Temporal patterns, sources, and sinks of C8-C16

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. D19, 8099, doi:10.1029/2000JD000232, 2002

Temporal patterns, sources, and sinks of C8-C16 hydrocarbons in the atmosphere of Mace Head, Ireland J. H. Sartin, C. J. Halsall, L. A. Robertson, R. G. Gonard, and A. R. MacKenzie Department of Environmental Science, Institute of Environmental and Natural Sciences, Lancaster University, Lancaster, UK

H. Berresheim Deutscher Wetterdienst, Meteorologisches Observatorium, Hohenpeissenberg, Germany

C. N. Hewitt Department of Environmental Science, Institute of Environmental and Natural Sciences, Lancaster University, Lancaster, UK Received 7 December 2000; revised 8 May 2001; accepted 15 May 2001; published 4 September 2002.

[1] During the 1999 New Particle Formation and Fate in the Coastal Environment (PARFORCE) field campaign, 16 C8-C16 volatile organic compounds (VOCs) were identified in the coastal atmosphere of Mace Head, Ireland. Sampling took place over 24 days, with 12 VOCs routinely quantified. Concentrations were observed in the low ⬍10 – 150 parts per trillion by volume range, with levels typically in the order of aldehydes ⬎ ketones ⱖ n-alkanes. Concentrations of these compounds were also measured in shoreline surface seawater. No relationship was observed between atmospheric concentrations and high/low tide events. Many VOCs revealed a temporal pattern in the atmosphere, with highest concentrations measured during the early morning and lowest concentrations in the afternoon. The strongest pattern was observed for the n-alkanes. However, this was dependent on the prevailing air mass direction and the local meteorology. A Lagrangian box model was applied to assess this diurnal cycle, using seawater emissions as a source (based on the seawater concentrations and observed wind speeds), and depletion via OH radicals and dilution by entrainment as sinks (using measured [OH] and boundary layer height data). The model gave good agreement to the observed concentrations for selected air mass types, predicting the daytime decrease in VOC concentrations due to OH radical chemistry and boundary layer growth, and the subsequent increase in VOC concentrations toward evening as both oxidation chemistry diminished and the mixing layer height fell. INDEX TERMS: 0312 Atmospheric Composition and Structure: Air/sea constituent fluxes (3339, 4504); 3339 Meteorology and Atmospheric Dynamics: Ocean/atmosphere interactions (0312, 4504); 4820 Oceanography: Biological and Chemical: Gases; KEYWORDS: Mace Head, n-alkanes, oxygenates, marine boundary layer (MBL), sea water emissions, hydroxl radical. Citation: Sartin, J. H., C. J. Halsall, L. A. Robertson, R. G. Gonard, A. R. MacKenzie, H. Berresheim, and C. N. Hewitt, Temporal patterns, sources, and sinks of C8-C16 hydrocarbons in the atmosphere of Mace Head, Ireland, J. Geophys. Res., 107(D19), 8099, doi:10.1029/2000JD000232, 2002.

1.

Introduction

[2] There is growing interest in the occurrence of biogenic volatile organic compounds (VOCs) and semivolatile VOCs (SVOCs) in the marine atmosphere [Baker et al., 2000; McKay et al., 1996; Ratte et al., 1998; Singh et al., 2001]. These compounds contribute to the mass of reactive hydrocarbons present in the troposphere, yet knowledge on their oceanic and 1 Also at Deutscher Wetterdienst, Meteorologisches Observatorium, Hohenpeissenberg, Germany.

Copyright 2002 by the American Geophysical Union. 0148-0227/02/2000JD000232$09.00

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coastal sources is scarce. Helmig et al. [1996] recently confirmed the presence of over 80 organic compounds ranging from C2-C15 in free tropospheric air at Mauna Loa, Hawaii. Sources for these compounds were attributed to both local release and long-range atmospheric transport. Interestingly, oxygenates, such as the straight chain n-aldehydes, were consistently observed in the majority of samples, suggesting that the marine environment is an important source of reactive hydrocarbons. [3] As part of the New Particle Formation and Fate in the Coastal Environment (PARFORCE) project, the presence of VOCs in the coastal atmosphere at Mace Head was investigated during two separate campaigns in September 1998 and

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June 1999. The objectives of PARFORCE were to examine the particle “bursts” previously observed at Mace Head during daylight hours [Allen et al., 1999; O’Dowd et al., 1998, 1999]. There is a possibility that biota exposed in the intertidal zone might release VOCs/SVOCs, resulting in condensable vapors which contribute to particle formation in the boundary layer. These condensable compounds are either released directly or formed through atmospheric reactions. Initially, emphasis was placed on the terpenoid class of compounds (monoterpenes (C10) and sesquiterpenes (C15)), as they are known to be released from terrestrial vegetation [Mariaca et al., 1997; Owen et al., 1997] and are involved in both photooxidant chemistry and secondary aerosol formation [Hoffmann et al., 1997; Kamens et al., 1999]. However, terpenes were not found at detectable levels in the first PARFORCE campaign [Sartin et al., 2001]. Instead, a range of C8-C16 VOCs were subsequently targeted. An overview of the complete PARFORCE project is included in this special section [O’Dowd et al., 2002a]. [4] Prior to the 1998 campaign, only C2-C6 nonmethane hydrocarbons (NMHCs) had been investigated in any detail at Mace Head [Lewis et al., 1997], while any previous work on heavier hydrocarbons (ⱖC7) had been limited to several much earlier open-ocean studies [e.g., Eichmann et al., 1979, 1980]. This paper reports temporal data for C8-C16 VOCs related to regional air mass origin measured during the June 1999 campaign. In addition, sources and chemistry that affect VOC concentrations are investigated. Shoreline seawater was also analyzed to assess its role as a possible source of these compounds.

2.

Methodology

[5] Atmospheric samples were collected systematically for the 24 days between June 5 and 28, 1999 (Julian day 156 –179). Over the same period, intermittent seawater samples were taken during daylight low and high tides. Other PARFORCE participants conducted meteorological observations and further physical-chemical measurements. The results were stored at a central database, located at http://macehead.physics. nuigalway.ie/parforce/. Data utilized for this paper included measurements of the marine boundary layer (MBL) height, wind speeds, and OH radical concentrations. 2.1.

Atmospheric Sampling

[6] Air samples were typically taken every hour and collected in stainless steel cartridges (6.1 mm OD, 90 mm length, Perkin Elmer) containing two separate adsorbents, Tenax TA (200 mg) and Carbotrap (100 mg). A manual collection system was employed, which allowed sampling during daylight hours coupled with more intensive measurements during periods of low tide. Continuous data for three 24-hour periods were obtained during the campaign. For each sample a total of 5 L of air was pumped through each tube at a rate of 250 mL min⫺1 (i.e., 20 min sampling time). Air was initially drawn through a Teflon line (5 mm ID) fitted with an ozone scrubber (constructed from MnO2-coated copper gauze). After the initial setup, the Teflon line was flushed for 24 hours and, from then on, at least half an hour before each sample. Further details of procedure and quality controls are presented by Sartin et al. [2001]. 2.2.

Seawater Sampling

[7] Surface seawater samples (n ⫽ 25) were manually collected from the shore using 2.5–5.0 L amber glass vessels.

VOC extraction was conducted using the two methods outlined below, resulting in optimization of recoveries for compounds covering a relatively wide range of physical-chemical properties. For both methods, sample extraction was carried out less than 15 min after collection. 2.2.1. Purge and Trap [8] A purge and trap or sparging system was utilized to strip dissolved VOCs from the seawater by bubbling N2 gas through a 10 L reservoir. This procedure has frequently been used to analyze dissolved VOC contaminants [e.g., Rabideau et al., 1999]. The methodology in this paper was based on the work of Chao et al. [1998], and utilized Tenax TA/Carbotrap cartridges, as used in the atmospheric measurements, to trap the VOCs in the N2 gas stream. 2.2.2. Liquid/Liquid Extraction [9] In conjunction with the purge and trap system, a liquid/ liquid microextraction technique was employed to extract the higher molecular mass organics (⬎C10) not efficiently sparged. The technique was based on that of Junk et al. [1981] to measure trace VOC contaminants in wastewater. Principles behind microextraction are given by Jeannot and Cantwell [1996]. Briefly, the procedure involved adding 2 mL of n-pentane to 100 mL seawater in a volumetric flask. This mixture was vigorously stirred for 15 min. The solution was then left to stand for 30 s before transferring a 1 mL aliquot of the separated pentane to a 2 mL vial, which was subsequently capped and stored at 4⬚C prior to analysis. 2.3.

Sample Analysis

[10] Adsorbent cartridges used for the atmospheric sampling and sparging system were desorbed with a Perkin-Elmer Automated Thermal Desorption unit (ATD-400). ATD analytical conditions were reported by Sartin et al. [2001]. The desorbed analytes were carried via helium flow through a heated transfer line to either a gas chromatographic (GC)mass spectrometer (MS) or GC-flame ionization detector (FID). Both the GC-MS and GC-FID systems operated with a 50 m Ultra 2 (5% phenyl) capillary column and an identical temperature program. The initial temperature was 48⬚C for 1 min, then to 160⬚C at 4⬚C min⫺1, 160 to 300⬚C at 35⬚C min⫺1, and then held at 300⬚C for 10 min. Analysis of the n-pentane seawater extracts was carried out under conditions similar to the air samples. The sample extracts were injected (2 ␮L) into the GC via a split/splitless capillary inlet. [11] Compound identification was based on mass spectral comparison with authentic standards. Confirmation was also aided by the use of a calculated retention index (RI). One unknown compound, which occurred routinely in the air samples, was tentatively identified using the Wiley Mass Spectral library. Full details of RIs and compound quantification for the air samples are presented by Sartin et al. [2001]. For the npentane seawater extracts, quantification was carried out using hexa-methyl benzene (HMB) as an internal standard. Calibration standards (n-pentane keeper solvent) containing the majority of the C8-C16 compounds identified in the air samples were run with each batch of samples. 2.4.

Quality Controls

[12] Standards and blanks were run with every six samples, resulting in detection limits for the air samples in the order of low parts per trillion by volume (pptv). For seawater analysis,

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Table 1. VOCs Routinely Identified in the Atmosphere of Mace Head, Ireland

Compound

GC-FID Retention Time, min

Arithmatic Mean (Median) n⫽221, pptv

Range, pptv

n-Nonane 6-Methy-5-hepten-2-one n-Decane Octanal 2-Ethyl-1-hexanol n-Undecane Nonanal n-Dodecane Decanal n-Tridecane 3-Hydroxy-2,4,4-trimethylpentyl-ester-2-methyl-propanoic acid n-Tetradecane Geranyl acetone 2,6-Di-tert-butyl-p-benzoquinone n-Pentadecane n-Hexadecane

8.85 11.23 11.54 11.64 12.63 14.86 15.04 18.40 18.64 21.94 24.60 25.37 27.21 27.75 28.55 30.49

29 (26) – – – 66 (58) 5 (4) 37 (32) 3 (2) 44 (36) 3 (2) 3 (2) 5 (2) 8 (6) 6 (2) 4 (2) –

⬍2–126 – – – ⬍2–427 ⬍2–53 3–132 ⬍2–15 3–183 ⬍2–25 ⬍2–18 ⬍2–72 ⬍2–92 ⬍2–77 ⬍2–28 –

detection limits ranged from 0.15– 0.30 ␮g L⫺1. Full details of quality control procedures, particularly with regard to reducing contamination artefacts, are given by Sartin et al. [2001]. Validation of both the sparging and the liquid/liquid extraction methods was carried out prior to the field campaign. This involved spiking the working standard into pure water (MilliQ) and undergoing the relevant extraction and analytical procedures. Mean recoveries for the sparging system (n ⫽ 3) ranged from ⬎80% (C8) (i.e., 2-ethyl-1-hexanol) to ⬍30% for (C15) (i.e., n-pentadecane). For the liquid extractions using npentane, recoveries ranged from 54 –108%, the lower recoveries occurring for the more polar oxygenated species. Sparging times and mixing times for the two techniques were established to achieve maximum recoveries for compounds in the standard [Sartin et al., 2000]. Concentrations of C8-C10 VOCs in seawater are reported from the sparging technique, whereas concentrations of ⬎C10 VOCs are based on the n-pentane extraction. Good agreement was obtained between the two techniques for n-decane spiked into artificial seawater. However, recoveries of n-decane from seawater samples were higher (ⱖ25%) for the n-pentane extraction, presumably due to the presence of suspended particulate matter (with associated organics) that was not filtered out of the samples prior to extraction. Owing to the small volumes of seawater extracted with pentane, every group of five extracts was bulked to obtain detectable concentrations.

3. 3.1.

Results and Discussion Atmospheric VOCs

[13] Table 1 presents the VOCs routinely identified in the Mace Head atmosphere, including their chromatographic retention times and range of concentrations. Of the 16 compounds identified, it was only possible to quantify 12 routinely. This was due to both coelution problems and poor peak definition for several of the early eluting, more polar VOCs (i.e., 6-methyl-5-hepten-2-one (6MHO) and octanal). In addition, partial coelution of n-hexadecane with an unknown compound prevented quantification of this n-alkane. Precision estimates for the data presented in Table 1 were calculated as a 2 s.d. error of 15% and a method detection limit (MDL) of 2 pptv. This MDL was an improvement on that of Sartin et al. [2001], due to a refined experimental technique and greater sample volumes.

3.2.

Temporal Trends of Atmospheric VOCs

[14] Atmospheric VOC samples were taken continuously during daylight hours for a period of 24 days (JD 156 –179). Observed particle formation events, largely dependent on regional air mass direction, were classified into three “types” [O’Dowd et al., 2002b]. To put this VOC study into context, a brief summary of these PARFORCE event types will be presented below, with respective air mass trajectories illustrated in Figure 1. The two broad categories of VOC, n-alkanes and oxygenates (see Table 1), will then be described with respect to these classifications. Expected atmospheric lifetimes of the nalkanes and oxygenates, due to reaction with OH, are 10 –18 hours and 1–2 hours, respectively (12-hour daytime average OH radical concentration of 1.0 ⫻ 106 molecule cm⫺3) [Aschmann and Atkinson, 1995; Fruekilde et al., 1998]. 1. Type I events occurred at low tide and had only one coastal source region, with winds coming directly off the Atlantic ocean (Figure 1a). The particles observed were very small (Dp ⬍10 nm). 2. Type II events occurred when the wind direction was from north to NW, the air mass having traveled over multiple tidal regions (Figures 1b and 1c). The particles observed were ⱖ10 nm, although the lower size ranges typical of a type I episode were observed as well. 3. Type III events were dominated by land sources, with wind directions from the east to SE (Figure 1d). Most of the particles observed were ⱖ5 nm diameter. 3.2.1. n-Alkanes [15] Concentrations of n-alkane (C9, C11-C15) over the first 8 days of the campaign (JD 156-163) are presented in Figure 2. n-Nonane, the lightest n-alkane of this series, consistently displayed the highest concentrations (see Table 1). However, levels of the two heavier compounds, n-tetradecane and npentadecane, both exceeded 20 pptv during the first 3 days. Continuous 24-hour sampling occurred over JD 161–162 produced evidence of a nighttime increase in concentrations. For the majority of the n-alkanes this was only observed at low concentrations close to or within the nominal MDL. However, further support for nighttime highs was also observed earlier in the week (JD 156 –159), where significant morning concentrations suggest comparable nocturnal levels. This was evident in

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Figure 1. Air mass trajectories during four contrasting 2-day periods at Mace Head (MH): (a) JD 178 –179, (b) JD 157–158, (c) JD 160 –161, and (d) JD 175–176.

the 24-day results for n-tetradecane (C14) and n-pentadecane (C15) presented in Figure 3, where notable levels indicate comparable concentrations are also likely to have occurred the preceding night. Figure 3 also reports the individual day “type” classifications. [16] None of the quantified VOCs showed a dependence on tidal cycles, and were therefore considered unlikely to play a direct role in the particle formation process. In addition, parameters that may have played minor roles in the production and loss of compounds, such as windspeed, relative humidity, air temperature, and sea temperature, presented no obvious correlation with these VOCs. However, as noted above, a diurnal pattern for these compounds was apparent (Figures 2 and 3), with the highest concentrations occurring during the early morning, followed by a subsequent decline throughout the day. These results suggested a diurnal cycle, primarily driven by photochemistry and dilution through atmospheric entrainment. The lighter n-alkanes (C9-C12) showed evidence of this diurnal pattern, but the strength of the pattern became more apparent with increasing mass (C13-C15 n-alkanes). This

is clear in Figure 3, where the similar chemical reaction rates of n-tetradecane and n-pentadecane assist in producing comparable diurnal trends over the 24-day campaign. The improved definition of trends with alkane mass may be partly due to increased rates of reaction with OH, as n-alkane rate constants increase with increasing carbon number [Aschmann and Atkinson, 1995]. The relatively high atmospheric lifetimes of the lighter n-alkanes, in combination with the possibility of additional coastal sources other than seawater or the littoral zone, may help to blur any diurnal pattern. Apart from n-nonane (C9), the n-alkanes displayed higher concentrations in coastal airflow (type II), as opposed to airflow off the open ocean (type I). Exceptions to this trend can be accounted for by changes in air mass direction occurring for short time periods (e.g., see JD 165–167 in Figure 3). The majority of type I days, influenced solely by air originating over the Atlantic, were characterized by the absence of high VOC concentrations in the morning. [17] These observations suggest that coastal sources of the n-alkanes exceed those from the open ocean. Anthropogenic sources of these n-alkanes cannot be excluded. Long chain

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Figure 2. Atmospheric n-alkane concentrations (pptv) at Mace Head: JD 156 –163 1999. n-alkanes are indicative of fuel/oil contamination of marine surfaces [Dutta and Harayama, 2000; Maldonado et al., 1999]. However, in both PARFORCE campaigns the coastal atmosphere was dominated by the odd-numbered carbon n-alkanes (C9, C11, C13, C15), suggesting a biogenic influence. Furthermore, seawater analysis (see later) did not reveal the heavier C20-C30 n-alkanes typical of diesel fuel. [18] Only 2 days (JD 175 and JD 176) were influenced by air of continental origin (i.e., type III event). Comparing the “oceanic” and “continental” air mass data from 1998, nalkanes were found at higher concentrations in air from the Atlantic. However, in this study, the lack of easterly airflows prevented a thorough comparison. Instead, the influence of the different coastal sectors on n-alkane behavior was investigated. The four different case studies presented in Figure 1 represent 8 of the 11 days for which OH radical concentrations, wind speeds, and MBL heights were available, allowing a modeling approach to be used to assess the sources, sinks, and behavior of n-alkanes (see below). 3.2.2. Oxygenates [19] As is typical for the remote MBL, oxygenates were found in higher concentrations than the n-alkanes [Singh et al., 2001; Yokouchi et al., 1990]. For example, nonanal and decanal displayed mean mixing ratios of 37 and 44 pptv, respectively (see Table 1). The concentrations of these two aldehydes were strongly correlated, with temporal trends that loosely agreed with the diurnal pattern displayed by the n-alkanes. Importantly, there was no dependence on event type for these aldehydes. This was probably due to a greater variety of significant sources that are of oceanic and terrestrial origin. [20] The only two detectable ketones (C8-C16) were geranyl acetone and an aromatic compound, 2,6-di-tert-butyl-pbenzoquinone (DBBQ). Concentrations of geranyl acetone showed little temporal variation, with a mean concentration of 8 pptv. However, three episodes of elevated concentrations (⬎55 pptv) occurred during JD 167, 169, and 170. The reasons for these episodes are unclear, as only one occurred in con-

junction with low tide and all three episodes occurred at different times of the day, usually at or post noon. Both JD 167 and 169 were typified by type I events, with airflow from the open ocean. A variation in air mass direction did occur during the morning of JD 167, however, when the airflow initially passed over the coastal region to the south of Mace Head, rather than directly from the open ocean. Geranyl acetone has previously been shown to be of plant origin [Fruekilde et al., 1998], but preliminary seaweed chamber studies revealed the absence of this compound [Sartin et al., 2001]. The temporal trend of DBBQ is presented in Figure 4 and was notably different to the temporal trend of geranyl acetone, probably indicating different source types. DBBQ displayed a similar temporal pattern to the heavier n-alkanes with characteristic morning maxima, but was less influenced by event type. Included in Figure 4 is the time series of 2-ethyl-1-hexanol, which had the highest concentrations of all the measured VOCs, with a mean of 66 pptv and a maximum of 427 pptv. This compound has previously been suggested to be a sampling artefact [Helmig et al., 1996]. However, the diurnal trends observed in this study coupled with the negligible levels found on system blanks, including sample blanks of clean air [see Sartin et al., 2001], appear to rule this out. More likely, the presence of this chemical may be due to its extensive industrial use, and it may be considered a ubiquitous anthropogenic pollutant [Ackman, 1997]. [21] An unknown peak that occurred in the majority of the air sample chromatograms was tentatively identified as 3-hydroxy-2,4,4-trimethylpentyl-ester-2-methyl-propanoic acid (C12H23O3) or “C12-propanoate.” This compound has been detailed previously [Sartin et al., 2001], with pseudoquantification based on the FID response to n-dodecane (C12H26). The interest in this unusual ester stems from its ubiquitous occurrence in the Mace Head atmosphere, albeit at low levels (⬍18 pptv). At present, the source of this compound is unclear, but it may be related to fatty acid esters present in plant tissues. An approximate diurnal trend was observed with this compound which was similar to other VOCs, indicating its

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Figure 3. Tetradecane and pentadecane atmospheric mixing ratios at Mace Head (1999, JD 156 –179).

susceptibility to depletion during daylight hours and the probable lack of strong point sources within the littoral zone. 3.3.

Seawater VOCs

[22] Table 2 presents the VOC concentrations determined in inshore surface seawater (⬍0.5 m). Changes in concentration with tide events were not apparent, and similarly no temporal pattern was evident from the grab samples taken over the course of 2 weeks. Possible fluctuations during a 24-hour cycle, however, were not investigated. As a result, comparison with other parameters, such as atmospheric VOC concentrations, was restricted and potential correlations undetectable. Seawater concentrations of many of the compounds, apart from the longer chain n-alkanes, ranged over an order of magnitude, with upper and lower concentrations not related to any observed event (e.g., high tide). The majority of compounds found in the atmosphere were present in the surface seawater. The presence of aromatics was evident in some of the seawater samples, notably the known environmental contaminants benzene, toluene, ethyl benzene, and xylene (BTEX). Comparison to other North Atlantic results was difficult due to the paucity of data. However, one much earlier study at Loop Head, also on the west coast of Ireland (52⬚35⬘N, 9⬚55⬘W), did conduct

both coastal and offshore measurements of seawater n-alkanes [Eichmann et al., 1979]. During this previous work, coastal concentrations were similar to those observed at Mace Head. However, offshore measurements were up to a factor of 10 lower (see Table 2). This difference between open-ocean and coastal seawater n-alkane concentrations was also apparent during a following study at Cape Grim [Eichmann et al., 1980]. This dependence of concentration on distance from the shore has also been reported by Maldonado et al. [1999], with a gradient in dissolved concentrations observed from river estuaries to open seawater in the Black Sea, for both fuel and nonfuel related n-alkanes and polycyclic aromatic hydrocarbons. Interestingly, sporadic elevated levels of aliphatic hydrocarbons occurred offshore in association with particulate organic carbon, attributed to patches of phytoplankton. Only limited studies have investigated the occurrence of heavier oxygenates in seawater, but comparison of mean concentrations for three aldehydes (octanal, nonanal and decanal) between this study and an offshore campaign centred in the Sargasso Sea [Zhou and Mopper, 1990] reveals notably higher levels in the coastal water off Mace Head (see Table 2). Note that samples in the present study were not filtered prior to extraction; therefore any particle-bound compounds would be included in the reported concentrations, par-

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Figure 4. 2-Ethyl-1-hexanol and DBBQ atmospheric mixing ratios at Mace Head (1999, JD 156 –179).

ticularly for the liquid extraction technique. Direct comparisons to those studies using filtration, such as the work of Zhou and Mopper [1990], must be viewed with caution. 3.4. Assessing the Influence of Hydrocarbon Sources and Sinks [23] Using surface seawater as a source of VOCs to the marine boundary layer (MBL), it was possible to describe the observed temporal patterns for type I and type II events utilizing a box model. This approach assessed the hypothetical source (seawater) and sink (deposition, entrainment, and atmospheric oxidation) functions in accounting for the observed diurnal trend during both event types. The two longest chain n-alkanes (n-tetradecane and n-pentadecane) were selected for this modeling approach, due to knowledge about their physical-chemical properties and the observed contrast in their ambient concentrations during type I and type II events.

3.4.1.

Box Model

[24] Source flux, atmospheric chemistry, deposition, advection, turbulent mixing, and entrainment of air across the top of the boundary layer will affect the temporal trend of VOCs measured at the surface. Some of these processes can be addressed in a Lagrangian model [see, e.g., Seinfeld and Pandis, 1998]. Here, however, the deposition term was disregarded. Wet deposition was discounted, as precipitation at the site and upwind was negligible during the time period modeled. Dry deposition was also considered insignificant as surface sorption and/or dissolution would be extremely low over the modeling time frame, given the low water solubilities, high Henry’s law constants, and high vapor pressures exhibited by these VOCs [Wesely and Hicks, 2000]. The relatively high surface seawater loading of these hydrophobic chemicals suggests that the seawater acts as a potential source, not sink, of these compounds to the atmosphere. The model also included the assumptions that the MBL was completely mixed, and the VOC concentra-

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Table 2. Inshore and Offshore Concentrations of Hydrocarbons in Seawatera

Mean (n⫽25)

Range

Sargasso Seab Filtered Subsurface Offshore Range

1.13 2.76 2.04 2.55 0.78 3.28 0.85 1.09 0.15 0.24 nd nd 0.38 0.25

0.37–3.86 1.21–4.60 0.42–3.07 0.20–5.22 0.21–1.08 0.57–6.63 nd to 0.98 0.39–1.13 nd to 0.50 0.23–0.34 nd nd 0.17–0.61 0.17–0.48

– – 0.025–0.089 – – 0.028–0.14 – 0.031–0.13 – – – – – –

This Study Unfiltered Surface Water Inshore

6-Methyl-5-hepten-2-one n-Decane Octanal 2-Ethyl-l-hexanol n-Undecane Nonanal n-Dodecane Decanal n-Tridecane n-Tetradecane Geranyl acetone DBBQ n-Pentadecane n-Hexadecane

Loop Headc Unfiltered

Cape Grimd Unfiltered

Subsurface 1 km Offshore Mean

Surface “Film” Inshore Mean

Subsurface 5 km Offshore Mean

Surface “Film” Inshore Mean

– 0.05 – – 0.05 – 0.06 – 0.06 0.10 – – 0.06 0.12

– 0.20 – – 0.30 – 0.70 – 0.15 1.00 – – 0.45 0.50

– 0.08 – – 0.01 – 0.01 – 0.01 0.01 – – 0.05 0.01

– 0.10 – – 0.11 – 0.22 – 0.28 0.70 – – 1.20 0.80

Units are in ␮g L⫺1; nd, not detected. Zhou and Mopper [1990]. c Eichmann et al. [1979]. d Eichmann et al. [1980]. a b

tion above the boundary layer was zero. The following formula was used to describe the rate of change of VOC concentration in an air parcel advected over a homogeneous surface (i.e., the ocean surface): d[VOC] q [VOC] dH ⫽ ⫹R⫺ dt H H dt d[VOC] q ⫽ ⫹R dt H

for

dH for ⬎0 dt

dH ⱕ 0, dt

(1)

where [VOC] is VOC concentration (pptv), d[VOC]/dt is the rate of change of the VOC concentration (pptv s⫺1), q is the source flux of the VOC (pptv m s⫺1), H is the MBL height (m), R is the rate of oxidation of the VOC (pptv s⫺1), and dH/dt is the rate of change in MBL height (m s⫺1). The third term of equation (1) is removed when dH/dt ⱕ 0 because VOC concentrations in the MBL are no longer influenced by entrainment when the mixing layer is decreasing. [25] Assuming that R ⫽ k⬘[VOC], where k⬘ is a psuedo first-order rate constant (s⫺1) dependent on the concentration of OH radicals, and that ⌬H/⌬t is known from measurements, equation (1) can be integrated using a Euler forward step: [VOC] n ⫽ [VOC] n⫺1 ⫹ ⫺

冉

q n⫺1 ⫺ k⬘n⫺1 ⫻ [VOC] n⫺1 H n⫺1

冊

[VOC] n⫺1 ⌬H ⌬t, H n⫺1 ⌬t

(2)

where [VOC]n is the VOC concentration after n time steps and is a function of the change in MBL height (⌬H) over time step ⌬t and the variables [VOC], q, H, and k⬘ after (n⫺1) time steps. The model was initialized with the observed VOC concentration at 0900 UT, which represented [VOC]0. Although the model results agreed to within 2% with 30 min time steps, the results reported here used 1 min time steps, as this simple model consumed negligible processor time on a standard PC. [26] Seawater n-alkane concentrations were used to determine the respective n-alkane fluxes (q) across the seawater-air

interface. The flux was calculated according to the following equation [Liss and Merlivat, 1986]: q ⫽ k tot⫺w

冉

Cw ⫺

冊

Ca , K aw

(3)

where Cw is the surface seawater concentration, Ca is the air concentration, Kaw is the dimensionless air-water partition coefficient derived from the Henry’s law constant (calculated from the temperature-dependent vapor pressures and aqueous solubilities for the two n-alkanes), and ktot-w is the waterside mass transfer coefficient. For the two n-alkanes, Ca/Kaw, effectively the dry deposition term, was ⬃6 orders of magnitude lower than Cw, and was therefore considered negligible, both quantitatively and for the related reasons previously discussed. The overall mass transfer coefficient ktot-w (m h⫺1) was calculated from the measured wind speed at 10 m above the ocean surface u10 (m s⫺1) using the equations of Liss and Merlivat [1986]. Cw values determined in this study were utilized, along with those published for open-ocean seawater [Cripps, 1992; Eichmann et al., 1979, 1980]. This part of the sensitivity study therefore addressed the possibility of open-ocean concentrations having a greater influence on an Atlantic air mass than coastal water concentrations. A third scenario, with emissions set to zero, was included to examine the influence of this parameter on the model. [27] Values of H, the MBL height, were measured at Mace Head by Kunz et al. [2002]. H typically tended to a maximum toward midday, but for a number of days lidar data were only available for morning and evening periods. Data were therefore extrapolated from comparable days (i.e., with similar meteorological conditions) to complete a full diurnal cycle of H. As monitoring of H was limited to a single site, possible differences between these coastal measurements and open-ocean mixing heights have to be accepted as a limitation of the model. The range accuracy of the lidar was better than 1 m, so any uncertainty in these measurements was dictated by natural variation in the MBL height. [28] OH radical concentrations, [OH] (molecules cm⫺3),

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Table 3. Final n-Alkane Concentrations and Associated Cumulative Errora Coastal Cw Model Run

Open-Ocean Cw

[VOC] at 1700

Error (⫹/⫺)

JD JD JD JD

171 171 178 178

tetradecane pentadecane tetradecane pentadecane

27.2 38.9 18.5 26.0

3.3 5.1 2.6 3.8

JD JD JD JD

158 158 160 160

tetradecane pentadecane tetradecane pentadecane

10.3 12.9 8.3 8.6

2.5 3.0 1.8 1.5

a

[VOC] at 1700 Type I

Type II

Zero Cw

Error (⫹/⫺)

[VOC] at 1700

Error (⫹/⫺)

6.2 9.6 4.4 6.0

0.8 2.5 0.8 0.9

0.2 0.3 0.4 0.2

0.1 0.1 0.2 0.1

5.3 5.9 4.8 3.5

1.9 2.2 1.4 1.5

3.9 3.8 3.8 2.0

1.8 1.9 1.3 1.0

Units are in pptv.

were also measured during the second PARFORCE campaign [Berresheim et al., this issue]. For this data a precision error of 48% was reported, with a detection limit of 5 ⫻ 105 molecules cm⫺3. As with the MBL results, the data were restricted to a single site at Mace Head. To calculate k⬘ from equation (2), OH concentrations were multiplied by the respective rate constant k(OH⫹VOC) (n-tetradecane: 1.68 ⫻ 10⫺11 cm3 molecules⫺1 s⫺1 and n-pentadecane: 1.83 ⫻ 10⫺11 cm3 molecules⫺1 s⫺1 [Aschmann and Atkinson, 1995]). As with u10 and H, [OH] was time-dependent, resulting in k⬘ also varying with time. The model was only run for daylight hours, so nighttime reaction with NO3 radicals was not considered. Halogen radicals, specifically chlorine, were not measured during the PARFORCE campaign but, due to their potentially high concentrations during the early morning [Keene and Jacob, 1996; Singh et al., 1996; Spicer et al., 1998], are considered in the following discussion. Cumulative errors associated with this model over the 8-hour run time are presented in Table 3. These estimates do not include the uncertainty within the seawater measurements, as this is discussed with respect to the three emission scenarios. 3.4.2. Type I Case: Oceanic Air Mass [29] The model was run using data from 2 type I days (JD 171 and JD 178), and compared with observed variations in n-tetradecane and n-pentadecane concentrations. JD 171 was characterized by relatively high wind speeds (mean of 9 m s⫺1), and low [OH] (fluctuating around 1.0 ⫻ 106 molecules cm⫺3 between 1000 and 1600 UT), whereas JD 178 had a lower mean wind speed (7 m s⫺1) and higher [OH] (peaking at ⬃2.5 ⫻ 106 molecules cm⫺3 at midday). As a result, JD 171 had relatively high seawater emissions (higher ktot-w; see equation (3)) coupled with relatively low depletion chemistry compared to JD 178. H was similar for both days, with midday maximums of ⬃1000 m and comparable diurnal trends. The observed and modeled n-alkane mixing ratios for these 2 days are presented in Figure 5. All the model runs indicate the same characteristics, with concentrations showing a greater dependence on emission than either chemistry or entrainment. This was apparent when comparing the model outputs using both the coastal Cw values (this study) and the open-ocean Cw values from the literature. Model results using emissions from the open ocean were a factor of ⬃4 lower than the results generated using coastal seawater emissions, and were more representative of the actual observed levels in the atmosphere. This was still true when minimum coastal seawater concentrations

were considered, with the model-predicted levels at least twice as great as from open-ocean emissions. The above results are consistent with classification as type I air trajectories that originated over the open ocean, and only crossed the coastal area directly in front of the Mace Head site. When zero seawater emissions were used in the model, air concentrations were also close to the observed values. However, a scenario of zero emissions could neither explain the nighttime highs observed in the atmospheric data during the campaign, nor why nalkanes were present at all. The relatively higher [OH] on JD 178 accounted for the differences in modeled data between JD 171 and JD 178 (see Figure 5). On JD178, modeled VOC concentrations reached a steady state in the late morning, and only began to increase as [OH] and H decreased toward the end of the day. 3.4.3. Type II Case: Coastal Air Mass [30] Two days (JD 158 and 160) were selected to represent type II conditions, when air masses had advected over multiple tidal zones. The specific air trajectories of these 2 days are presented in Figures 1b and 1c, with wind directions from the north and northwest for JD 158 and JD 160, respectively. The only significant difference between the 2 days, with respect to model input, was H, which reached a maximum of ⬃3000 m on JD 158 and a maximum of ⬃1500 m on JD 160. During both days, wind speeds ranged between 4 and 5 m s⫺1, and diurnal trends of [OH] were comparable, with early afternoon concentrations reaching around 2.0 ⫻ 106 molecules cm⫺3. For comparative purposes, the three emission scenarios (coastal, open ocean, and zero) were included in the model, even though the coastal air masses had crossed over land as well as water. This provided a definite minimum flux (zero emissions), and, assuming that land emissions of n-alkanes are lower than those from coastal waters, a definite maximum (coastal emissions) flux to the atmosphere. Justification for the above assumption of negligible land emissions was provided by type III data from this campaign and data from the 1998 campaign [Sartin et al., 2001]. The resulting model scenarios for the type II days are presented in Figure 6. Modeled concentrations were in good agreement with the observed data, suggesting that halogen radicals were not in sufficient concentrations to play a role in early morning loss of the VOCs. This was still true when the calculated uncertainties presented in Table 3 were considered. The data also indicated that, compared to type I, type II days were less sensitive to the different emission scenarios (i.e.,

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Figure 5. Modeled n-alkane data from 2 type I days (JD 171 and JD 178) measured Cw versus literature Cw). Depletion due to OH chemistry and dilution through increased H were the dominant processes, while the limited effects of different Cw were only apparent after 1500 UT, when [OH] and H decreased (Figure 6). However, it is likely that the relatively high seawater concentrations measured in this study do have an impact on the coastal air mass, possibly accounting for the higher concentrations typically observed for the type II events. The differences between JD 158 and JD 160 were largely dependent on initial concentrations and H. Variations in H had the effect of diluting the atmospheric concentrations, particularly when H was large (i.e., on JD 158, when maximum H reached 3000 m, compared to JD 160, when maximum H was only 1500 m). [31] The model has not been integrated for longer than 8 hours, due to a lack of measurements with which to compare. Nevertheless, it is straightforward to show from equation (1) what the model would predict for hypothetical nocturnal conditions. When R is negligible, as will be the case during the night if NO3 oxidation is insignificant, and q and H are assumed constant, the integrated form of equation (1) is a linear equation: [VOC] t ⫽ [VOC] 0 ⫹

qt H

(4)

which we use with assumed type II conditions of [VOC]0 ⫽ 5 pptv, q ⫽ 0.35 pptv m s⫺1, t ⫽ 28800 s (8 hours), and H ⫽ 500 m. Five parts per trillion by volume is a typical n-alkane concentration observed in the early evening, 500 m is the assumed height of the nocturnal MBL, and q is calculated using the characteristic wind speed of 5 m s⫺1 and the coastal seawater concentrations of n-tetradecane. Under these condi-

tions the predicted concentration of n-tetradecane after 8 hours is 25 pptv. In the case of n-pentadecane, when q ⫽ 0.52 pptv m s⫺1, the comparable concentration is 35 pptv. These two values compare well with observed morning levels, inferring that the morning highs observed on type II days may be largely due to the nocturnal characteristics of both the MBL and oxidation chemistry.

4.

Conclusions

[32] Twelve VOCs were routinely quantified in the atmosphere of Mace Head, with several carbonyls displaying the highest concentrations. Diurnal patterns were apparent for some of the compounds, notably the n-alkanes, typically characterized by early morning “highs” and post-midday “lows.” Limited night sampling indicated that nighttime levels were generally higher than the daytime concentrations, explaining the elevated morning concentrations often observed. The diurnal pattern was strongest for type II events, when the prevailing air mass had tracked the coastal region to the north or south of Mace Head, rather than originating directly from the open ocean to the west. Comparing n-alkane concentrations with the air mass origin suggested that coastal environments promote higher atmospheric n-alkane levels than open-ocean environments. Shoreline surface seawater (unfiltered) showed notably higher concentrations than previous studies conducted in open-ocean locations. Consequently, VOC emissions from Mace Head coastal water may be greater than emissions from open-ocean water. The oxygenates produced the same diurnal pattern as the n-alkanes, although not as well defined. A weaker dependence on air mass origin was also apparent, with

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Figure 6. Modeled n-alkane data from 2 type II days (JD 158 and JD 160).

both oceanic and coastal trajectories producing comparable atmospheric concentrations. The use of a box model revealed that photochemistry (i.e., decay via OH) and changes in the height of the MBL adequately explained the observed diurnal pattern for the heavier n-alkanes. It was not necessary to invoke VOC-halogen chemistry as a route for removal of VOCs from the atmosphere, as VOC behavior could be adequately explained without their involvement. [33] Acknowledgments. This work was supported by the European Commission Environment and Climate Research Programme (Framework 4) under contract ENV4-CT97-0526. J.H.S. was supported by the Natural Environment Research Council. The authors would like to thank G. de Leeuw et al. for access to their data on MBL height in advance of their publication in this special issue. The authors would also like to express gratitude to Tom Harner of the Meteorological Service of Canada for supplying air mass trajectories and Christian Plass-Du ¨lmer for helpful discussions.

References Ackman, R. G., 2-ethyl-1-hexanol: Contaminant or sex pheromone in Arenicola marina?, Mar. Ecol. Prog. Ser., 152(1–3), 311–312, 1997. Allen, A. G., J. L. Grenfell, R. M. Harrison, J. James, and M. J. Evans, Nanoparticle formation in marine airmasses: Contrasting behaviour of the open ocean and coastal environment, Atmos. Res., 51, 1–14, 1999. Aschmann, S. M., and R. Atkinson, Rate constants for the gas-phase reactions of alkanes with Cl atoms at 296⫹/⫺2 K, Int. J. Chem. Kinet., 27(6), 613– 622, 1995. Baker, A. R., S. M. Turner, W. J. Broadgate, A. Thompson, G. B. McFiggans, O. Vesperini, P. D. Nightingale, P. S. Liss, and T. D. Jickells, Distribution and sea-air fluxes of biogenic trace gases in the eastern Atlantic Ocean, Global Biogeochem. Cycles, 14(3), 871– 886, 2000.

Berresheim, H., T. Elste, H. G. Tremmel, C. O’Dowd, A. G. Allen, H. Hansson, M. DalMaso, and J. M. Makela, Gas-aerosol relationships of H2SO4, MSA, and OH: Observations in the coastal marine boundary layer at Mace Head, Ireland, J. Geophys. Res., this issue. Chao, K. P., S. K. Ong, and A. Protopapas, Water-to-air mass transfer of VOCs: Laboratory-scale air sparging system, J. Environ. Eng., 124(11), 1054 –1060, 1998. Cripps, G. C., Base-line levels of hydrocarbons in seawater of the Southern Ocean - Natural variability and regional patterns, Mar. Pollut. Bull., 24(2), 109 –114, 1992. Dutta, T. K., and S. Harayama, Fate of crude oil by the combination of photooxidation and biodegradation, Environ. Sci. Technol., 34(8), 1500 –1505, 2000. Eichmann, R., P. Neuling, G. Ketseridis, J. Hahn, R. Jaenicke, and C. Junge, n-Alkane studies in the troposphere, I, Gas and particulate concentrations in North Atlantic air, Atmos. Environ., 13, 587–599, 1979. Eichmann, R., G. Ketseridis, G. Schebeske, R. Jaenicke, J. Hahn, P. Warneck, and C. Junge, n-Alkane studies in the troposphere, II, Gas and particulate concentrations in Indian Ocean air, Atmos. Environ., 14, 695–703, 1980. Fruekilde, P., J. Hjorth, N. R. Jensen, D. Kotzias, and B. Larsen, Ozonolysis at vegetation surfaces: A source of acetone, 4-oxopentanal, 6-methyl-5-hepten-2-one, and geranyl acetone in the troposphere, Atmos. Environ., 32(11), 1893–1902, 1998. Helmig, D., W. Pollock, J. Greenberg, and P. Zimmerman, Gas chromatography mass spectrometry analysis of volatile organic trace gases at Mauna Loa observatory, Hawaii, J. Geophys. Res., 101(D9), 14,697–14,710, 1996. Hoffmann, T., J. R. Odum, F. Bowman, D. Collins, D. Klockow, R. C. Flagan, and J. H. Seinfeld, Formation of organic aerosols from the oxidation of biogenic hydrocarbons, J. Atmos. Chem., 26(2), 189 – 222, 1997. Jeannot, M. A., and F. F. Cantwell, Solvent microextraction into a single drop, Anal. Chem., 68(13), 2236 –2240, 1996. Junk, J. A., I. Ogama, and H. J. Sjeck, Extraction of organic compounds from water using small amounts of solvent., in Advances in the Identification and Analysis of Organic Micropollutants in Water,

PAR

4 - 12

SARTIN ET AL.: C8-C16 HYDROCARBONS IN THE ATMOSPHERE OF MACE HEAD

vol. 1, edited by L. H. Keith, pp. 281–292, Butterworth-Heinemann, Woburn, Mass., 1981. Kamens, R., M. Jang, C. J. Chien, and K. Leach, Aerosol formation from the reaction of alpha-pinene and ozone using a gas-phase kinetics aerosol partitioning model, Environ. Sci. Technol., 33(9), 1430 –1438, 1999. Keene, W. C., and D. J. Jacob, Reactive chlorine: A potential sink for dimethylsulfide and hydrocarbons in the marine boundary layer, Atmos. Environ., 30(6), i–iii, 1996. Kunz, G. J., G. de Leeuw, E. Becker, and C. O’Dowd, Lidar studies of the atmospheric boundary layer and locally generated sea spray aerosol plumes at Mace Head, J. Geophys. Res., 107, 10.1029/ 2001JD001240, in press, 2002. Lewis, A. C., K. D. Bartle, D. E. Heard, J. B. McQuaid, M. J. Pilling, and P. W. Seakins, In situ, gas chromatographic measurements of non-methane hydrocarbons and dimethyl sulfide at a remote coastal location (Mace Head, Eire) July-August 1996, J. Chem. Soc. Faraday Trans., 93(16), 2921–2927, 1997. Liss, P. S., and L. Merlivat, Air-sea exchange rates: Introduction and synthesis, in The Role of Air-Sea Exchange in Geochemical Cycling, edited by Buat-Menard, pp. 113–127, D. Reidel, Norwell, Mass., 1986. Maldonado, C., J. M. Bayona, and L. Bodineau, Sources, distribution, and water column processes of aliphatic and polycyclic aromatic hydrocarbons in the northwestern Black Sea water, Environ. Sci. Technol., 33(16), 2693–2702, 1999. Mariaca, R. G., T. F. H. Berger, R. Gauch, M. I. Imhof, B. Jeangros, and J. O. Bosset, Occurrence of volatile mono- and sesquiterpenoids in highland and lowland plant species as possible precursors for flavor compounds in milk and dairy products, J. Agric. Food Chem., 45(11), 4423– 4434, 1997. McKay, W. A., M. F. Turner, B. M. R. Jones, and C. M. Halliwell, Emissions of hydrocarbons from marine phytoplankton - Some results from controlled laboratory experiments, Atmos. Environ., 30(14), 2583–2593, 1996. O’Dowd, C. D., M. Geever, and M. K. Hill, New particle formation: Nucleation rates and spatial scales in the clean marine coastal environment, Geophys. Res. Lett., 25(10), 1661–1664, 1998. O’Dowd, C. D. et al., On the photochemical production of new particles in the coastal boundary layer, Geophys. Res. Lett., 26(12), 1707–1710, 1999. O’Dowd, C. D., et al., A dedicated study of New Particle Formation and Fate in the Coastal Environment (PARFORCE): Overview of objectives and achievements, J. Geophys. Res, 107, 10.1029/ 2001JD000505, in press, 2002a. O’Dowd, C. D., et al., Coastal new particle formation: Environmental conditions and aerosol physico-chemical characteristics during nucleation bursts, J. Geophys. Res., 107, 10.1029/2000JD000206, in press, 2002b. Owen, S., C. Boissard, R. A. Street, S. C. Duckham, O. Csiky, and

C. N. Hewitt, Screening of 18 Mediterranean plant species for volatile organic compound emissions, Atmos. Environ., 31, 101–117, 1997. Rabideau, A. J., J. M. Blayden, and C. Ganguly, Field performance of air sparging system for removing TCE from groundwater, Environ. Sci. Technol., 33(1), 157–162, 1999. Ratte, M., O. Bujok, A. Spitzy, and J. Rudolph, Photochemical alkene formation in seawater from dissolved organic carbon: Results from laboratory experiments, J. Geophys. Res., 103(D5), 5707–5717, 1998. Sartin, J. H., C. J. Halsall, and C. N. Hewitt, C8-C16 volatile organic compounds (VOCs) in the coastal environment of Mace Head, Ireland, Rep. Ser. Aerosol Sci., 48, 122–133, 2000. Sartin, J. H., C. J. Halsall, B. Davison, S. Owen, and C. N. Hewitt, Determination of biogenic volatile organic compounds (C8-C16) in the coastal atmosphere at Mace Head, Ireland, Anal. Chim. Acta, 428, 61–72, 2001. Seinfeld, J. H., and S. N. Pandis, Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, pp. 1193–1244, John Wiley, New York, 1998. Singh, H. B., et al., Low ozone in the marine boundary payer of the tropical Pacific Ocean: Photochemical loss, chlorine atoms, and entrainment, J. Geophys. Res., 101(D1), 1907–1917, 1996. Singh, H., Y. Chen, A. Staudt, D. Jacob, D. Blake, B. Heikes, and J. Snow, Evidence from the Pacific troposphere for large global sources of oxygenated organic compounds, Nature, 410, 1078 –1081, 2001. Spicer, C. W., E. G. Chapman, B. J. Finlayson-Pitts, R. A. Plastridge, J. M. Hubbe, J. D. Fast, and C. M. Berkowitz, Unexpectedly high concentrations of molecular chlorine in coastal air, Nature, 394(6691), 353–356, 1998. Wesely, M. L., and B. B. Hicks, A review of the current status of knowledge on dry deposition, Atmos. Environ., 34(12–14), 2261– 2282, 2000. Yokouchi, Y., H. Mukai, K. Nakajima, and Y. Ambe, Semi-volatile aldehydes as predominant organic gases in remote areas, Atmos. Environ., Part A, 24(2), 439 – 442, 1990. Zhou, X., and K. Mopper, Apparent partition coefficients of 15 carbonyl compounds between air and seawater and between air and freshwater; implications for air-sea exchange., Environ. Sci. Technol., 24, 1864 –1869, 1990. H. Berresheim, Deutscher Wetterdienst, Meteorologisches Observatorium, Albin-Schwaiger-Weg 10, D-82383 Hohenpeissenberg, Germany. ([email protected]) R. G. Gonard, C. J. Halsall, C. N. Hewitt (corresponding author), A. R. MacKenzie, L. A. Robertson, and J. H. Sartin, Department of Environmental Science, Institute of Environmental and Natural Sciences, Lancaster University, Lancaster LA1 4YQ, UK. ([email protected])