets
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Absolute levels, NMHC ratios, and variability of NMHC were compared with previously report-
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ed data from selected other locations for characterization of the influence of upwind emission
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sources and long-range transport on air composition and chemistry at the Pico Mountain station.
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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 described
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with sinusoidal fit curves (Rudolph, 1995), however higher resolution data have also shown that
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with decreasing NMHC lifetime observed seasonal cycles deviate increasingly from this behav-
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ior, where the winter maxima become increasingly narrow and the summer minima increasingly
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broad (Goldstein et al., 1995). The Pico data do not quite have the temporal resolution and high
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number of data points to clearly demonstrate this behavior. A further constraint is that with
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increasing molecule size an increasing fraction of the data (in particular of summer values) fall
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below the detection limit. Therefore we only applied fit a sinusodidal regression function (least-
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square fit regression to the diurnal mean data), defined by y = A + B sin (day+C), to all available
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ethane and propane measurements. These regression functions are the best description of the
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seasonal behavior of NMHC at the observatory and calculated A-values of 985 pptv for ethane,
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and 185 pptv for propane are our best estimates for the annual mean mixing ratios of these two
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NMHC at the station.
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Measured NMHC show a distinct seasonal cycle with highest mixing ratios in the late winter and
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lowest values in the summer. This behavior is driven by the seasonal changes in NMHC remov-
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al rate by the OH radical, whose concentration is 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. These features increase with increasing
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molecule size (shorter OH lifetime). The seasonal maximum and minimum of ethane occur the
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latest of all compounds (early March and early September, respectively, determined from the
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timing of the minimum and maximum of the best fit curve). Due to its slower OH reaction,
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ambient ethane levels respond with a longer delay to the seasonal OH cycle. Heavier NMHC
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were found to maximize as early as mid January and minimize as early as mid July. This behav-
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ior in the Pico NMHC data is in agreement with reported seasonal cycles from other northern
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hemisphere sites, which have been discussed in detail (e.g. Jobson et al., 1994a; Goldstein et al.,
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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 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 Septem-
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ber 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., 1994a).
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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. 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) flow. Upslope data for MLO typically were higher, with relative enhancements increasing
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with decreasing molecule liftetime. Pico data are consistently higher for all NMHC and during
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all seasons. The differences between Pico and MLO mixing ratios increase with decreasing
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NMHC lifetime, e.g. while ethane mixing ratios are ~20% higher, n-butane values at Pico are
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more than 5 times higher than at MLO. In contrast to MLO, the comparison with Fraserdale
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shows that Pico experiences overall lower NMHC mixing ratios than this low elevation continen-
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tal site. Again, differences in these two data sets become more pronounced with increasing
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molecular weight, but in this case with the Pico data becoming increasingly lower.
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The general trend with [NMHCMLO] < [NMHCPico] < [NMHCFraserdale] is likely due to several
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reasons. Probably of greatest importance is the distance of these sites from NMHC sources, in
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particular to continental areas, which is about two times greater for MLO than Pico. The longer
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transport results in longer photochemical processing times and more depleted NMHC concentra-
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tions at MLO compared to Pico. Secondly, aircraft profiles have shown that NMHC mixing
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ratios generally decline with height within the free troposphere (e.g. Blake et al., 1997). Both
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MLO and Pico, due to the small island size, and high elevation behave to some extent like tower
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platforms. MLO is about 1200 m higher in elevation than the Pico Mountain station. Conse-
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quently NMHC mixing ratios are expected to be lower at MLO than at Pico. Thirdly, NMHC
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mixing ratios in the lower troposphere decrease towards lower latitude (Rudolph, 1995). Again,
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this dependency implies higher NMHC levels at Fraserdale (50oN), followed by Pico (38oN) and
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MLO (19oN). This spatial distribution also reflects chemical oxidation since OH has a latitudinal
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gradient. Further comparisons of the Pico data with several other NMHC data sets from higher
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northern latitudes in Canada, the Atlantic Region and Europe (as summarized by Gautrois et al.
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(2003)) show that Pico NMHC levels are without exception lower, both during the winter and the
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summer, compared to the further northern locations. One other point to consider is that possible
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temporal trends in NMHC may bias this site comparison as both the MLO and Fraserdale data
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are 12-15 years older than our Pico measurements. Unfortunately, reports of NMHC trends at
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remote background sites are scarce and do not allow a conclusive evaluation of long-term trends
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of NMHC concentrations. Measurements made in Finland have shown decreasing levels of
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shorter-lived compounds and increasing trends of longer-lived NMHC (Hakola et al., 2006). In
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source regions in Europe and the U.S. NMHC emissions and resulting ambient air mixing ratios
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have generally been decreasing over the past decade (EPA, 2003; Stemmler et al., 2005; Plass-
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Duelmer and Berresheim, 2006).
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Figure 2 compares the cumulative distributions of the NMHC mixing ratios during fall 2004
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(September 22 to December 20), winter 2004-2005 (December 21 to March 19), spring 2005
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(March 20 to June 20) and summer 2005 (June 21 to September 21). In these analyses the medi-
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an value is located at the center of the y-axis and with a logarithmic scale extending both to
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higher and lower values such that log-normally distributed data define a linear distribution on the
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graph. Also, the y-axis scale is stretched such that data within one standard deviation of the
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median fall within half the distance from the median as data within two standard deviations and
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so forth (i.e. the y-axis scale in essence is a linear scale of the standard deviation). Lacking data
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for particular NMHC in the lower percentage ranges result from respective fractions of these data
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falling below the instrument detection limit (for instance, i- and n-pentane were below the detec-
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tion limit in ~4% of the measurements during fall 2004, whereas during summer 2005 ~85% and
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60% of chromatograms could not be quantified for i- and n-pentane, respectively). The regres-
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sion line slopes through these distributions indicate the variability of the atmospheric mixing
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ratios of the respective compounds. Linear behavior indicates a Gaussian distribution of the log-
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transformed data, while deviations from linearity indicate higher mode contributions to the dis-
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tribution, which may imply different behavior of NMHC data in air sampled from different
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sources or at different times. Calculated regression coefficients for best fit linear regressions
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ranged from 0.95 – 0.99, indicating that most of the variability is log-normally distributed.
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There are no obvious differences in the quality of the fit between the seasons, which may indi-
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cate similarity in source strengths and removal mechanisms between seasons. An interesting
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feature is that at the high end of the concentration range of each NMHC, measured mixing ratios
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are lower than what would be expected from a purely log-normal distribution. At this point we
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are not certain of the interpretation of this feature. Steeper slopes are observed for longer-lived
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NMHC (e.g. ethane) as these compounds have longer lifetimes, which reduces the relative varia-
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bility caused by emission and aging influences. Regression line slopes are consistently lower for
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the summer, which can be attributed to a higher variability resulting from the shorter seasonal
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atmospheric lifetime, lower absolute concentrations, and the relatively stronger influence of
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perturbations from different histories of transport and photochemical aging.
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3.2 Biomass Burning Influences on NMHC Concentrations at Pico
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As discussed in detail in other contributions to the ICARTT issue, the summers of 2004 and
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2005 were characterized by an unusually high occurrence of boreal wild fires at high northern
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latitudes. Two previous publications (Val Martin et al., 2006; Lapina et al., 2008) investigated
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nitrogen oxides, carbon monoxide emission ratios and ozone chemistry in identified boreal bio-
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mass fire plumes originating in North America and Siberia and transported over 6-15 days to the
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Azores. The majority of the fire plumes observed at Pico had a well defined, detailed structure
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and could be identified from the short-term variability in CO and NOy. Comparison of NMHC
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data from within and outside of the 2004 and 2005 fire events consistently show enhancements
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of NMHC levels during these identified episodes of biomass burning. A summary and compari-
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son of the C2-C4 NMHC for 2005 is given in Table 1. Mixing ratios for the NMHC in fire
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plumes increased significantly, with medians up to a factor of 3 higher for propane and the bu-
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tanes. Comparison of propane fire event data for summer 2004 (not shown) with the data from
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fire events in 2005 shows overall higher mixing ratios during 2004, suggesting that the identified
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fire events in 2004 brought air with higher NMHC enhancements to the station than in the fol-
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lowing year. The enhanced NMHC mixing ratios in the fire plumes during both years under-
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score the conclusions derived from observations of CO, NOy and black carbon, that boreal bio-
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mass burning emissions continued to affect atmospheric composition and oxidation chemistry
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after 1-2 weeks of transport to the Azores region.
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3.3 Evaluation of the Influence of NMHC Sources and Their Distance from Pico Using
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Observations of 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 degree of influence of local emissions on the air composition at a given site (Jobson et al.,
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1998, 1999). Here we use this analysis to further investigate potential local emissions and
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transport from other Azores islands versus those from distant regions on air composition at Pico.
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The variability of NMHC (expressed as lnx, the standard deviation of the natural logarithm of all
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measurements) has been found to show linear behavior when plotted against the estimated at-
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mospheric lifetime (in a double-logarithmic plot. The regression line through these data gives
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the relationship lnx = A -b. The derived A and b coefficients from the regression line analysis
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have been used to characterize the exposure to emission sources or remoteness of measurement
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locations (Jobson et al., 1998, 1999). Using one other atmospheric component with known at-
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mospheric lifetime, best fit analysis through the combined data has also yielded estimates for
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mean OH radical fields during transport of air to the measurement site (Ehhalt et al., 1998; Wil-
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liams et al., 2000, 2001).
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While previous studies have applied this relationship to characterize data sets from mostly short-
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er campaigns, the wealth of the Pico data offers an opportunity to test for possible seasonal
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variations in this behavior using a full year of data. In Figure 2 the variability of each NMHC
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during each of the four seasons is reflected by the slopes of the regression lines fit to the cumula-
375
tive distribution plots, with the inverse of each slope giving the corresponding lnx. An estimate
376
of the seasonal lifetime, of each NMHC was obtained by averaging the lifetime corresponding
377
to each NMHC measurement made during the season. The lifetime was calculated from the
12
378
product of the NMHC OH reaction constant and the OH radical concentration estimated accord-
379
ing to (Goldstein et al., 1995):
380
381
[OH] = A [1-B cos(2 t/365)],
(1)
382
383
using values of A=1.6*106 and B=0.80 for 800 hPa, 36.0oN, 27.5oW (Spivakovsky et al., 2000).
384
Reaction rate constants were adjusted to the temperatures measured at the Pico Mountain station
385
during the respective time when the measurements were made. Note that this local lifetime
386
represents an estimate for the conditions at the receptor site. The true NMHC lifetime may
387
deviate from this estimate dependent on the conditions encountered during transport to Pico.
388
389
The variability-lifetime relationships with linear regressions providing solutions for lnx = A -b
390
are shown in Figure 3. In each seasonal data set NMHC lnx values are well correlated with the
391
seasonal lifetime estimates; R2 values range from 0.91-0.99. The calculated A- and b-values for
392
the five subsets of seasonal data range from 1.4-2.3 and 0.39-0.60, respectively; the later four
393
seasonal data sets, which have the full set of all C2-C5 NMHC and which allow for more accurate
394
determinations, gave an even narrower range of 1.4-1.9 for A, and 0.39-0.44 for b. The absence
395
of detectable differences between the seasonal data sets suggests a similar behavior of the influ-
396
ences that determine NMHC variability throughout the year. The best fit linear regression analy-
397
sis through the data for all seasons yielded lnx = 1.56 -0.38, the mean b-value of the five season
398
periods was 0.46 ± 0.08. The exponent b in this equation has been taken to describe the relative
399
importance of source and sink terms in determining the regional variability of species concentra-
400
tions. Interpretation of observed values of b from different sites has shown that b approaches 0
401
near urban areas, where the variability is strongly influenced by differences in the strength of
402
local emission sources. Values of b close to 1 were found in stratospheric data, where the varia-
403
bility is dominated by chemical loss alone (Jobson et al., 1998, 1999). Interestingly, the Pico
404
value is close to results from three other free tropospheric data sets from marine environments,
405
even though those data resulted from shorter observational periods; b-values in data from the
406
Arctic Boundary Layer (ABLE3A), the equatorial Atlantic (TRACE-A), and the western Pacific
407
(PEM-West B) experiment all ranged from 0.46–0.53 (Jobson et al., 1999). This comparison
408
illustrates a rather high similarity between the continuous, seasonal Pico surface data and the
13
409
results from the comparatively short aircraft campaigns in the marine troposphere. It also under-
410
scores that the atmosphere at the Pico Mountain observatory behaves similar to other areas that
411
have been shown to be remote from local influences.
412
413
3.4 Analysis of Atmospheric Processing of NMHC Using NMHC Ratios
414
415
The relationships between concentrations of different NMHC observed at Pico are dependent
416
upon the NMHC source emission ratios and upon the atmospheric processing that occurs during
417
transport from the emission region to Pico. In this section we investigate several of these rela-
418
tionships with the goal of elucidating the seasonal and species dependence of NMHC oxidation.
419
The interrelationships of all NMHC are briefly discussed, followed by a more in-depth analysis
420
of several NMHC ratios that are particularly useful for providing atmospheric processing infor-
421
mation. NMHC ratios are less sensitive to changes from mixing during transport; consequently
422
NMHC ratios are better indicators for studying chemical processing during transport than using
423
NMHC concentrations alone. However, such analyses rely on the assumption that variations of
424
emission ratios of NMHC pairs are relatively small in source regions. A good body of NMHC
425
data from urban areas supports this assumption, however there have been a few reports that point
426
towards seasonal changes of NMHC emission ratios in source regions (Greenberg et al., 1996;
427
Swanson et al., 2003; Lee et al., 2006). Thus, a crucial step in each analysis will be the evalua-
428
tion of emission ratios. In the ratio analyses presented here we will follow a common convention
429
– the less-reactive NMHC will be placed in the denominator of the ratio, so that the ratio will
430
decrease as NMHC removal processes progress.
431
432
3.4.1 Correlation between NMHC and with CO
433
Correlation plots of all saturated C2-C5 NMHC combinations as well as of NMHC with CO are
434
provided in the Supplement to this paper. Results for the 2-sided linear regression analyses of
435
these correlation plots are given in Table 2. A general feature in all of these plots is that regres-
436
sion line slopes of NMHCa/NMHCb (with carbon number NMHCa < NMHCb) increase monoton-
437
ically with increasing carbon number of NMHCb. This behavior is expected as the atmospheric
438
mixing ratios (and lifetimes) of NMHC generally decrease with increasing carbon number. For
439
an individual pair of NMHC, the regression line slopes become larger towards the summer, as
14
440
the longer-chain NMHC are removed from the atmosphere faster than the more stable, shorter-
441
chain NMHC. Regression coefficients generally decrease towards the summer, as shorter
442
liftetimes, lower concentrations, and higher relative variability cause the correlation between
443
individual compounds to become weaker. Even though CO and ethane have similar lifetimes
444
their correlation is not as strong as correlations between ethane and shorter-lived NMHC. This is
445
attributed to differences in their primary and secondary sources; although fossil and biofuel
446
combustion is a common source, both gases have other unique sources, e.g. natural gas produc-
447
tion and distribution is a major source of ethane, but not of CO; CO in contrast is a degradation
448
product of hydrocarbons in the atmosphere.
449
450
3.4.2 Ratios of isomeric pairs of NMHC
451
The OH reaction rate constants of the isomeric pairs i-butane and n-butane are very similar;
452
consequently, the ratio of these two compounds is expected to change comparatively little during
453
oxidation by OH. The i-butane/n-butane correlation in the data from Pico, differentiated by the
454
time of year is shown in Figure 4 (upper left panel). A tight relationship between the concentra-
455
tions of two isomers is obvious. Other than for a few outliers and a slight increase in scatter at
456
lower mixing ratios (which likely is due to a loss of precision at lower mixing ratios), the two
457
butane isomers show no obvious change in their correlation throughout the course of the year.
458
Subsets of the data for the four seasons showed no difference in their correlation. A two-sided
459
linear regression analysis of the four seasonal data subsets gave slope values (with 95% confi-
460
dence interval and R2 value) of 0.49 ± 0.04 (0.91), 0.49 ± 0.01 (0.98), 0.49 ± 0.02 (0.89), and
461
0.44 ± 0.08 (0.55) for fall-summer. The regression line slope for all data was 0.52 ± 0.01 (R2 =
462
0.96), and the geometric mean ratio (and standard deviation) was 0.46 ± 0.12. Similar values
463
(range 0.37–0.55) have been reported in data from a multitude of other sites in both continental
464
and marine environments (e.g. Bottenheim and Shepherd, 1995; Bottenheim et al., 1997; Green-
465
berg et al., 1996; Parrish et al., 1998).
466
467
The upper right panel of Figure 4 investigates the behavior of the [i-butane]/[n-butane] ratio as a
468
function of total concentration (of n-butane). The 10-percentile bins of the data do not show any
469
significant difference in the [i-butane]/[n-butane] ratio, indicating a constant behavior over the
470
dynamic range of mixing ratios observed at the station. It is noteworthy that this consistency in
15
471
the butane isomer ratio contrasts with results from the polar marine boundary layer where either
472
episodic or concentration-dependent increases in [i-butane]/[n-butane] have been observed dur-
473
ing springtime solar sunrise. This deviation has been explained by the influence of chlorine atom
474
oxidation, as the ~50% faster reaction of the chlorine atom with the n-isomer can shift the [i-
475
butane]/[n-butane] ratio to higher values (Jobson et al., 1994b; Hopkins et al., 2002). The ab-
476
sence of increasing [i-butane]/[n-butane] ratio with increased processing in the Pico data indi-
477
cates that chlorine atom oxidation plays no significant role during transport of air from continen-
478
tal source regions to Pico.
479
480
The results of a similar analysis for the two pentane isomers indicates that these NMHC behave
481
somewhat differently than the butane isomers. The 2-sided regression analysis through the data
482
in Fig. 4 yielded a slope (and 95% confidence interval) of 0.69 ± 0.08 (R2 = 0.93); the geometric
483
mean ratio (and standard deviation) was 0.78 ± 0.31 for all of the included data. The geometric
484
mean ratio is higher than the regression slope because increasingly higher [n-pentane]/[i-
485
pentane] ratios were observed moving from the higher mixing ratio percentiles (mostly winter
486
data) towards the lower percentiles of the data (mostly spring and summer data). The geometric
487
mean ratio increased from 0.64 ± 0.11 in the upper 10 percentile of the data to a value of 1.15 ±
488
0.51 in the lowest 10 percentile. A change in the seasonal behavior of the pentane isomers is
489
also apparent in the cumulative distribution plots (Fig. 2). While the i-pentane distribution
490
shows the higher values in the fall and winter samples, the summer distribution is reversed, with
491
i-pentane being found at lower mixing ratios. The pentane isomers are the only pair of com-
492
pounds that exhibit such a switch of positions during the course of the year. Also, during the
493
summer there were a considerable number of samples that had relatively high n-pentane (8-45
494
pptv), but with i-pentane below the detection limit of 1 pptv. A loss of correlation of pentanes
495
with other NMHC and between the two pentane isomers is also evident in the low R2-values for
496
the summer data in Table 2. For samples with both i- and n-pentane data (n=34) above the de-
497
tection limit, the i-pentane mixing ratio and standard deviation was 3.7 ± 2.0 pptv, and the corre-
498
sponding value for n-pentane was 3.0 ± 1.5 pptv. Unfortunately, the interpretation of the pentane
499
data is somewhat limited by the measurement sensitivity, which was not sufficient to detect
500
either one or both of the pentane isomers in many of the summer samples. For n-pentane, only
501
41%, and for i-pentane an even small fraction (14%) of the summer measurements were above
16
502
the detection limit. Only 11% of the samples had detectable levels of both i- and n-pentane.
503
Consequently, interpretations are based upon a small fraction of the overall data set. Nonethe-
504
less, the available pentane data from Pico point towards a similar behavior as observations from
505
other studies. Enhanced [n-pentane]/[i-pentane] ratios during the summer months and in low-
506
concentration (well aged) samples were also evident in the Fraserdale data (Jobson et al., 1994a)
507
as well as during ICARTT in the WP-3D data set and a 2006 data set from the Research Vessel
508
Ronald H. Brown (D. Parrish, unpublished data).
509
510
The following discussion investigates if seasonal changes in the n-pentane/i-pentane emission
511
ratio from anthropogenic sources may play a role in the seasonal changes seen in the n-
512
pentane/i-pentane concentration ratio seen at Pico and in other data sets from remote marine
513
regions. The [n-pentane]/[i-pentane] ratio in the year-round (2005) data set from Hohenpeissen-
514
berg, Germany averaged 0.58 ± 0.15 (median ± standard deviation) for January and 0.37 ± 0.10
515
for July (Plass-Duelmer and Berresheim, 2006; and 2007, personal communication). A similar
516
behavior is seen in data from North American sites, e.g. measurements in the Southeastern Unit-
517
ed States gave a ratio of 0.60 ± 0.05 for winter, and 0.43 ± 0.13 for summer (mean ± standard
518
deviation of median seasonal data from four sites, Hagerman et al., 1997) and the extensive
519
NMHC data from Harvard forest indicate a ratio of 0.45 in winter and approximately 0.35 in
520
summer (Lee et al., 2006). There are two striking features in the comparison of these emission
521
ratios to the observed ambient concentration ratio at Pico. First, the Pico [n-pentane]/[i-pentane]
522
ratios are higher than in the source region data, and second the seasonal tendency is reversed.
523
While the urban/continental [n-pentane]/[i-pentane] data generally decrease towards the summer,
524
an increase is seen at Pico and at the other above-mentioned remote sites. Clearly seasonal
525
differences in emission ratios from anthropogenic sources do not play a dominant role in the
526
magnitude and trend of the observed [n-pentane]/[i-pentane] ratios.
527
528
Differences in the photochemical processing of the two pentane isomers also might play a role in
529
the evolution of their concentration ratio. However, if OH oxidation dominates as is generally
530
expected, a decrease in the [n-pentane]/[i-pentane] ratio would be infered during summer and
531
with increasing transport time, due to the slightly higher n-pentane OH reaction rate constant (3.8
532
x 10-12 cm3 molecule-1s-1 at 298 K) compared to i-pentane (3.6 x 10-12 cm3 molecule-1s-1) (Atkin-
17
533
son and Arey, 2003). Reports in the literature have suggested significant roles for chlorine atoms
534
and NO3 radicals in NMHC oxidation. However, chlorine atoms, as OH, generally react with n-
535
isomers more rapidly than branched isomers. Thus, any contribution from chlorine reactions
536
would contribute to a further reduction of [n-pentane]/[i-pentane] during photochemical pro-
537
cessing. In contrast, oxidation of NMHC by NO3 would shift the isomer ratio to larger values
538
because the reaction of the i-pentane is about two times faster (Atkinson and Arey, 2003). How-
539
ever, a first-order estimate of the [NO3] needed to account for the observed shift in [n-
540
pentane]/[i-pentane] resulted in unrealistically high NO3 levels. In summary, NMHC processing
541
in the atmosphere cannot account for the pentane isomer concentration ratio behavior.
542
543
Although anthropogenic emission sources are generally thought to dominate the atmospheric
544
concentrations of the light alkanes, there are other sources that could possibly alter the relative
545
ambient concentrations of these species, especially at the low concentrations characteristic of the
546
Pico summertime data set. A stronger influence of biomass burning activities during the summer
547
could shift the pentane isomer ratio more towards the n-isomer. During summer 2005 the geo-
548
metric mean (with standard deviation) of [n-pentane]/[i-pentane] in 22 samples that had both
549
isomers above the detection limit outside of fire events was 0.62 ± 0.33; this value is similar to
550
the anthropogenic emission ratio. In contrast, in 12 samples that had both isomers above the
551
detection limit during fire events, the ratio was 1.37 ± 0.55, more than twice as high. For com-
552
parison, the [i-butane]/[n-butane] ratio outside of fire events was 0.37 ± 0.12 (n = 75) compared
553
to 0.38 ± 7 (n=25) during fire events. The n-pentane/i-pentane emission factor ratio is about 2
554
for extratropical forest fires (Andreae and Merlet, 2001), which account for the primary impact at
555
Pico (Val Martin et al., 2006; Lapina et al., 2007). This ratio is 3-4 times higher than the report-
556
ed emission ratios from urban areas. In contrast the i-butane/n- butane emission factor ratio from
557
these fires is about 0.32, in much closer agreement with the emission ratio from anthropogenic
558
sources. Thus, a strong influence of forest fire emissions in the summer is a plausible explana-
559
tion for the summertime shift in the pentane isomer distribution, without a corresponding shift in
560
the butane isomer ratio.
561
562
Oceanic emissions of NMHC is another source that could conceivably effect the [n-pentane]/[i-
563
pentane]. Hopkins et al. (2002) measured NMHC concentrations in the Arctic marine boundary
18
564
layer. In air masses that had been isolated from anthropogenic sources for extended periods,
565
they found [n-pentane]/[i-pentane] ratios near 2. They attribute these relatively high ratios to the
566
influence of emissions of these compounds from the ocean. The butane isomer ratio was not
567
markedly different from either anthropogenic emissions or forest fire emissions. Although (as
568
discussed above) the Pico site is not influenced by local boundary layer sources, there is substan-
569
tial exchange of marine boundary layer air with the free troposphere over the days of transport
570
upwind of Pico. Therefore it is possible that oceanic emissions may play a role in determining
571
the elevated [n-pentane]/[i-pentane] ratios observed at Pico. However, it should be noted that
572
Hopkins et al. (2002) did not directly measure NMHC fluxes from the ocean; they simply argued
573
that oceanic emissions were the likely source in the isolated marine Arctic region. They also did
574
not consider the possible effect of the transport of boreal forest fire emissions to the Arctic re-
575
gion.
576
577
As mentioned above only a small fraction of the summer NMHC measurements at Pico had both
578
pentanes above the detection limit, so the preceding analysis is based on a small fraction of the
579
measurements. At these low measured concentrations biases in the analyses may be possible.
580
Subsequent measurements with higher sample volume collections and additional analytical tests
581
were conducted at Pico in 2006 and are planned for 2008-2009. With the higher sensitivity
582
expected in these new measurements a stronger data set is anticipated, which, together with
583
FLEXPART transport studies, will be applied for a re-evaluation of the seasonal trends in the [i-
584
butane]/[n-butane] and [n-pentane]/[i-pentane] ratios. One focus on the analysis of those data
585
will be quantifying the relative contribution of anthropogenic, forest fire and oceanic emissions.
586
587
3.4.3 Photochemical processing as indicated by NMHC ratios
588
By working with the ratio of pairs of hydrocarbons the influence of photochemical processing
589
can be most clearly examined and the influence of mixing on the composition of a given air
590
parcel can be minimized. Figure 5 more sensitively investigates the seasonal dependence of
591
NMHC oxidation by plotting [propane] and [n-butane] against ln [propane]/[ethane] and ln [n-
592
butane]/[ethane]. Smaller values of these ratios, along with overall lower absolute NMHC mix-
593
ing ratios, are seen in the summer, and larger values occur in the winter. A notable feature is that
594
at a given [NMHCa]/[NMHCb] ratio value, lower absolute mixing ratios of the faster reacting
19
595
NMHC are observed during the earlier part of the year than during the later part. This hysteresis
596
behavior illustrated the different seasonal behavior of individual NMHC. If the seasonal cycle
597
of all NMHC were in phase, then data in these plots would be expected to fall on one line
598
throughout the year. The phase difference between the NMHC plotted on the y-axis and the ratio
599
that is plotted on the x-axis determines the spread of the data in the x-y domain. This analysis
600
further exemplifies the shift in the seasonal maxima and minima for individual NMHC (moving
601
towards earlier in the year with decreasing atmospheric lifetime) as discussed above in section
602
3.1.
603
604
The distribution of NMHC in a double natural logarithm plot of [n-butane]/[ethane] versus [pro-
605
pane]/[ethane] is particularly useful to investigate photochemical processing and mixing that
606
occurs during atmospheric transport from source regions to remote sites (Rudolph and Johnen,
607
1990; McKeen and Liu, 1993). Photochemical processing of NMHC in the atmosphere is ex-
608
pected to follow first-order kinetics, in which case atmospheric NMHC mixing ratios will de-
609
crease exponentially with aging. Thus, when the logarithm of the NMHC ratios are plotted the
610
axes scales are linearly proportional to processing time. The Pico data shown in Figure 6 fall
611
between two theoretical limits. The steeper “kinetic” line with a slope of 2.61 indicates the
612
evolution of isolated air parcels from the assumed source region emission ratios of these com-
613
pounds (indicated by the black diamond) when subjected to oxidation by OH radicals. Here, we
614
used the emission ratios of 0.63 for [propane]/[ethane] and 0.35 for [n-butane]/[ethane] (Parrish
615
et al., 2007), which was derived from the mean of the Goldstein et al. (1995), and Swanson et al.
616
(2003) results. Applied rate constants were k = 0.18 x 10-12, 0.89 x 10-12 and 2.05 x 10-12 mole-
617
cules-1 cm3 s-1 for ethane, propane and n-butane, respectively (Atkinson and Arey (2003) with T
618
= 273 K). The less steep “dilution” line indicates the effect of dilution of an air parcel starting at
619
the same emission ratio when mixing with “background” air that has aged to the point that there
620
are negligible concentrations of the more-reactive NMHC (propane, n-butane) in the numerators
621
of the ratios.
622
623
The seasonal differences in the degree of NMHC oxidation are clearly visible in these plots.
624
During winter, most data have larger ratios, fall closer to the assumed emission ratios, and are
625
less variable. This behavior indicates the lower degree of photochemical processing. In contrast,
20
626
spring and summer data are more scattered, and the lower [NMHC]/[ethane] ratios are indicative
627
of the higher degree of NMHC oxidation that occurred during transport in these seasons. The
628
two-sided regression analysis of the four seasonal subsets of these data resulted in slopes (with
629
95% confidence interval) of 1.65 ± 0.16, 1.57 ± 0.07, 1.57 ± 0.11, and 1.46 ± 0.18 for the fall,
630
winter, spring, and summer, respectively. The regression through all data yielded a slope (and
631
95% confidence interval) of 1.60 ± 0.04. All of these slopes fall within the range of slopes (1.44
632
- 1.78) from the eleven data sets summarized by Parrish et al., (2007).
633
634
The Pico data closely converge to the assumed emission ratios; consequently the chosen values
635
can be considered as a good representation for the year-round data. A number of available addi-
636
tional determinations of regional emission ratios are shown in Figure 6. It is notable that while
637
there is close agreement between the convergence point of the Pico data and several of the de-
638
terminations (see figure caption for details), data from a number of other studies (i.e. Seila et al.,
639
1989; Warnecke et al., 2007) deviate to a greater extent, with the earlier data from the Seila et al.
640
(1989) study falling markedly above the Pico regression lines and most of the recent data from
641
the Boston area and from Los Angeles (Warnecke et al., 2007) falling below it. In the latter
642
study recent NMHC observations off the coast of New England were normalized to observed
643
levels of acetylene and carbon monoxide and source region emission factors were calculated by
644
plotting these ratios against estimated photochemical age of the air mass and extrapolating this
645
dependency to zero processing time. A closer investigation of the seasonal behavior of the Pico
646
data provides some possible insight into this discrepancy. The seasonal regression lines added in
647
color in Fig. 6 show that of all seasonal data the Pico summer measurements yield the a smallest
648
slope value, and the regression through the Pico summer data most closely approaches the emis-
649
sion ratios of Warnecke et al., (2007), which were based on mid-summer observations in the
650
Northeastern U.S. The better agreement between the Pico summer data and the Warnecke et al.
651
summer emission rates suggest that Pico data do, to some degree, reflect emission ratio changes
652
in NMHC seen in the Northeastern U.S. A lower summertime [n-butane]/[ethane] emission ratio
653
is also supported by the recent analysis of Lee et al. (2006). These researchers attributed these
654
changes to lower summertime butane emissions from gasoline use as a result of the mandated
655
reduction of gasoline composition (volatility) during the summer, which was implemented in the
656
early 1990s as a measure to curb summertime ozone production in the Eastern U.S. The higher
21
657
[n-butane]/[ethane] ratios in the data by Seila et al. (1989), which resulted from measurements
658
prior to the changes in gasoline composition, further support this conclusion. It has also been
659
pointed out that [propane]/[ethane] emission ratios are higher in New England data than com-
660
pared to other locations (de Gouw et al., 2005; Parrish et al., 2007; Warneke et al., 2007). If
661
such emission ratio changes occurred in all source regions, then a downwards shift of the regres-
662
sion line would be expected rather than a change in the slope as observed in the Pico data. The
663
lower summer slope possibly points towards a larger inhomogeneity in source region emission
664
ratios during the summer, where air with lower [butane]/[ethane] and/or higher [pro-
665
pane]/[ethane] emission ratios is transported from regions (e.g. the Northeastern U.S.) that are
666
within a relatively short transport distance to Pico.
667
668
3.5 Relationship Between NMHC Processing and Ozone
669
670
In general, understanding the temporal variability of tropospheric ozone at any particular loca-
671
tion is complex because several processes can have significant impacts, and these impacts vary
672
strongly on different time scales. In-situ photochemical production and destruction proceed at
673
rates that vary with the ambient levels of ozone precursors, and variables such as sunlight and
674
water vapor concentrations. Surface deposition and destruction by reaction with local emissions
675
of NO or reactive NMHC can drastically reduce near-surface ozone concentrations at rates that
676
vary with the flux of local emissions and the structure and evolution of the planetary boundary
677
layer. Transport of ozone from the stratosphere or from upwind regions of strong photochemical
678
production can greatly increase ozone concentrations. However, if the effects of local influences
679
(surface deposition, reaction with emissions) and transport of stratospheric ozone can be elimi-
680
nated, then concurrent measurements of NMHC ratios and ozone provide information on the net
681
effect of photochemical ozone production or loss; Parrish et al. (2004) show that the relative
682
change of ozone with NMHC ratio evolution is an indication of ozone production or loss during
683
long-range transport.
684
685
The Pico Mountain site is a good location to isolate the effects of the regional photochemical
686
production and destruction in the central North Atlantic. Previous discussion in this paper and
687
Kleissl et al. (2007) have shown that air sampled at the site is characteristic of the lower free
22
688
troposphere with no significant effects from surface deposition, destruction by reaction with
689
local emissions, or photochemical production from locally emitted precursors. The varying
690
influence of the transport of stratospheric ozone often dominates the variability of ozone in free
691
tropospheric data sets. Since ozone from the stratosphere has a steep, negative correlation with
692
CO (see e.g., Danielsen et al., 1987) the influence of stratospheric ozone transport can be evalu-
693
ated from the correlation of ozone with CO. Honrath et al. (2004) discuss the strong, positive
694
ozone-CO correlation observed at Pico Mountain; their Figure 7, which presents these correla-
695
tions, shows only a few scattered points with relatively high ozone at low CO that may be due to
696
stratospheric influence. Thus, consistent with other Pico analyses (Lapina et al., 2006; Owen et
697
al., 2006; and Val Martin et al., 2006), Honrath et al. (2004) concluded that the ozone variability
698
at Pico is dominated by the influence of the regional photochemical production and destruction
699
in the central North Atlantic.
700
701
Parrish et al. (1992, 2004) demonstrate that NMHC aging correlates with increasing ozone when
702
photochemical ozone production dominates, and correlates with decreasing ozone when ozone
703
destruction processes dominate. These previous studies focused upon marine boundary layer
704
observations to avoid the confounding influence of stratospheric ozone. Given the low strato-
705
spheric influence at Pico the same analysis can be directly applied to the Pico data. Figure 7
706
shows the dependence of ozone concentrations on the natural logarithm of [propane]/[ethane] as
707
the indicator of the photochemical processing. During fall and winter both the [pro-
708
pane]/[ethane] ratio and the ozone concentrations are relatively constant with no clear depend-
709
ence on the NMHC ratios. In spring and summer ozone has a higher variability, both in the
710
higher and lower range of photochemical processing. An increasing correlation between the
711
degree of NMHC processing (as indicated by the [propane]/[ethane] ratio) and the ozone concen-
712
trations is observed during the spring and summer. Higher ozone levels were more consistently
713
observed in air that had relatively ‘fresh’ photochemical signatures (i.e. ln [propane]/[ethane] > -
714
2.5), and overall lower ozone levels were seen in more processed air (i.e. ln [propane]/[ethane] <
715
-2.5).
716
717
The short-term variability of this behavior further illustrates how, in addition to the seasonal
718
dependency, changes in transport time influence the ozone chemistry-NMHC relationship. In
23
719
Figure 8 the seasonal record of ln [propane]/[ethane] is plotted, with the data points being color-
720
coded according to the ozone mixing ratio observed at the time of measurement. This analysis
721
shows the dynamic range of photochemical processing at a given time of year (on the order of 1-
722
3 in ln([propane]/[ethane]) and it reemphasizes how ozone variability and the dynamic range of
723
NMHC ratios increases from winter to spring-summer. It furthermore shows that lowest ozone
724
levels are observed during events when air that is well aged (as indicated by low ln ([pro-
725
pane]/[ethane]) values) is transported to the site, whereas ozone levels are higher when fresher
726
air (indicating faster transport) is encountered. Significant changes in the degree of NMHC
727
processing and ozone are observed on 1-5 days timescales. Conclusively, these analyses indi-
728
cate that in spring and summer the highest ozone concentrations were observed in air masses
729
transported rapidly from continental source regions to Pico, and lower concentrations were ob-
730
served in air masses that had been processed for longer times. Thus, on the whole, the photo-
731
chemical environment of the lower free troposphere over the North Atlantic leads to net ozone
732
destruction.
733
734
The regression lines for the spring and summer included in Figure 7 were calculated by using a
735
linear, least-squares weighted regression algorithm that allows for uncertainties in both the x and
736
y variables. Each variable was weighted by 1/σ2, where σ is the estimated uncertainty in each
737
measurement, as obtained from the specified precision and accuracy of the analysis for the
738
NMHC (discussed in Section 2.2) and using the greater of 1 ppbv or the standard deviation of the
739
ozone measurements over the NMHC sampling period. The positive slope values for the spring,
740
and even more so during summer, reflect the degree of the photochemical destruction of ozone.
741
Even though these dependencies account for a relatively small fraction of the variance of ozone
742
in spring and summer (approximately equal to the R2 values given in Figure 7), the relationship
743
is highly significant (as indicated by the 95% confidence limits of the slopes given in Figure 7).
744
The same analysis was conducted on a subset of these data that excluded periods with suspected
745
upslope conditions according to the criteria given in Kleissl et al., (2007); this analysis yielded
746
regression line slopes for the ozone-ln ([propane]/[ethane]) relationships that were within 5% of
747
those in Fig. 7 and not statistically different.
748
24
749
During the summer of 2004 the NASA DC-8 aircraft conducted flights over the western North
750
Atlantic Ocean as part of the INTEX-A field study (Singh et al., 2006). Data collected over the
751
North Atlantic during those flights have isolated the vertical distribution of ozone chemical
752
sources and sinks, and were used to conduct box model calculations initialized with observed
753
concentrations of measured species (J.R. Olson and J.W. Crawford, private communication,
754
2007). These calculations found that net ozone destruction dominated in the lower troposphere,
755
while net production characterized the upper troposphere. Thus, the model calculations based
756
upon the INTEX aircraft measurements are consistent with the present analysis based on the
757
lower troposphere monitoring at the Pico Mountain station.
758
759
Table 3 compares the springtime and summertime slope of the ozone - ln ([propane]/[ethane])
760
relationship found at Pico with those reported from the north temperate Pacific marine boundary
761
layer. Based on the earlier data included in Table 3, Parrish et al. (2004) argued that the recent
762
Pacific studies (ITCT-2K2, TRACE-P) indicated only weak net ozone destruction (small positive
763
slopes) in the more remote Pacific marine boundary layer, or no evidence for net ozone destruc-
764
tion (PHOBEA). This weak photochemical destruction is in sharp contrast with the much
765
stronger photochemical destruction indicated by a study at Point Arena from nearly two decades
766
earlier. The exception to this picture is the strong photochemical production (large negative
767
slope) in the PEM West-B study, which investigated the western North Pacific region of strong
768
outflow of ozone precursor emissions from Asia. Comparison of these results from the Pacific
769
region to those from Pico suggest that, at the present time, spring- and summertime photochemis-
770
try more effectively destroys ozone in lower troposphere over the central North Atlantic than in
771
the springtime marine boundary layer of the central North Pacific.
772
773
4. Summary and Conclusions
774
775
Concentrations of NMHC at Pico Mountain are lower than at remote, higher northern latitude
776
sites, but higher than at MLO. The observed NMHC levels at Pico reflect the increased influ-
777
ence of the adjacent continents on air composition in the central Atlantic region in comparison to
778
the Northern Mid-Pacific (MLO), as well as the station’s latitude and elevation above sea level.
779
Analyses of the NMHC variability-lifetime relationship provided evidence for the remote charac-
25
780
ter of the Pico site and the lack of significant local influences on NMHC concentrations, with the
781
exception of isoprene. These analyses are in accord with previous conclusions that air encoun-
782
tered at Pico reflects the composition and chemistry of the lower free troposphere above the
783
North Atlantic Ocean. Subsets of seasonal data showed similar behavior in NMHC variability as
784
a function of OH lifetimes. This finding suggests similarity in NMHC transport regimes and
785
source regions throughout the year. Ratios of butane isomers behave as expected from OH
786
chemistry.
787
788
NMHC remained elevated in air masses that had been influenced by either anthropogenic emis-
789
sions or injections from biomass burning after time scales of 1-2 weeks during their transport
790
from source regions to Pico (see more discussion on this topic in Honrath et al., 2008). Increases
791
in the [n-pentane]/[i-pentane] ratio during the summer, and in particular in identified biomass
792
burning plumes further reemphasize the influence of boreal biomass fires on atmospheric com-
793
position and chemistry in the North Atlantic region. It is possible that oceanic emissions of the
794
pentanes may contribute to their concentrations at Pico. Future analyses will more fully investi-
795
gate the biomass burning and possible oceanic emission influences.
796
797
Spring- and summertime ozone levels at a given [propane]/[ethane] ratio showed a higher varia-
798
bility, indicating, more extensive photochemical processing and variable ozone chemistry than
799
during winter and fall. During spring and summer, average ozone decreased with increased
800
photochemical processing. This relationship implies that during the spring and summer the
801
photochemical environment of the lower troposphere over the North Atlantic is characterized by
802
net ozone destruction during transport of air masses to Pico. This behavior is in contrast to the
803
springtime North Pacific, where the photochemical processing is closer to ozone neutral, i.e. it
804
neither produces nor destroys ozone.
805
806
Acknowledgments
807
808
We thank P. Goldan, NOAA Earth System Research Laboratory, Boulder, CO for the reference
809
analysis of the primary NMHC standard prior and after its use at Pico, M. Dziobak and M. Val
810
Martin, Michigan Technological University, for GC instrument maintenance tasks at Pico, and T.
26
811
Jobson, Washington State University, for making available the Fraserdale data. The anonymous
812
reviewers provided valuable comments that helped to further develop discussions in this manu-
813
script. This research was funded by a grant from the NOAA Office of Global Programs (award #
814
NA03OAR4310072). REH and RCO acknowledge support from NOAA grant
815
NA03OAR4310002 and National Science Foundation grant ATM-0535486.
816
817
818
819
820
821
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30
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
31
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 were 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) are shown for comparison. Sinusoidal
fit curves are included for ethane and propane, but not for the higher alkanes as their annual cycle increasingly deviates from this
behavior.
32
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 mixing ratios at the Pico Mountain station during the four measurement seasons.
33
2
Summer '04
Fall '04
Winter '05
Spring '05
Summer '05
-0.60
ethane
propane
i-butane
n-butane
i-pentane
n-pentane
1
lnx
2
y=2.3x
(R =0.93)
-0.39
2
y=1.8x
(R =0.91)
-0.44
2
y=1.9x
(R =0.99)
-0.44
2
y=1.5x
(R =0.93)
y=1.4x-0.43 (R2=0.99)
0.6
0.5
0.4
0.3
0.2
1
2
3
4 5 6
10
20
30 40 50 60
100
200
400
OH lifetime (d)
Figure 3
The standard deviation of the natural logarithm of NMHC mixing ratios during the four seasons at their estimated seasonal OH lifetime. In the upper right corner results for the 2-sided linear regression analysis on seasonal subsets of the data are given in the same
colors as the corresponding data points in the graph. The summer 2004 analysis excludes ethane as it was not quantified during that
period.
34
200
Jan
2
Jan
Dec
Dec
100
1
Oct
Sep
Sep
Aug
Jul
10
Jun
3
4 5
10
100
400
Aug
Jul
Jun
May
May
1
Nov
Oct
i-butane/n-butane
i-butane (pptv)
Nov
0.1
Apr
Apr
Mar
Mar
Feb
Feb
0.03
Jan
3
n-butane (pptv)
10
100
400
100
Jan
4
Jan
3
Dec
Dec
Nov
Nov
2
Oct
n-pentane (pptv)
Sep
Aug
10
Jul
Jun
n-pentane/i-pentane
Oct
Sep
Aug
1
Jul
Jun
May
0.5
May
Apr
0.4
Apr
Mar
0.3
Mar
Feb
1
1
10
i-pentane (pptv)
Jan
n-butane (pptv)
100
200
Jan
Feb
0.2
1
10
i-pentane (pptv)
100
200
Jan
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 npentane versus i-pentane (left) and ratio of n-pentane/i-pentane versus i-pentane (right) in the lower graphs The circles with the error
bars in the right-hand graphs show the geometric mean and standard deviation of the data within 10-percentile bins of the data.
35
400
800
Jan
Jan
Dec
Dec
Nov
Nov
100
Oct
Oct
100
Aug
Jul
Jun
Sep
[n-butane] (pptv)
[propane] (pptv)
Sep
Aug
Jul
Jun
10
May
May
Apr
Apr
10
Mar
Mar
Feb
Feb
4
-5
-4
-3
-2
ln([propane]/[ethane])
-1
0
800
1
-5
Jan
-4
-3
-2
ln([propane]/[ethane])
-1
0
400
Jan
Jan
Dec
Dec
Nov
Nov
100
Oct
100
Aug
Jul
Jun
10
Oct
Sep
[n-butane] (pptv)
[propane] (pptv)
Sep
Aug
Jul
Jun
10
May
May
Apr
Apr
Mar
Mar
Feb
Feb
4
-7
-6
-5
-4
-3
ln([n-butane]/[ethane])
-2
-1
Jan
Jan
1
-7
-6
-5
-4
-3
ln([n-butane]/[ethane])
-2
-1
Jan
Figure 5
Relationship of propane and n-butane as a function of the ln ([propane]/[ethane]) (upper graphs) and ln ([n-butane]/[ethane]) (lower
graphs) with the color coding representing the time of the measurement.
36
Jan
0
-1
ln([n-butane]/[ethane])
-2
-3
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
fall 2004
winter 2005
spring 2005
summer 2005
Dec
Nov
Oct
Sep
Aug
Jul
-4
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 fall 2004-summer 2005 Pico NMHC data with the seasonal dependency illustrated by the
color bar. The filled, black diamond marks the assumed source emission ratio. The data are
bound by two lines representing 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. Four two-sided regression lines for the four
seasonal subsets were added in colors corresponding to the times of year indicated by the color
bar. 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, July 12 - August 10, 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,
April - May, 2002, Warneke et al. (2007), (5) Boston, New York City, August 2003, Warneke et
al. (2007), (6) average of 39 U.S. Cities, June-September 1984 - 1986, Seila et al.
(1989)/Warnecke et al. (2007), (7) Harvard Forest, summer, August 1992 - July 1994, Goldstein
et al. (1995); (8) Harvard Forest, August 1992 - July 1994, winter, Goldstein et al. (1995); (9)
Summit, Greenland, June 1997 - 1998, Swanson et al. (2003), (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).
37
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 natural logarithm-transformed NMHC ratios 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.
38
0
80
70
50
-3
40
70
-1
30
ln([propane]/[ethane])
-4
20
-2
80
-5
Apr
60
May
50
-3
ozone (ppbv)
0
60
-2
ozone (ppbv)
ln([propane]/[ethane])
-1
40
30
-4
20
-5
Nov
Dec
Jan
Feb Mar
Apr
May
Jun
Jul
Aug
Figure 8
Ozone measured at Pico Mountain (indicated by the color coding) as a function of ln([propane]/[ethane]) and time of year during
2004-2005. The insert shows a blowup of the spring 2005 period which was used for the case transport study presented in the companion paper by Honrath et al. (2008).
39
Table 1
Comparison of ethane, propane and butane isomer mixing ratios (in pptv) seen outside and within fire events during summer of 2005.
Ethane
Propane
i- Butane
n -Butane
2005
not fire
fire
events events
2005
not fire
fire
events events
2005
not fire
fire
events events
2005
not fire
fire
events events
number of samples:
315
33
292
31
290
25
290
25
mean mixing ratio:
527
834
36
80
1.0
4.5
8.9
11.9
259
400
507
642
846
592
664
857
973
1051
9
19
27
45
83
51
61
77
95
123
<2
<2
<2
2
4
2
3
4
6
10
<2
<2
5
12
32
5
8
10
16
22
Percentiles
5
25
50
75
95
40
Table 2
Correlation between NMHC and carbon monoxide with number of samples available (n), 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
41
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
Dates
24 April - 9 May 1984
8 February - 14 March 1994
22 April - 19 May 2002
March – May 1997-1999; 20012002
TRACE-P – West to central N. Pacifica
26 February - 10 April 2001
b
Pico-Mountain
20 March - 20 June 2005
21 June - 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
42
Supplemental Materials
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)
43
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)
44
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)
45
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)
46
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)
47
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)
48
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)
49
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)
50
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)
51
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)
52
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)
53