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Analysis of Air Transport and Oxidation Chemistry in the NorthAtlantic Region from Interpretations of Non-Methane Hydrocarbon
(NMHC) Measurements at Pico Mountain, Azores
D. Helmig1*, D. M. Tanner1, R.C. Owen2, R. E. Honrath2 and D.D. Parrish3
1
Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, CO
80309, USA
2
Department of Civil and Environmental Engineering, Michigan Technological University,
Houghton MI 49931, Michigan, USA
3
Chemical Sciences Division, National Oceanic and Atmospheric Administration, Boulder, CO
80303, USA
*corresponding author: Detlev.Helmig@colorado.edu
Manuscript submitted to Journal of Geographical Research
Revised Version
Oct. 31, 2006
Abstract
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One year of continuous measurements (summer 2004-2005) of non-methane hydrocarbons
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(NMHC) at the mountaintop PICO-NARE observatory on Pico Island, Azores were used to
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investigate seasonal oxidation chemistry and transport patterns in the central North Atlantic
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Region. NMHC exhibited short-term variations and seasonal cycles that reflect the distance of
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the island from continental sources of NHMC emissions and oxidation of NMHC by the
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seasonally highly variable OH radical. Substantially enhanced NMHC levels during the summer
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of 2004 were attributed to the impact of long-range transport of biomass burning plumes
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resulting from Northern Canada and Alaskan wildfires. During summer, NMHC absolute levels
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and ratios were indicative of a higher degree of photochemical processing than during other
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times. NMHC concentrations and their relative ratios were valuable in identifying transport
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situations where anthropogenically influenced air from the mid and western U.S. was transported
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to Pico in 5-8 days. Interpretations of NMHC ratios for use as a relative scale for photochemical
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processing (‘photochemical clock’) was shown to yield results that were in qualitative agreement
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with trajectory analyses and interpretations derived from the particle dispersion model
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FLEXPART.
Ozone in excess of 35 ppbv was measured at PICO-NARE 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). Ozone at lower levels (< 40 ppbv) was observed in
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more processed air (‘older’ air with ln [propane]/[ethane] < -2.5). These studies contributed
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towards research in the North Atlantic region in context of the International Consortium on
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Atmospheric Research on Transport and Training, (ICARTT).
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1. Introduction
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Non-methane hydrocarbons (NMHC) in the atmosphere show considerable variations on spatial
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and temporal scales, their concentrations being determined by the strength of emission sources
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and atmospheric removal processes. Atmospheric oxidation is mostly due to reaction with the
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OH radical, with reaction rate constants increasing significantly with the molecular size within a
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given class of NMHC.
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longer lifetimes. Their atmospheric concentrations decline at slow enough rates for NMHC
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concentrations to remain high enough after several days of transport to impact air chemistry at
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remote downwind locations. Since many individual NMHC have common emission sources and
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since their emission ratios vary comparatively little, changes in absolute concentrations and
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NMHC ratios can be used as tools to decipher atmospheric transport and oxidation chemistry.
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Several researchers have investigated this utility and have presented a framework for the
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interpretation of atmospheric light saturated (C2-C6) NMHC observations.
Lighter, saturated NMHC, having the slowest reaction rates, exhibit
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Selected NMHC can be used as tracers for specific emission sources or events. For instance,
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isoprene is a selective tracer for biogenic emissions (Fehsenfeld et al., 1992). While methyl
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chloride and acetonitrile are tracers of biomass burning plumes, acetylene and benzene over light
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n-alkanes, due to their relatively high emission ratios over n-alkanes in biomass burning
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compared to anthropogenic combustion processes, and due to their relatively long atmospheric
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lifetime and low natural backgrounds, are other potential indicators of influence from biomass
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burning (Andreae and Merlet, 2001; DeGouw et al., 2004). Light, saturated and unsaturated
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NMHC have been used to identify influences from urban energy use and petrochemical
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industries (Blake and Rowland, 1995; Jobson et al., 2004). Diurnal concentration changes of
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light, unsaturated NMHC (ethene, propene) allowed the identification of occurrences of upslope
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and downslope flow conditions at Mauna Loa Observatory (Greenberg et al., 1996).
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Certain NMHC (e.g. butanes) have similar atmospheric removal rates and hence their,
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atmospheric ratios show little variations during atmospheric transport and processing. Their
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analytical data can therefore be used as a quality control tool in NMHC measurements (Parrish et
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al., 1998).
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The variability of NMHC concentrations can provide information on the impact or distance of a
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measurement site from pollution sources. A ‘remoteness’ scale has been proposed, that is
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derived from a plot of the natural logarithm (ln) of the standard deviation of ambient NMHC
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concentrations at a given site versus their estimated lifetime (Jobson et al., 1999). Relative
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changes of the ratio of branched versus straight n-alkanes have been used to infer the importance
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of halogen and nitrate radical versus OH radical chemistry as the reaction rates of these two
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different oxidation routes are significantly different enough to cause shifts in the atmospheric
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concentration ratios of these isomeric compounds (Penkett et al., 1993; Finlayson-Pitts, 1993).
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NMHC ratios and concurrent measurements of ozone were also applied to investigate potential
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changes in the oxidation chemistry of the atmosphere. In particular, the relative change of ozone
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with observed NMHC ratio evolution was used as an argument for an increased ozone
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production (or reduced ozone loss) rate in long-range transport across the Pacific Ocean (Parrish
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et al., 2004). NMHC measurements from remote, marine environments have also been applied
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for estimating mean OH radical fields during transport of air in the marine boundary layer and
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lower free troposphere (Ehhalt et al., 1998; Williams et al., 2000, 2001).
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The possibilities for using NMHC data for interpretations of atmospheric oxidation processes are
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particularly promising in situations where observations can be obtained in air that has traveled
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for extended periods of time without being influenced by 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 aforementioned considerations motivated the monitoring of NMHC at the
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mountaintop PICO-NARE site on Pico Island, Azores. These measurements commenced in the
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summer of 2004, in overlap with the International Consortium for Atmospheric Research on
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Transport and Transformation (ICARTT) campaign in the North Atlantic Region, and have been
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continuous for most times when the station was on power. These measurements provided one of
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the very few continuous annual records of NMHC from a lower free-troposphere measurement
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site. In this paper we present data from the first year of these new observations. Interpretations
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demonstrate the utility of the NMHC data for interpretations of oxidation and transport processes
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in the North Atlantic region.
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2. Methods
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2.1. PICO-NARE Station
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The PICO-NARE observatory is located in the summit caldera of the inactive Pico Mountain
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volcano (38.47N, 28.40W), the highest mountain on Pico Island, and in the Azores, Portugal. At
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2225 m asl, lower, free tropospheric air is sampled at the station during most times. More
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information on the geo- and topographical features are provided by Kleissl et al. (2006). A
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detailed analysis of boundary layer height, and of mechanical uplifted and buoyant flow
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conditions showed that air from lower elevations was potentially lifted up to the station height up
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to 50% of the days during some months, but to a lesser extent (~ 25% of the days) during the
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summer. However, chemical measurements of nitrogen oxides and carbon monoxide at the
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observatory showed very little influence from island emission sources even during uplifting
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events (or upslope flow), the station is negligibly impacted by inland emissions (Kleissl et al.,
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2006). Data and interpretations from other research, including studies of oxidized nitrogen
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species, ozone, carbon monoxide and of aerosol properties at PICO-NARE have been presented
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previously (Honrath et al., 2004; Fialho et al., 2006) and in other contributions to this special
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issue (Val Martin et al., 2006).
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2.2 NMHC Measurements
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The remoteness of the PICO-NARE site and the limitations for power and for supply of cryogen
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and consumable gases determined the design of an analytical system that was tailored towards
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this unique situation. All consumable gases and blank air were prepared at the site with low-
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power gas generators. The instrument was designed to follow automated startup and shutdown
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procedures and could be remotely controlled from our Boulder, CO, offices.
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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 mulit-stage solid
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adsorbent trap, NMHC were analyzed by thermal desorption with gas chromatography (GC)
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separation and flame ionization detection (FID). The instrument was calibrated by regular
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injections of a multi-component, rural air standard that was quantified prior to shipment against a
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gravimetric hydrocarbon standard scale in the NOAA Earth System Research Laboratory. A
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second, remote, ambient air standard (collected at Niwot Ridge, Colorado) was injected every 3-
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4 days for quality control.
Ozone was
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Sample volumes of 600 ml (10 min collection time) and 3000 ml (50 min collection time) were
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collected semi-continuously (every few hours) and sample volumes were alternated for
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quantification of ethane (in the 600 ml sample) and NMHC > C2 (in the 3000 ml sample),
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respectively. Typically, a total of 12 ambient air samples, one standard and one blank sample
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were analyzed daily. Data were transferred daily for instant quality control and analysis. The
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primary calibration standard was returned to Boulder in spring 2006 and the control analysis on
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the independently calibrated NOAA GC system showed that mixing ratios for all C2-C5 NMHC
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reported in this study were within +/-5% (which is within the accuracy range of the NOAA
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measurements) of the values determined two years earlier, prior to the shipment to Pico. NMHC
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were quantified using compound-specific FID response factors, as determined from the primary
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standard injections. Quantified NMHC included ethane, propane, n-butane, i-butane, i-pentane,
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n-pentane and isoprene (the ethane record doesn’t begin until in fall 2004 when some
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modifications in the focusing procedure allowed its quantitative analysis).
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described experiments analytical precision and accuracy were estimated to be better than 10%
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for mixing ratios > 100 pptv and approximately a factor of 2 higher for levels between the
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detection limit (which typically were ~ 30, 11, and 1-2
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respectively) and 100 pptv. Ethene, propene, benzene, and toluene, while captured with this
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system, where excluded from the analysis because of higher and inconsistent blanks which made
From the above
pptv for C2, C3, and C4-C6,
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their quantification at low pptv levels not feasible. More instrumental details have been provided
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elsewhere (Tanner et al., 2006).
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2.3 Trajectory Analysis
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Backward trajectories were calculated with the Hybrid Single-Particle Lagrangian Integrated
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Trajectories (HYSPLIT) model (Draxler and Rolph, 2003). HYSPLIT uses meteorological data
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from the National Weather Service’s National Center for Environmental Prediction (NCEP) final
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analysis (FNL). Data are available at 6-hour resolution with 13 pressure altitude levels. A set of
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six trajectories were calculated, one terminating at the station, four terminating at grid points
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adjacent to the station and separated by 1o, and one terminating directly below that station at
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2000 m asl. Trajectories were run 10 days backward in time.
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2.4 FLEXPART Simulations
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Besides the trajectory analysis, the FLEXPART particle dispersion model (versions 5.2 and 6.2,
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(Stohl et al., 1998, 2005; Stohl and Thomson, 1999)) was used to evaluate derived NMHC
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transport times with synoptic transport modeling results. FLEXPART version 6.2 was driven
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with data from the European Centre for Medium Range Weather Forecasts (ECMWF) (ECMWF,
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2005) with a 1 degree horizontal resolution, 60 vertical levels and a temporal resolution of 3
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hours, using meteorological analyses at 0000, 0600, 1200, and 1800 UTC, and ECMWF 3-hr
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forecasts at intermediate times (3, 9, 15, 21 UTC). FLEXPART version 5.2 was driven with data
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from wind fields from the NOAA NCEP FNL. The FNL data were downloaded from the
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National Center for Atmospheric Research data archive, available every 6-hours with a
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horizontal grid spacing of 1 x 1o and 21 vertical levels between 1000 and 100 hPa.
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Version 5.2 of the model was run in its forward mode to simulate CO enhancements at the PICO-
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NARE station resulting from the transport of North American and Asian emissions. These
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emissions were divided into one day age classes, which was also useful for determining the time
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since emission. CO emissions were released into the lowest 300 m of the atmosphere over North
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America and Asia. Emissions were based on the EDGAR 3.2 Fast Track 2000 dataset (Olivier et
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al., 2001) for anthropogenic sources only with a 1 degree resolution.
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Version 6.2 of the model was run in its backward mode to create “retroplumes”, similar to
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backward trajectories. Retroplumes are simulated from the release of thousands of particles at
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the receptor that are advected backwards in time. Retroplumes are superior to trajectories in that
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they allow for an assessment of the deformation of an air mass as it travels and for determining
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source regions for observed enhancements (Seibert and Frank, 2004).
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initiated every three hours with 20,000 particles released over a three hour time interval into a 1
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degree x 1 degree grid box centered on the PICO-NARE station, over an altitude range of 1750
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m asl to 2750 m asl. Particles were followed backward in time for 20 days.
Retroplumes were
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3. Results and Discussion
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3.1 NMHC Mixing Ratios
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Plots with the individual sample data (representing a total of 1958 analyzed air samples) for
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ethane, propane, and n-butane from Aug. 2004–Sept. 2005 were presented by Tanner et al.
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(2006). Here we combined these data to monthly whisker plots that show the minimum, 5, 25,
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50, 75, and 95 percentile, and the maximum values of measured mixing ratios during each month
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of available measurements (Fig. 1). Sinusoidal best fit curves were calculated from the (diurnal
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resolution) data and are included in these graphs to illustrate a smoothed seasonal NMHC cycle.
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These data show the typical Northern Hemisphere seasonal cycle of NMHC with lower mixing
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ratios in the summer and maximum values in late winter. This behavior to a large extent is
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driven by the annual concentration changes of the OH radical, which are closely linked to the
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latitudinal solar radiation cycle. High variability in NMHC mixing ratios was observed at any
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given time of year. It is noteworthy that all of these features show relations and dependencies
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upon the individual NMHC reactivity with OH and the resulting NMHC lifetime. The longest-
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lived NMHC, ethane, shows the relatively smallest amplitude between the mean winter and
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summer mixing ratios and the smallest relative variability on short (e.g. weeks) time scales. All
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of these features increase with increasing molecule size (respectively shorter OH lifetime). The
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seasonal maximum and minimum of ethane occurs the latest of all compounds (March 3 and
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September 3, respectively), as due to its slower OH reaction, ambient levels respond with a
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longer delay to the seasonal OH cycle. Heavier NMHC were found to maximize earlier, up to
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around January 20 for the most reactive compounds, and also had their seasonal minimum
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earlier, around July 18. These features in the Pico NMHC data are in agreement with data from a
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number of other sites, which, along with their seasonal OH dependencies, have been presented
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and discussed in detail in the literature (e.g. Jobson et al., 1994; Goldstein et al., 1995; Gautrois
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et al., 2003).
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Comparison of the NMHC >C2 for summer 2004 with data from the corresponding period during
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2005 shows a higher variability as well as overall higher mixing ratios during 2004.
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discussed in detail in other contributions to the ICARTT issue, the summer of 2004 was
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characterized by an unusually high occurrence of boreal wild fires in Northern Canada and
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Alaska, outflow of which was frequently observed at Pico (Val Martin et al., 2006). Substantial
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enhancements in NMHC, at times increasing to twice their seasonal background levels, were
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observed in these boreal fire plumes. Overall, during times when the station was impacted by
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boreal fire plumes (as defined in Val Martin et al., 2006), e.g. propane 25/50/75 percentile
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mixing ratios were 65/81/143 pptv, whereas outside of fire events, they were 25/51/87 pptv
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during the 2004 summer.
As
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A number of data sets have been presented in the literature that allow comparisons with the
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PICO-NARE measurements. We included two related data series in Figure 1, notably from two
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years of measurements at the continental, remote boreal site in Fraserdale, Ontario (50oN, 82oW)
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(Jobson et al., 1994) and from the Mauna Loa Observatory Photochemical Experiment-2 (19oN,
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155oW) (Greenberg et al., 1996), which, as shown below, bracket the Pico measurements and
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allow a further interpretation of the particular conditions encountered at Pico.
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Probably the most closely related study is the MLOPEX-2 NMHC measurements. Similar to
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PICO-NARE, the Mauna Loa site is a remote mountaintop island location, where, during
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downslope conditions, free tropospheric air is sampled that has traveled over the ocean for
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several days. MLO has a more prominent diurnal upslope-downslope cycle and data presented
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by Greenberg et al. were broken up into the occurrences of these two flow regimes.
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data in Figure 1 are the 25/50/75 percentiles for the time periods spanned by the width of the
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boxes.
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Upslope data for MLO typically were higher, with relative enhancements increasing with
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decreasing molecule liftetime. MLOPEX-2 data are consistently lower for all NMHC and during
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all seasons. The differences in Pico and MLO NMHC mixing ratios increases with decreasing
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lifetime, e.g. while ethane mixing ratios compare to within ~20%, n-butane values at MLO are
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more than 5 times lower than at Pico. In contrast to MLO, Fraserdale, a remote, low elevation
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continental forest site, experiences overall higher NMHC values than Pico. Again, differences in
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these two data series become more pronounced with molecular weight, although this time the
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PICO-NARE data are the ones becoming increasingly lower.
Included
The shown MLO data are from downslope (e.g. free tropospheric air) conditions.
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Higher NMHC levels at Fraserdale than at Pico, and higher NMHC levels at PICO-NARE than
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at MLO are likely due to several reasons. 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), with
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larger concentration changes being observed for shorter-lived compounds. Pico is higher than
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Fraserdale, and MLO is at about 1200 m higher altitude than the PICO-NARE station,
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consequently NMHC mixing ratios would be expected to be highest at Fraserdale, followed by
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PICO-NARE and MLO. This is simply a manifestation of aging, since the NMHC sources are
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primarily at the surface. Secondly, NMHC mixing ratios in the lower troposphere decrease
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towards lower latitude (Rudolph, 1995). Again, this dependency would infer higher NMHC
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levels at Fraserdale (50oN), followed by Pico (38oN) and MLO (19oN) This distribution reflects
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chemical oxidation since OH has a latitudinal gradient. Of further importance is the distance to
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the adjacent continents, which is about two times as much for MLO, and which will cause
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transport and photochemical processing times from continental sources to be longer, resulting in
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more depleted NMHC ratios at MLO compared to PICO-NARE. Further comparisons of the
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PICO-NARE data with other data sets from higher (than Pico) northern latitudes in Canada, the
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Atlantic Region and Europe (as summarized by Gautrois et al. (2003)) shows that Pico NMHC
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levels are without exception lower, both during the winter and in the summer compared to these
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locations.
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The cumulative distributions of NMHC during fall 2004 (September 22 to December 20), winter
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2004-2005 (December 21 to March 19), spring 2005 (March 20 to June 20) and summer 2005
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(June 21 to September 21) are shown in Figure 2. Data series that do not extend in the lower
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percentage range resulted from respective fractions of these data being reported below the
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instrument detection limit (for instance, i- and n-pentane were below the detection limit in about
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4% of the measurements during fall 2004, whereas during summer 2005, ~85% and 60% of
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chromatograms did not have i- and n-pentane peaks that were large enough to quantify). The
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regression line slopes through these individual data series indicate the variability of the
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atmospheric concentrations of a given compound. Steeper slopes are observed for longer-lived
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NMHCs (e.g. ethane) as these compounds have a higher atmospheric background concentration
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which reduces the relative variability caused by emission and aging influences. It is noteworthy
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that regression line slopes are lower for the summer, which can be attributed to the shorter
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seasonal atmospheric lifetime and resulting higher relative variabilities driven by a larger range
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in the degree of photochemical aging.
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Results for isoprene measured at the station were presented by Kleissl et al. (2006). Isoprene
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was not detected (< 1 pptv) in winter and nighttime samples.
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occasionally observed in samples collected during morning to evening hours. Occurrences and
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mixing ratios of isoprene increased towards late summer.
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detected on 60% of all days in the afternoon with maximum mixing ratios reaching up to 27
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pptv. The isoprene data clearly show seasonal and diurnal dependencies that are determined by
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both the expected seasonal changes in isoprene emission rates from vegetation growing at lower
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elevation on Pico Island, and by seasonal changes of frequency of buoyant and mechanical uplift
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flow that transports air from lower parts of Pico to the observatory (Kleissl et al., 2006). A
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correlation analysis between NMHC and isoprene in identified upslope events did not show any
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discernable increases of other NMHC in those samples. From that analysis it was concluded that
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emissions of other NMHC from island sources have a negligible influence on the NMHC
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composition in air sampled at the station and no further attempts were undertaken to filter the
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NMHC data for possible occurrences of upslope flow.
During spring, isoprene was
During August 2005, isoprene was
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3.2 NMHC Variability
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As mentioned above, the variability of NMHC can be used to characterize the importance of
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local emissions on the air composition at a given site. The variability of NMHC during each of
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the four seasons is reflected by the slopes of regression lines through the data in Figure 2, where
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high slope values are representative for low, and low slope values indicate high variability of
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NMHC mixing ratios within a given season. On that cumulative distribution plot, the inverse of
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the slope of the regression line is the standard deviation of the natural log of the data (lnx),
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assuming the data (including points falling below the detection limit) are log-normally
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distributed.
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The seasonal lifetime, of each NMHC can be determined from its OH reaction constant and
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the seasonal OH radical concentration. [OH] can be estimated at a 1-day resolution according to
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(Goldstein et al., 1995):
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[OH] = A [1-B cos(2*pi * t/365)],
(1)
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where A=1.6*106 and B=0.80. This relationship was derived from monthly average OH values
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(Spivakovsky et al., 2000) for 800 hPa, 36.0oN, 27.5oW. Here, daily OH concentrations from
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this equation were averaged to seasonal OH values within the defined time periods. Reaction
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rate constants were adjusted to the temperatures measured at the PICO-NARE station during the
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respective season. Please note that this local lifetime represents an estimate for the conditions at
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the receptor site; the actually encountered lifetime during transport of a NMHC to Pico may have
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deviated from this estimate dependent on the geographical and atmospheric conditions during the
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transport path. Also note that this analysis is not very sensitive towards the applied [OH], but
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more so towards the relative reactivity differences between individual compounds.
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Consequently, errors in the estimated, total [OH] will only have little effect on the results for the
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regression coefficients (Jobson et al., 1999). Figure 3 shows that lnx estimates from Figure 2 are
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well correlated with lifetime estimates for all four seasons. Regression line slopes, yielding lnx
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= A -b. for each seasonal data set are included in the figure. Seasonal differences in the b-values
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were not statistically significantly different at P > 95% for seasons where data of all C 2-C5
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NMHC were included. The best fit linear regression analysis through the data for all seasons
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yielded lnx = 1.60 -0.44.
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The exponent b in this equation has been noted to describe the importance of sink terms in the
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regional variability budget whereas the coefficient A can be related to the degree of
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photochemical aging; A-values have been used to derive estimates of transit times for different
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sample sets (Jobson et al., 1998).
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Interpretation of observed values for b from different sites has shown that b approaches 0 near
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urban areas, where the variability is strongly influenced by differences in the strength of local
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emission sources, whereas b-values close to 1 are found in stratospheric data sets, where the
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variability is low and dominated by chemical loss alone. The mean Pico value of 0.44 +/- 0.03.
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compares well with data from three aircraft data sets collected over other diverse remote areas,
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including the Arctic Boundary Layer (ABLE3A), the equatorial Atlantic (TRACE-A) and the
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western Pacific (PEM-West B) experiment, which resulted in b-values of 0.46–0.53 (Jobson et
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al., 1999). This comparison illustrates a rather high similarity between the continuous, seasonal
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Pico data and the results from the comparatively short aircraft campaigns that have been
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previously presented in the literature.
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3.3 Ratios of NMHC
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Correlation plots of all saturated C2-C5 NMHC as well as of these NMHC with CO are provided
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in the Appendix section to this paper. Results for the linear regression analyses of correlation
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plots are given in Table 1. The common feature in these data is that regression line slopes of
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(NMHCA/NMHCB with carbon number NMHCA < NMHCB) increase monotonically with
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increasing carbon number of NMHCB. This behavior is expected as the atmospheric mixing
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ratios (and lifetimes) of NMHC generally decrease with increasing carbon number. For an
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individual pair of NMHC, the regression line slopes become larger towards the summer, as the
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longer-chain NMHC are removed from the atmosphere faster than the more stable, shorter-chain
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NMHC. Regression coefficients generally decrease towards the summer, as shorter liftetimes,
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lower concentrations and higher relative variability cause the correlation between individual
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compounds to become weaker. In general, compounds with similar lifetimes generally show
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better correlations than compound pairs with much different lifetimes. However, the correlation
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between CO and ethane (which have very similar lifetimes) is notably weaker, likely because of
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their differences in primary and secondary sources.
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common source of both gases, CO is also a degradation product of hydrocarbons (mainly
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methane) in the atmosphere and natural gas production and distribution is a major source of
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ethane, but not for CO.
Although fossil fuel combustion is a
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The OH reaction rate constants of the isomeric pairs iso-butane and n-butane, and of iso-pentane
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and n-pentane are very similar; consequently, the atmospheric ratios of these two compound
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pairs is expected to change very little during transport and photochemical oxidation. Their
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correlations in the data from Pico, differentiated by the four seasons, as well as their ratio against
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the absolute levels of n-butane and iso-pentane, respectively, are shown in Figure 4. The tight
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correlation between these pairs of compounds is clear. Deviations and larger scatter at lower (e.g
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< 50 pptv) mixing ratios to some extent can be attributed to the loss of precision when mixing
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ratios approach the detection limit.
393
394
For the butanes, no statistically significant difference was found in the regression line slope
395
between the four seasonal data series. The regression line slope for all data was calculated to be
396
0.51 +/- 0.01 (R2 = 0.96). Similar values (range 0.37–0.55) have been reported in data from a
397
multitude of other sites in both continental and marine environments (e.g. Bottenheim and
398
Shepherd, 1995; Bottenheim et al., 1997; Greenberg et al., 1996; Parrish et al., 1998). The same
399
analysis yielded a slope of 0.69 +/- 0.08 (R2 = 0.93) for the n-pentane/i-pentane data. The graphs
400
on the right side of Figure 4 investigate possible changes in the oxidation chemistry of these
401
compound pairs by season as well as by their absolute concentrations. Other than for the
402
increase in scatter at lower mixing ratios, the butanes do not show any systematic seasonal
403
changes. The pentane plot is somewhat different, as higher n-pentane/iso-pentane ratios are
404
observed for the spring and summer data as well as during fall, when total mixing ratios are low.
405
This point is also visible in the cumulative distribution plots (Fig. 2) where, only for the summer
406
data, the i-pentane data distribution falls above the n-pentane values. This behavior points to
407
either different source region emission ratios or to different oxidation chemistry during the
13
408
summer months. Changes in n-pentane ratios have previously been investigated by several other
409
researchers. Similarly enhanced n-pentane/i-pentane ratios during summer months and in low-
410
concentration (well aged) samples were also evident in the Fraserdale data (Jobson et al., 1994)
411
as well as during ICARTT in the WP-3D data set (D. Parrish, unpublished data). Notably, the
412
n/i-pentane ratios in the Pico data overall seem to be higher than in the data collected closer to
413
the continental emission regions. This behavior is contrary to expected OH kinetics, as a relative
414
decrease of n-pentane/i-pentane would be expected during summer, due to the slightly higher n-
415
pentane OH reaction rate constant at 298 K (4.00 x 10-12 cm3 molecule-1s-1) compared to iso-
416
pentane (3.7 x 10-12 cm3 molecule-1s-1) (Atkinson, 1997), which should result in lower n/i-
417
pentane ratios in summer. These dependencies could conceivably point towards seasonally
418
changing competition between alternative destruction pathways, such as by the NO3 radical or by
419
chlorine chemistry (Penkett et al., 1993).
420
421
As a note of caution, it should be pointed out that our current interpretations of the summer
422
pentane data are somewhat limited. Even though the cumulative distribution plot shows n-
423
pentane data about twice as high as i-pentane, the pentane ratio plot shows the summertime value
424
to be around 1, with quite a substantial degree of scatter. For i-pentane, only 14% of the summer
425
data, and for n-pentane, only 41% of the summer data were above the detection limit. Only 11%
426
of the samples had both i- and n-pentane. So, any conclusions about these data are from the
427
highest 11% of concentrations observed during the summer. Also, it may be possible that at such
428
low concentrations (< 5 ppt) increasing sampling or measurements artifacts have to be taken into
429
account. The pentane observations and their preliminary interpretations presented here are
430
nonetheless a motivation for future, more thorough and accurate studies of pentane chemistry at
431
Pico.
432
433
The distribution of NMHC data in a double natural logarithm plot of [n-buante]/[ethane] versus
434
[propane]/[ethane] (Figure 5) can be used to investigate the degree of photochemical processing
435
that occurred in air reaching Pico. Data in these plots is distributed between two theoretical
436
limits that are determined by the assumptions that air with a common ratio of these compounds at
437
a source would have only been altered by OH photochemistry (kinetic line) or by dilution with
438
air that has zero concentrations of both compounds in the numerators of the ratios (dilution line).
14
439
The seasonal differences in the degree of NMHC oxidation are clearly visible in these data.
440
During winter, most data have larger ratios and are less variable, indicative of less photochemical
441
processing. In contrast, spring and summer data are more scattered (weaker R2 values), and the
442
lower [NMHC]/[ethane] ratios are indicative of the higher degree of air processing that occurred
443
during transport. The regression through all data yields a slope of 1.60 (±0.04), which is within
444
the range of slopes reported for this analysis from several other experiments (Parrish et al.,
445
2006).
446
447
3.4 NMHC Processing and Ozone
448
449
In general, understanding the temporal variability of tropospheric ozone at any particular
450
location is complex because several processes can have significant impacts, and these impacts
451
vary strongly on different time scales. In situ photochemical production and destruction proceed
452
at rates that vary with the ambient levels of ozone precursors and variables such as sunlight and
453
water vapor levels. Surface deposition and destruction by reaction with local emissions of NO or
454
reactive NMHC can drastically reduce near-surface ozone concentrations at rates that vary with
455
the characteristics of the planetary boundary layer and the flux of local emissions. Transport of
456
ozone to the site from the stratosphere or from upwind regions of strong photochemical
457
production can greatly increase ozone concentrations.
458
459
The PICO-NARE site is ideally situated to isolate the effects of the regional photochemical
460
production and destruction in the central North Atlantic from the effects of the other processes.
461
Kleissl et al. (2006) show that air sampled at the site is characteristic of the lower free
462
troposphere with essentially no opportunity for significant effects from surface deposition,
463
destruction by reaction with local emissions, or photochemical production from locally emitted
464
precursors. The varying influence of the transport of stratospheric ozone often dominates the
465
variability of ozone in free tropospheric data sets. Since ozone from the stratosphere has a steep,
466
negative correlation with CO (see e.g., (Danielsen et al., 1987)) the influence of stratospheric
467
ozone transport can be evaluated from the correlation of ozone with CO. Honrath et al. (2004)
468
discuss the ozone-CO correlation at PICO-NARE; their Figure 6 shows only a few scattered
469
points with such correlation (relatively high ozone at low CO). Consistent with other PICO-
15
470
NARE analyses (Honrath et. al., 2004; Lapina et al., 2006; Owen et al., 2006; and Val Martin et
471
al., 2006) the ozone variability is expected to mostly reflect the influence of the regional
472
photochemical production and destruction in the central North Atlantic.
473
474
The evolution of NMHC ratios through photochemical processing provides a means to
475
investigate the photochemical evolution of ozone (Parrish et al., 1992; 2004). Figure 6 shows the
476
dependence of ozone concentrations on the natural logarithm of [propane]/[ethane] as the
477
indicator of the photochemical processing in each season.
478
concentrations are relatively constant with no dependence on the NMHC ratios. In spring and
479
summer ozone has higher variability, both toward higher and lower concentrations.
480
relationships in Figure 6 indicate that higher ozone levels were consistently observed in air that
481
had relatively ‘fresh’ photochemical signatures (e.g. ln [propane]/[ethane] > -2.5), and that lower
482
ozone correlated with more processed air (i.e. ln [propane]/[ethane] < -2.5). These relationships
483
suggest that in spring and summer the highest ozone concentrations are observed when air
484
masses most recently transported from continental source regions impact the site, and lower
485
concentrations are observed in air masses that have been processed for longer times in the marine
486
troposphere. Evidently the photochemical environment of aged air masses in the central North
487
Atlantic is characterized by net photochemical destruction of ozone in spring and even more
488
strongly in summer.
During fall and winter ozone
The
489
490
Table 2 compares the springtime and summertime slope of the ozone - ln [propane]/[ethane]
491
relationship found at PICO-NARE with those reported from the north temperate Pacific marine
492
boundary layer (data for fall and winter were excluded because there was no correlation (Fig. 6)).
493
(Note that the same analysis was conducted for data that was filtered of suspected upslope
494
conditions (Kleissl et al., 2006). This analysis yielded regression line slopes that were within 5%
495
and not statistically different. This finding further confirms our above assumption that local
496
island emissions have little influence on PICO-NARE measurements of NMHC (and ozone).
497
Based on these results Parrish et al. (2004) argue that the recent Pacific studies (ITCT-2K2,
498
PHOBEA, TRACE-P) find evidence for only weak net ozone destruction (small positive slopes)
499
in the more remote Pacific marine boundary layer. This weak photochemical destruction is in
500
sharp contrast with the much stronger photochemical destruction indicated by a study at Point
16
501
Arena from nearly two decades earlier. The exception to this picture is the strong photochemical
502
production (large negative slope) in the PEM West-B study, which focused on the region of
503
strong outflow of ozone precursor emissions from Asia to the western North Pacific.
504
Comparison of the Pacific results to those from PICO-NARE suggest that spring- and
505
summertime photochemistry more effectively destroys ozone in the central North Atlantic than
506
in the North Pacific.
507
508
3.5 Transport Event Case Study
509
510
In Figure 7a six weeks of data for four NMHC during spring 2005 are shown (note the
511
logarithmic concentration scale). NMHC concentrations are highly variable, close to 10-fold
512
increases were observed several times during this observation window. The high correlation,
513
with concurrent minima and maxima of these four individual NMHC is noteworthy.
514
amplitudes of relative mixing ratio increases are highest for the shorter-lived compounds.
515
Underneath these variable data, the springtime decline in the NMHC mixing ratios can be
516
discerned. The ln [propane]/[ethane] and ln [butane]/[ethane] analysis for the same data (Figure
517
7b) can be used to investigate the short-term changes in the inferred photochemical age of air
518
reaching PICO-NARE. Again, a high variability is found, with high ln [NMHCi]/[ethane] ratios
519
(indicating ‘fresh’, e.g. little processed air) coinciding with periods of enhanced absolute NMHC
520
mixing ratios and low ln [NMHCi]/[ethane] ratios (indicating ‘old’, e.g. well processed air)
521
coinciding with periods of low absolute NMHC mixing ratios.
The
522
523
Three periods when air switched from a ‘fresh’ signature to an ‘old’ signature and back to ‘fresh’
524
were subjected to a closer investigation. These three, 1-2 day intervals are indicated by the
525
colored circles in Figure 7b. The corresponding data points are marked by the same colors and
526
compared with all data during this April-May period in the ln [butane]/[ethane] versus ln
527
[propane]/[ethane] plot in Figure 8a. This presentation shows that data from these three periods
528
center around different regions in this plot, which indicates distinct differences in photochemical
529
history of these air masses.
530
17
531
The FLEXPART retroplume results for contributions of enhanced CO at PICO-NARE from
532
North American emissions are shown in Figure 7c.
533
occurring concurrently with the timing of the NMHC (and ln [NMHC]/[ethane]) increases during
534
both the 4/17-4/19 and 4/21-4/23 periods. The later event, with the overall highest NMHC and
535
ln [NMHC]/[ethane] ratios (please note that the NMHC ratio plots are logarithmic data), is
536
paralleled by similarly overall higher CO enhancements. The identified minimum in the NMHC
537
data on 4/19-4/20 shows comparatively low CO contributions from North America. The derived
538
average CO ages shown on the right side y-axis are 7-8, >15 and 6-9 days for these three case
539
studies.
This analysis shows increases in CO
540
541
Parrish et al. (2006) show that approximate NMHC ratios can be calculated for a sampled air
542
parcel from the corresponding FLEXPART age spectrum for an estimated average [OH] if
543
emission ratios and reaction rate constants are known. Molar emission ratios were set to 0.63 for
544
propane:ethane and 0.35 for n-butane:ethane (which are the averages of results from Goldstein,
545
et al. (1995), and Swanson et al. (2003)), k = 0.18 x 10-12, 0.89 x 10-12 and 2.05 x 10-12 molecules
546
cm-3s-1 for ethane, propane and n-butane, respectively (Atkinson and Arey (2003) with T=273
547
K). The 24-hour [OH] was estimated as 0.8 x 106 molecules cm-3 to approximately match the
548
dynamic range of the NMHC ratios in Figure 8a. The NMHC ratios calculated for all age spectra
549
in Figure 7c are shown in Figure 8b with the points from the approximate time periods indicated
550
in Figure 7b similarly marked here. The age spectra are extrapolated to times earlier than the 20
551
days covered by the FLEXPART calculations in the manner described by Parrish et al. [2006].
552
553
Equation 3 of Parrish et al. (2006), using the above parameters, allows the assignment of an
554
approximate average photochemical age to each sampled air parcel from the n-butane/ethane
555
ratio. This photochemical age is expected to be an approximation of the length of time that the
556
sampled propane has been transported through the troposphere since emission. These times are
557
marked in the diamonds (in days) on the regression line of the data depicted in Figure 8b. Under
558
these assumptions, the data of the three episodes marked in Figure 7b are defined with ages of
559
about 20, 30-40, and about 10 days, respectively. Comparison of the NMHC ratios in the air
560
samples measured at PICO-NARE (Fig. 8a) with the FLEXPART derived values (Fig. 8b) shows
561
a qualitatively similar distribution pattern, however it is apparent that the calculated values in the
18
562
‘fresh’ air region are underestimated (too little aging) and that calculated values in the ‘old’ air
563
region are somewhat overestimated (too much aging).
564
565
Previous attempts of deriving photochemical transport times, or ‘photochemical clocks’ from
566
analysis of NMHC ratios have raised the question to what extent quantitative information can be
567
derived from this analysis. Trajectories and FLEXPART transport and footprint analyses were
568
used to further expand upon and interpret the information derived from the NMHC ratio analysis.
569
570
FLEXPART retroplumes for the three episodes marked in Figure 78b are shown in Figure 9. The
571
left hand column shows horizontal pathways taken by the plume, derived from total column
572
residence time of the plumes particles. The right hand column shows the source contribution,
573
which is the product of the residence time in the footprint layer (0-300 m) folded with emissions.
574
The source contribution map for the first episode (Figure 9b) indicates that the bulk of the
575
emissions originated over central Colorado about 6 days prior to arrival, with significant but
576
smaller contributions originating over the western US up to 8 days earlier. Figure 7c indicated
577
that the average age during the first event was 7-8 days. Interestingly, the retroplume pathway is
578
similar to typical warm conveyor belt transport, a mechanism that has been observed to transport
579
emissions from North America directly to the PICO-NARE station (Owen et al., 2006). Indeed,
580
there was a cold front located over this region of the U.S. on April 13-15, indicating frontal
581
transport was partially responsible for this episode. The transport pathway took the emissions
582
from the source region to high altitudes above regions with little emissions, which resulted in
583
little mixing with polluted air masses of different ages and composition, giving the emissions
584
observed at the station a fairly small range of CO ages.
585
586
In contrast, trajectory and FLEXPART results show that air sampled during 4/19-20 originated
587
over the southeastern Northern Pacific, traveled briefly over Mexico and the Gulf of Mexico,
588
before spending 10-12 days within the mid-Atlantic region, circulating around the Azores High
589
at relatively low altitudes, before arriving at the PICO-NARE station (Figure 9c). The CO time
590
series (Figure 7c) shows no CO present at the station less than 10 days old, with most falling into
591
the 15-20 day old bin. The stronger emission signal from southwestern Mexico (Figure 9d)
592
occurred in the 15-20 day window, while the signals over the eastern U.S. and the Caribbean
19
593
occurred in the 10-15 day window. The wide range of ages indicates significant mixing of
594
several polluted air masses as well as dilution with unpolluted marine air. The average age of the
595
CO in the resulting mixture, which only has a small modeled CO enhancement, ranges from 16-
596
19 days old.
597
598
Finally, air sampled during the 4/21-23 episode had passed over large portions of North America
599
before arriving at the PICO-NARE station (Figure 9e). The path for the retroplume is consistent
600
with another typical transport pathway to the station, export from the U.S. and subsequent
601
transport at relatively low levels in the westerly wind (Owen et al, 2006). The height of the
602
plume during transit over the U.S. was relatively low, generally less than 3 km, and remained
603
low during transport over the Atlantic to the station. The contribution map indicates that sources
604
across the eastern U.S. were responsible for the enhancements observed at this time (Figure 9f),
605
with a wide range of CO ages present. The resulting average age of CO for this episode ranges
606
from 6-9 days, though CO fell into a wide range of age bins, from 4-15 days old (Figure 7c).
607
Both the retroplumes and distribution of CO ages indicate the mixing of many air masses of
608
relatively fresh emissions (particularly compared to the air sampled during 4/19-20) and little
609
mixing with unpolluted marine air.
610
611
The comparison of the NMHC interpretations with the trajectories and FLEXPART results imply
612
that periods identified with ‘photochemically fresh’ air coincided with air transport over
613
populated, U.S. continental regions, where, most likely, an injection of recent anthropogenic
614
emissions had occurred. In contrast, the period that was identified as ‘photochemically old’ was
615
attributed to conditions where air had resided over the Atlantic Ocean for an extended (> 10
616
days) period of time.
617
618
Comparison of the NMHC data with average CO ages in the same events as well as comparison
619
with the trajectory analyses allows assigning transport times to observed NMHC ratios for the
620
photochemical conditions in the North Atlantic region during this April period. For instance,
621
ratios of ln [propane]/[ethane] ≈ -1.6 correspond to ~ 6-7 days of transport, while ln
622
[propane]/[ethane] ≈ -2.7 would correspond to a photochemical age of > 15 days. Due to the
623
faster photochemical oxidation of n-butane, the ln [n-butane]/[ethane] ratio is a somewhat more
20
624
sensitive scale, as here the ratio drops from -3.0 to -5.2 for the increase from 6-7 days to > 15
625
days.
626
627
The photochemical age classification derived from the calculated NMHC ratios, which were
628
based the CO age classes and assumptions in emission ratios of NMHC in source regions and
629
photochemical processing yielded transport estimates that generally were higher than the results
630
from the trajectory and CO age class analysis. These comparisons can be used to further
631
investigate potential improvements in this photochemical model and its input variables. Also,
632
better descriptions are needed to account for the varying degree of dilution (and spatially and
633
temporally variable background concentrations) and reaction history of air parcels during the
634
transport path.
635
636
4. Summary and Conclusions
637
638
Air sampled at PICO-NARE shows high variability in NMHC and their ratios during all times of
639
the year. This observation is indicative of the atmospheric transport conditions that bring air
640
with variable flow, origin and photochemical history to the station. Overall, concentrations of
641
NMHC at PICO-NARE are lower than at remote, higher northern latitude sites. In contrast,
642
NMHC mixing ratios at PICO-NARE are higher than at MLO). The observed NMHC levels at
643
PICO-NARE reflect the station’s latitude, elevation above sea level and the increased influence
644
of the adjacent continents to air composition in the central Atlantic region in comparison to the
645
Northern Mid-Pacific (MLO).
646
647
Short-chain NMHC remain elevated in air plumes that have been influenced by either
648
anthropogenic injections or biomass burning after time scales in excess of 1 week during their
649
transport to the PICO-NARE station. Isoprene data convincingly describe summertime (mostly
650
buoyant) upslope flow occurrences. Isoprene was found as the best of all chemical tracers to
651
identify upslope flow.
652
653
A good correlation was determined between seasonally differentiated NMHC variability and the
654
NMHC OH lifetimes. Regression analysis of the lnx=A-b relationship for these data yields a b-
21
655
value of -0.44, which confirms the remote island and free tropospheric character of the Pico site
656
and the lack of major local influences on NMHC levels.
657
658
Spring- and summertime ozone-(ln [propane]/[ethane]) correlations show higher variability,
659
indicating, as expected, more variability in photochemical conditions than during wintertime,
660
when photochemical processing becomes increasingly weaker the decline in available solar
661
radiation and with increasing latitude.
662
photochemical processing has reduced ln [propane]/[ethane] to values < -2.5; a condition that is
663
only observed during the spring and summer.
Net ozone destruction typically only occurs after
664
665
Qualitative agreement was found between derived relative photochemical ages of NMHC in air
666
plumes sampled at PICO-NARE and inferred ages of synoptic transport from potential NMHC
667
source regions. This approach of comparing NMHC data with trajectories and FLEXPART-
668
derived CO average age classification offers a possibility for calibrating a photochemical clock
669
scale that then can be applied to calculate transport times from observations of NMHC ratios.
670
671
Acknowledgments
672
673
We thank P. Goldan, NOAA Aeronomy Laboratory, Boulder, CO for the reference analysis of
674
the primary NMHC standard prior and after its use at Pico, M. Dziobak and M. Val Martin,
675
Michigan Technological University, for GC instrument maintenance tasks at the Pico, D.
676
Henriques, Institute of Meteorology, Ponta Delgada, Portugal for retrieving the ECMWF data
677
used in this work, Andreas Stohl, Norsk Institutt for Luftforskning (NILU), Kjeller, Norway for
678
providing and assisting in running the FLEXPART mode, the Data Support Section of NCAR’s
679
Scientific Computing Division for making the NCEP FNL analyses available for download and
680
T. Jobson, Washington State University, for the Fraserdale data. This research was funded by a
681
grant from the NOAA Office of Global Programs (award # NA03OAR4310072).
682
683
684
685
686
687
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25
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
___Pico
___Mauna Loa
___Fraserdale
i-butane
100
40
30
20
10
1000
400
300
200
jul aug sep oct nov dec jan feb mar apr may jun jul aug
summer '04
fall '04
winter '05
spring '05
summer '05
___Pico
___Mauna Loa
___Fraserdale
n-butane
100
40
30
20
10
4
3
2
4
3
2
1
1
jul aug sep oct nov dec jan feb mar apr may jun jul aug
summer '04
fall '04
winter '05
spring '05
summer '05
mixing ratio (pptv)
mixing ratio (pptv)
2000
jul aug sep oct nov dec jan feb mar apr may jun jul aug
summer '04
fall '04
winter '05
spring '05
summer '05
1
jul aug sep oct nov dec jan feb mar apr may jun jul aug
summer '04
fall '04
winter '05
spring '05
summer '05
26
1000
i-pentane
100
40
30
20
10
4
3
2
1
1000
400
300
200
mixing ratio (pptv)
mixing ratio (pptv)
400
300
200
___Pico
___Mauna Loa
___Fraserdale
___Pico
___Mauna Loa
___Fraserdale
n-pentane
100
40
30
20
10
4
3
2
jul aug sep oct nov dec jan feb mar apr may jun jul aug
summer '04
fall '04
winter '05
spring '05
summer '05
1
jul aug sep oct nov dec jan feb mar apr may jun jul aug
summer '04
fall '04
winter '05
spring '05
summer '05
Figure 1
Whisker plots of monthly data for ethane, propane, i-butane, n-butane, i-pentane and pentane. The 5, 25, 50, 75, 95 percentiles are
indicated by the horizontal lines of each box, the vertical lines extend to the minimum and maximum observed values. The width of
the box indicates the time period over which data was acquired for a given month. The vertical dotted lines show the windows that
were applied in the seasonal (winter, spring, summer, fall) analysis. Data from a remote boreal forest site in Canada (Jobson et al.,
1994) and 25, 50, and 75 percentile data from Mauna Loa Observatory (Greenberg et al., 1996) were added for comparison.
27
Fall '04
i-pentane
i-butane
n-butane
propane
y = 0.89x - 2.2 y = 1.3x - 4.4 y = 1.6x - 7.0 y = 1.8x - 9.7
R2 = 0.99
R2 = 0.99
R2 = 0.99
R2 = 0.99
Winter '05
ethane
y = 3.5x - 24
R2 = 0.99
n-pentane
y = 1.2x - 3.4
R2 = 0.99
99
99
95
90
95
90
Percent of distribution
Percent of distribution
n-pentane
y = 0.94x - 2.1
R2 = 0.98
80
70
50
30
20
10
5
1
ethane
i-butane
n-butane
propane
y = 1.7x - 6.9 y = 1.6x - 7.2 y = 2.5x - 12 y = 4.5x - 32
R2 = 0.99
R2 = 0.99
R2 = 0.99
R2 = 0.99
80
70
50
30
20
10
5
1
1
10
100
Mixing ratio (ppt)
1
1000
n-pentane
y = 0.93x - 1.2
R2 = 0.98
10
100
Mixing ratio (ppt)
1000
Summer '05
Spring '05
ethane
n-pentane
n-butane
propane
i-butane
y = 0.97x - 1.7 y = 1.4x - 4.7 y = 2.7x - 17
y
=
0.96x
+
0.02
y
=
0.80x
+
0.13
y = 0.77x + 0.76
R2 = 0.99
R2 = 0.99
R2 = 0.99
R2 = 0.99
R2 = 0.96
R2 = 0.95
i-pentane
ethane
i-butane
n-butane
propane
y = 3.2x - 32
y = 0.82x - 1.0 y = 0.84x - 1.9 y = 1.4x - 3.5 y = 1.2x - 6.0
R2 = 0.98
R2 = 0.96
R2 = 0.99
R2 = 0.95
R2 = 0.98
i-pentane
99
99
95
90
95
90
Percent of distribution
Percent of distribution
i-pentane
y = 1.2x - 3.9
R2 = 0.98
80
70
50
30
20
10
5
1
80
70
50
30
20
10
5
1
1
10
100
Mixing ratio (ppt)
1000
1
10
100
1000
Mixing ratio (ppt)
Figure 2
Cumulative distributions of NMHC at the PICO-NARE station during the four measurement seasons.
28
2
1
lnx
y=2.3x-0.60 (R2=0.93)
y=1.7x-0.43 (R2=0.92)
y=1.6x-0.43 (R2=0.99)
y=1.4x-0.44 (R2=0.93)
y=1.4x-0.43 (R2=0.99)
Summer '04
Fall '04
Winter '05
Spring '05
Summer '05
0.6
0.5
y = 1.60x-0.44
R2 = 0.91
0.4
ethane
propane
i-butane
n-butane
i-pentane
n-pentane
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 the NMHC mixing ratio during the four seasons at its seasonal OH lifetime.
29
2
200
40
30
i-butane / n-butane
i-butane (pptv)
1
fall 04
winter 05
spring 05
summer 05
100
20
10
0.4
0.3
0.2
fall 04
winter 05
spring 05
summer 05
0.1
4
3
2
0.04
0.03
1
3
4
10
20
30 40
n-butane (pptv)
100
200
300 400
3
10
20
30 40
n-butane (pptv)
100
200
300 400
4
100
Fall '04
Winter '05
Spring '05
Summer '05
40
Fall '04
Winter '05
Spring '05
Summer '05
3
2
n-pentane/i-pentane
30
n-pentane (pptv)
4
20
10
4
3
1
0.4
2
0.3
1
1
2
3
4
10
20
i-pentane (pptv)
30 40
100
200
0.2
1
2
3
4
10
20
30 40
i-pentane (pptv)
100
200
Figure 4
Mixing ratio of i-butane versus n-butane (left) and ratio of i-butane/n-butane versus n-butane
(right) in the upper graphs and of n-pentane versus i-pentane (left) and ratio of n-pentane/ipentane versus i-pentane (right) in the lower graphs The error bars in the right graph show the
standard deviation of the data within 10-percentile bins of the data distribution. Dotted lines
illustrate the estimated uncertainties in the measurement above the mean ratio of all data.
30
0
0
slope=1.58±0.15
slope=1.58±0.07
winter 2005 r2=0.91
-1
-1
-2
-2
ln(butane/ethane)
ln(butane/ethane)
fall 2004 r2=0.82
-3
-4
-3
-4
-5
-5
-6
-6
kinetic slope = 2.61
kinetic slope = 2.61
-7
-7
-5
-4
-3
-2
-1
0
-5
-4
ln(propane/ethane)
0
0
slope=1.61±0.11
-2
-1
0
-1
0
slope=1.40±0.18
spring 2005 r2=0.72
summer 2005 r2=0.55
-1
-1
-2
-2
ln(butane/ethane)
ln(butane/ethane)
-3
ln(propane/ethane)
-3
-4
-5
-3
-4
-5
-6
-6
kinetic slope = 2.61
kinetic slope = 2.61
-7
-7
-5
-4
-3
-2
ln(propane/ethane)
-1
0
-5
-4
-3
-2
ln(propane/ethane)
Figure 5
Relationship between the natural logarithms of [n-buante]/[ethane] versus [propane]/[ethane] for
the fall, summer, spring and summer data as defined in the text.
31
100
90
80
70
60
fall
50
50
40
40
O3 (ppbv)
O3 (ppbv)
100
90
80
70
60
30
20
10
-5.0
30
20
-4.0
-3.0
-2.0
ln(propane/ethane)
-1.0
10
-5.0
0.0
100
90
80
70
60
slope = 0.72±0.07 R2=0.43
spring
50
50
40
40
O3 (ppbv)
O3 (ppbv)
100
90
80
70
60
30
20
10
-5.0
winter
-4.0
summer
-3.0
-2.0
ln(propane/ethane)
-1.0
0.0
slope = 1.02±0.12 R2=0.22
30
20
-4.0
-3.0
-2.0
ln(propane/ethane)
-1.0
0.0
10
-5.0
-4.0
-3.0
-2.0
ln(propane/ethane)
-1.0
0.0
Figure 6
Ozone in relation to the natural logarithm of [propane]/[ethane] (indicating degree of
photochemical processing) during the fall, winter, spring and summer. The lines indicate the
linear least-squares fits to the log-transformed data, and their slopes with 95% confidence limits
and correlation coefficients are annotated. Remove regression line in A and B
32
ethane
propane
n-butane
n-pentane
10000
mixing ratio (pptv)
1000
100
10
1
Mar-27
Apr-3
Apr-10
Apr-17
Apr-24
May-1
May-8
May-15
A
0
4/17 16:00 to
4/19 1:00
4/21 5:00 to
4/23 15:30
ln(propane/ethane)
ln(n-butane/ethane)
-1
4/19 17:00 to
4/20 15:30
ln([HC]/[ethane])
-2
-3
-4
-5
-6
Mar-27
Apr-3
Apr-10
Apr-17
Apr-24
May-1
May-8
May-15
B
33
60
25
3
4
5
6
8
10
15
20
mean age
50
CO (ppbv)
40
15
30
10
20
5
10
0
Mar-27
Mean CO Age (days)
20
0
Apr-3
Apr-10
Apr-17
Apr-24
May-1
May-8
May-15
C
Figure 7
Mixing ratios of four NMHC (A) and the natural logarithms of [propane]/[ethane] and
[butane]/[ethane] for 21 days in spring 2005 (B). Panel (C) shows for the same time window the
Flexpart calculations for the enhancement of CO at Pico from contributions of emissions of CO
over North America. Contributions to the overall enhancement are broken up in age groups of
integrated 0-3, 0-4, 0-5, 0-6, 0-8, 0-10, 0-15 and 0-20 days transport classes. The derived
average CO enhancement transport time is shown on the secondary y-axis.
34
0
3/27 - 5/16
slope = 1.85 ± 0.03
r2 = 0.82
-1
ln(butane/ethane)
-2
-3
-4
-5
-6
kinetic slope = 2.61
-7
-5
-4
-3
-2
-1
0
ln(propane/ethane)
Figure 8a and b
Distribution of the marked data points in Fig. 8B in the ln-ln photochemical age plot in comparison to all spring 2005 data (left hand
graph) B) ln-ln photochemical age plot calculated from the FLEXPART age spectra plotted in Figure 8C. The colored points indicate
approximately the same time periods indicated in Figure 8B.
35
Figure 9
Results for retroplumes initiated at 00-03 on April 18 (A&B), April 20 (C&D) and April 22
(E&F). The left column shows the total column (0-10km) residence times. The right column
shows the foot print layer (0-300m) response to emissions sources (residence time folded with
emission strength). Colors are logarimithically scaled (100-1%) according to the maximum value
for each plot type (14000 seconds for total residence time, 4700 grams of CO for foot print layer
response), as shown by the scale on the bottom.
36
Table 1
Correlation between NMHC with regression line slopes (m), intercept (b) and regression coefficient (r2) during fall (F), winter (W),
spring (SP) and summer (S).
CO
m
b
r2
ethane
m
b
r2
propane m
b
r2
i-butane m
b
r2
n-butane m
b
r2
i-pentane m
b
r2
n-pentane m
b
r2
CO
F
W
SP
SU
1
0
1
14.6
1.91
0.378
8.044
0.979
0.413
2.368
0.218
0.551
4.081
0.414
0.503
1.903
0.151
0.623
1.186
0.098
0.604
1
0
1
11.33
0.579
0.608
4.095
0.348
0.359
1.075
0.076
0.446
2.119
0.164
0.405
0.882
0.068
0.407
0.543
0.048
0.339
1
0
1
18.3
0.797
0.624
5.47
0.252
0.597
0.78
0.038
0.576
1.576
0.086
0.514
0.415
0.023
0.5
0.312
0.017
0.506
1
0
1
7.35
0.703
0.256
1.636
0.126
0.347
0.093
0.014
0.13
0.3
0.041
0.144
0.03
0.005
0.087
-0.03
0.018
0.007
ethane
F
W
0.026 0.054
0.003 0.003
0.378 0.608
1
1
0
0
1
1
0.405 0.409
0.03 0.016
0.626 0.724
0.088 0.101
0.009 0.003
0.455 0.788
0.176 0.204
0.015 0.007
0.555 0.759
0.072 0.083
0.007 0.003
0.52 0.71
0.046 0.052
0.004 0.003
0.539 0.614
SP
0.034
0.001
0.624
1
0
1
0.286
0.006
0.873
0.039
0.001
0.775
0.078
0.003
0.658
0.021
8E-04
0.651
0.015
6E-04
0.633
SU
0.035
0.003
0.256
1
0
1
0.11
0.009
0.33
0.006
9E-04
0.099
-0
0.003
2E-04
0.001
4E-04
0.028
0.002
0.001
0.01
propane
F
W
0.051 0.088
0.006 0.007
0.413 0.359
1.546 1.772
0.115 0.069
0.626 0.724
1
1
0
0
1
1
0.227 0.229
0.011 0.004
0.79 0.941
0.433 0.48
0.015 0.005
0.879 0.972
0.169 0.192
0.009 0.004
0.76 0.883
0.109 0.125
0.005 0.004
0.816 0.823
SP
0.109
0.005
0.597
3.05
0.064
0.873
1
0
1
0.142
0.002
0.945
0.287
0.007
0.843
0.074
0.002
0.799
0.057
0.001
0.824
SU
0.212
0.016
0.347
3.005
0.239
0.33
1
0
1
0.078
0.003
0.695
0.108
0.015
0.144
0.009
0.002
0.063
0.013
0.006
0.013
i-butane
F
W
0.233 0.415
0.021 0.029
0.551 0.446
5.152 7.825
0.542 0.257
0.455 0.788
3.474 4.107
0.173 0.065
0.79 0.941
1
1
0
0
1
1
1.723 2.038
0.052 0.019
0.91 0.978
0.742 0.843
0.015 0.012
0.957 0.95
0.459 0.541
0.011 0.014
0.94 0.86
SP
0.738
0.036
0.576
19.72
0.583
0.775
6.672
0.088
0.945
1
0
1
2.034
0.038
0.897
0.538
0.01
0.894
0.408
0.008
0.892
SU
1.389
0.202
0.13
17.63
2.969
0.099
8.927
0.33
0.695
1
0
1
1.166
0.157
0.146
0.118
0.021
0.092
0.07
0.068
0.003
n-butane
F
W
0.123 0.191
0.013 0.015
0.503 0.405
3.15 3.727
0.271 0.133
0.555 0.759
2.029 2.025
0.072 0.022
0.879 0.972
0.528 0.48
0.016 0.005
0.91 0.978
1
1
0
0
1
1
0.398 0.407
0.013 0.006
0.9 0.942
0.254 0.265
0.006 0.006
0.938 0.876
SP
0.326
0.018
0.514
8.46
0.334
0.658
2.934
0.069
0.843
0.441
0.008
0.897
1
0
1
0.241
0.006
0.828
0.188
0.004
0.877
SU
0.48
0.066
0.144
-0.23
1.025
2E-04
1.333
0.181
0.144
0.125
0.017
0.146
1
0
1
0.027
0.007
0.045
0.05
0.022
0.015
i-pentane
F
W
0.327 0.462
0.026 0.035
0.623 0.407
7.261 8.59
0.671 0.348
0.52 0.71
4.492 4.602
0.243 0.106
0.76 0.883
1.289 1.127
0.026 0.016
0.957 0.95
2.259 2.313
0.072 0.036
0.9 0.942
1
1
0
0
1
1
0.613 0.646
0.011 0.012
0.966 0.918
SP
1.204
0.068
0.5
31.73
1.274
0.651
10.77
0.296
0.799
1.66
0.031
0.894
3.431
0.086
0.828
1
0
1
0.723
0.013
0.909
SU
2.917
0.53
0.087
23.89
7.914
0.028
6.926
1.485
0.063
0.78
0.136
0.092
1.67
0.427
0.045
1
0
1
0.268
0.175
0.007
n-pentane
F
W
0.51 0.625
0.042 0.055
0.604 0.339
11.85 11.84
1.054 0.596
0.539 0.614
7.462 6.589
0.341 0.193
0.816 0.823
2.048 1.59
0.05 0.041
0.94 0.86
3.697 3.307
0.091 0.079
0.938 0.876
1.575 1.421
0.028 0.027
0.966 0.918
1
1
0
0
1
1
SP
1.624
0.09
0.506
41.24
1.722
0.633
14.42
0.365
0.824
2.186
0.042
0.892
4.655
0.095
0.877
1.257
0.022
0.909
1
0
1
SU
-0.26
0.175
0.007
4.656
2.533
0.01
0.992
0.483
0.013
0.046
0.045
0.003
0.309
0.138
0.015
0.027
0.018
0.007
1
0
1
37
Table 2
Ozone relationships to ln[propane]/[ethane] in the marine troposphere in spring at northern temperate latitudes.
Location
Point Arena, Californiaa
PEM West-B – Asian outflow in Western N. Pacifica
ITCT-2K2 – Eastern N. Pacifica
PHOBEA – Eastern N. Pacifica
TRACE-P – West to central N. Pacifica
PICO-NAREb
Year
1984
1994
2002
1997-2002
2001
2005 spring
2005 summer
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
38
Appendix
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)
39
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)
40
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)
41
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)
42
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)
43
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)
44
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)
45
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)
46
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)
47
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)
48
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)
49