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Shouldn’t snowpacks be sources of monocarboxylic acids?
Atmospheric Environment 36 (2002) 2513–2522
Shouldn’t snowpacks be sources of monocarboxylic acids? Jack E. Dibb*, Matthew Arsenault Climate Change Research Center, Institute for the Study of Earth, Oceans and Space, University of New Hampshire, Durham, NH 03824, USA Received 4 June 2001; received in revised form 28 August 2001; accepted 11 January 2002
Abstract We report the first measurements of the mixing ratios of acetic (CH3COOH) and formic (HCOOH) acids in the air filling the pore spaces of the snowpacks (firn air) at Summit, Greenland and South Pole. Both monocarboxylic acids were present at levels well above 1 ppbv throughout the upper 35 cm of the snowpack at Summit. Maximum mixing ratios in Summit firn air reached nearly 8 ppbv CH3COOH and 6 ppbv HCOOH. At South Pole the mixing ratios of these acids in the top 35 cm of firn air were also generally >1 ppbv, though their maximums barely exceeded 2.5 ppbv of CH3COOH and 2.0 ppbv of HCOOH. Mixing ratios of the monocarboxylic acids in firn air did not consistently respond to diel and experimental (fast) variations in light intensity, unlike the case for N oxides in the same experiments. Air-to-snow fluxes of CH3COOH and HCOOH apparently support high mixing ratios (means of (CH3COOH/HCOOH) 445/460 and 310/159 pptv at Summit and South Pole, respectively) in air just above the snow during the summer sampling seasons at these sites. We hypothesize that oxidation of carbonyls and alkenes (that are produced by photo- and OH-oxidation of ubiquitous organic compounds) within the snowpack is the source of the monocarboxylic acids. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Snowpack chemistry; Acetic acid; Formic acid
1. Introduction The monocarboxylic acids, formic acid (HCOOH) and acetic acid (CH3COOH), (HFo and HAc, respectively, herein) are ubiquitous in the atmosphere and in precipitation. It has long been known that these acids can contribute a significant, or even dominant, fraction of the free acidity in precipitation falling in remote regions (Galloway et al., 1982; Keene and Galloway, 1988). In a recent review article, Chebbi and Carlier (1996) conclude that, despite over 20 years of active research, much still remains uncertain about the atmospheric chemistry of the carboxylic acids. In addition to being directly emitted by a variety of anthropogenic and biogenic sources, HAc and HFo are produced from volatile organic carbon (VOC) precursors in the atmo*Corresponding author. Tel.: +1-603-862-3063; fax: +1603-862-2124. E-mail address: [email protected] (J.E. Dibb).
sphere. Oxidation of alkenes by O3 and reactions of peroxy acyl radicals with other peroxy radicals can produce HAc and HFo (and additional carboxylic acids) but the exact mechanisms and their relative importance remain unclear (Chebbi and Carlier, 1996). Oxidation of HCHO in the aqueous phase (cloud water) has been proposed as a potential source of both monocarboxylic acids in the atmosphere. Production of HAc by the proposed mechanism has never been observed in the laboratory, arguing against its importance in the atmosphere. Formic acid is formed from HCHO in cloud drops, but the magnitude of this source is a point of contention (Chebbi and Carlier, 1996). Glasius et al. (2001) used 14C to determine that biogenic emissions of VOC precursors of the monocarboxylic acids dominate anthropogenic sources throughout Europe, even in urban areas with major pollution sources. Legrand and his collaborators pioneered investigations of the monocarboxylic acids in polar snow and ice. Stringent precautions must be taken to avoid
1352-2310/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 2 ) 0 0 1 3 1 - 0
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contamination of snow, and particularly firn core, samples by the sampling tools and storage containers (Legrand and Saigne, 1988). Concentrations of HAc and HFo are very low in polar snow, unlike rain in remote regions. These low concentrations appear to result from both poor preservation and inefficient incorporation of HAc and HFo within snowflakes. DeAngelis and Legrand (1995) demonstrated that the monocarboxylic acids can be lost from surface snow at Summit, Greenland in days to weeks after deposition. Similarly, Dibb et al. (1994) observed nearly complete disappearance of HAc and HFo within a period of 6 h after the deposition of ice fog had enriched their concentrations in the surface layer of snow by factors of 3–5. Cloud microphysics may also lead to more efficient scavenging of the monocarboxylic acids by rain than by snow (Legrand and DeAngelis, 1995). Legrand and DeAngelis (1996) pointed out that volcanic horizons in ice cores with low pH are depleted in Ac and, to a greater extent, Fo . They suggested that both the incorporation and preservation of Fo in snow increase with pH and hypothesized that enhanced concentrations of Fo in winter layers of the Greenland snowpack largely reflect the lower acidity of winter than summer snow. Because Antarctica is so far away from continental biogenic and anthropogenic sources of VOCs, Legrand and coworkers hypothesized that only long-lived precursors are important sources of monocarboxylic acids in the atmosphere above the snow. Specifically, they suggested that HCHO from the oxidation of CH4 must be the principal source of HFo in the Antarctic (Legrand and Saigne, 1988). Hydrolysis of peroxyacetyl nitrate in cloud water was suggested as a potentially important source of HAc at high northern latitudes, though emissions from boreal forest fires, and boreal vegetation more generally, are hypothesized as the dominant sources of monocarboxlic acids in Greenland snow (Legrand and DeAngelis, 1995, 1996). Osada and Langway (1993) inferred that biogenic emissions in Canada are the dominant source of Fo in preanthropogenic Greenland snow, based on their observation of decreasing concentrations in cores further north and east from Dye 2 (in the southwest). In contrast to the low concentrations in snow, mixing ratios of HAc and HFo in the atmosphere at Summit are significant. Dibb et al. (1994) pointed out that the average mixing ratios of HAc and HFo (715 and 1100 pptv, respectively) at Summit in summer 1993 were essentially the same as those in Schefferville, Ontario (in the middle of the boreal forest) during summer 1990. Although the average monocarboxylic acid mixing ratios at Summit in 1994 and 1995 (665 and 504 pptv HAc, 650 and 750 pptv HFo, respectively (Dibb et al., 1998)) were lower than in 1993, they remained far higher than average values at other Arctic sites (Dibb et al., 1994) and all of the free tropospheric and rural
boundary layer values compiled by Chebbi and Carlier (1996) and Glasius et al. (2001). (It should be noted that Talbot et al. (1995) reported median HFo and HAc mixing ratios (5.4 and 2.1 ppbv, respectively) from a mountain top site in rural Virginia that exceed values at Summit and all other rural sites tabulated in the recent compilations. The enhanced levels of these acids in Virginia compared to other rural sites did not appear to be due to strong anthropogenic or biomass burning sources during the study period, leaving mixed biogenic emissions as the likely dominant source (Talbot et al., 1995). High mixing ratios in Virginia do not negate the fact that HAc and HFo mixing ratios frequently >0.5 ppbv just above the snow at Summit are difficult to reconcile with a dominant biogenic source, given the great distance to vegetated regions.) In each of these previous studies, Dibb and coworkers tentatively suggested fluxes out of the snow to explain the high mixing ratios, but largely ruled out this possibility due to lack of a mechanism to sustain the necessarily large fluxes for periods of months. The discovery that NOx is apparently produced, in large amounts, in the snowpack at Summit (Honrath et al., 1999) suggested that active photochemistry in the snow could be producing a variety of reactive compounds. To explore this possibility we have conducted extensive sampling of the firn air at Summit in the summers of 1999 and 2000. In the latter campaign we included measurements of the moncarboxylic acids, using a firn air sampling probe developed to allow mistchamber sampling for HONO and HNO3 (Dibb et al., 2002). The techniques developed at Summit were also employed during a sampling campaign at South Pole in December 2000. This paper focuses on our measurements of HAc and HFo in polar firn air, with some speculation about their source and potential impact on the atmosphere above the snow.
2. Methods Gas phase sampling of the monocarboxylic acids was accomplished with the mistchamber technique, followed by ion chromatographic (IC) analysis in a field laboratory (Dibb et al., 1994, 1998). Keene et al. (1989) found this sampling technique to be the most reliable for gas phase collection of the monocarboxylic acids. We use IC columns and conditions developed by Talbot et al. (1995), and use aqueous standards provided by Talbot’s research group. Talbot et al. (1995) estimated that the overall uncertainty of mixing ratios during their Virginia campaign was 10–15% for 1-h integration times. At Summit and South Pole we generally sampled for 30 min at flow rates of 15–20 l min 1 (STP) and analyzed the samples within 1 h of collection (hence did not add biocide to preserve
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the samples). We estimate, by propagation of uncertainties in the IC analyses and measurement of air flow, that our overall uncertainty is no higher than 20%. Two samplers were operated simultaneously at all times. One sampler collected ambient air above the snow while the second employed a 2-m long heated inlet that could pull air from the pore spaces within the snowpack or at various heights ranging from 1 cm up to 2 m above the snow (Dibb et al., 2002). During the 2000 campaigns at Summit and South Pole the heated inlet was fitted with a firn probe constructed from PVC tubing during all experiments that sampled air in the pores between snow grains. This probe only allowed air to enter the inlet filter from the bottom of the hole in the snowpack into which it was inserted, and also insulated the surrounding snow from the heat applied to the tubing (the inlet was heated to facilitate passing of HNO3). During the Summit campaign 72 paired samples with both inlets B85 cm above the snow were collected to test the passing efficiency of the heated inlet/firn probe combination. At South Pole 26 such ‘‘null gradient’’ tests were performed. Average values of the ratio (mixing ratio through the heated inlet/mixing ratio through the standard configuration) for HAc and HFo were 0.99 and 0.95, respectively, at Summit and 0.84 and 0.97, respectively, at South Pole. These results indicate that the inlet and firn probe do not introduce a consistent bias in the measurement of the monocarboxylic acids. However, this ratio varied enough within the 98 null gradient tests to preclude confidence in small gradients of HAc and HFo we observed between 2 and 85 cm above the snow in both campaigns (unlike HONO and HNO3, see Honrath et al., 2002). To sample firn air from a given depth, a second length of PVC was used to ‘‘core’’ a cylindrical hole. In experiments examining the impact of changing light levels (both natural diel cycles and artificial shading studies) the firn probe was installed in such a hole and remained there for at least 15 h (up to several days in some instances). To measure depth profiles of the mixing ratios in firn air, a shallow hole was cored and samples were collected. The same hole was then made deeper and the probe repositioned, down to a maximum depth of 35 cm in the experiments discussed here. To repeat the depth profile the original hole was backfilled with loose surface snow and the process was started again at a new spot at least 50 cm away from the previous hole. Note that 35 cm represents roughly half the annual accumulation of snow at Summit, but is equivalent to almost 2 years of accumulation at South Pole. As discussed by Dibb et al. (2002), the snowpack is anisotropically permeable, so it is not well known which horizons contribute to the sampled flow. During the 2001 Summit field season we conducted extensive tracer tests to specifically examine this issue and will present the results and implications in a future publication.
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At South Pole three shallow snowpits were sampled to depths of 40 cm. Samples were collected into airtight glass bottles and then allowed to melt in the field laboratory that evening. Aliquots for the monocarboxylic acids were analyzed on the same ion chromatograph used for gas sampling as soon as the samples were completely liquid.
3. Results 3.1. Summit Mixing ratios of the monocarboxylic acids 85 cm above the snow were once again high for a remote site (Fig. 1). Both acids often exceeded 1 ppbv and averaged more than 0.4 ppbv (445 and 460 pptv of HAc and HFo, respectively). These means are lower than we measured in 1993–1995 (Dibb et al., 1994, 1998), but are still more than 2-fold higher than Talbot et al. (1992) found in the summer Arctic free troposphere during ABLE 3A. The 2000 Summit means are nearly equal to mean mixing ratios in ‘‘background’’ free tropospheric air above the Canadian boreal forest during ABLE 3B (Lefer et al., 1994). However, ambient mixing ratios of HAc and HFo at Summit are dwarfed by their mixing ratios in firn air. Both monocarboxylic acids were 5–7 times more abundant 10 cm below the air–snow interface than 85 cm above it (Fig. 2). In this experiment, and similar ones with the firn probe at 10 or 20 cm (not shown), mixing ratios in ambient and firn air decreased through the evening, perhaps reflecting decreasing sun elevation. It should be noted that the evening decreases of HAc and HFo in firn air shown in Fig. 2 are modest, especially compared to those of NOx and HONO which drop to nearly ambient levels around midnight (see Dibb et al., 2002). Deeper in the firn the monocarboxylic acid mixing ratios were even more elevated, and no sun synchronous behavior was apparent (Fig. 3). It is striking how constant HAc stayed through this 18-h interval, varying o10% from a mean of 5910 pptv. Formic acid did slowly increase nearly 2 ppbv between 9:00 and 16:00, but was then nearly constant for 9 h. During this experiment, ambient mixing ratios dropped to very low levels between 22:00 and about 2:00. Throughout the season most sampling was conducted between 8:00 and 22:00, but the few experiments that extended through the night are obvious in Fig. 1, showing up as the clusters of samples with mixing ratios near and below 100 pptv. When the patch of snow surrounding the firn probe was alternately shaded and exposed to full sun, the mixing ratios of HAc and HFo did not vary (Fig. 4). This combination of experiments suggests that the influence of sunlight on monocarboxylic acid mixing
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Fig. 1. Mixing ratios of acetic and formic acids 85 cm above the snow at Summit, Greenland in summer 2000.
Fig. 2. Mixing ratios of acetic and formic acids 85 cm above the snow (J) and 10 cm into the snow (K) at Summit on 1 July 2000. Times in this and all Summit plots are local time, 2 h behind GMT.
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Fig. 3. Mixing ratios of acetic and formic acids 85 cm above the snow (J) and 30 cm into the snow (K) at Summit on 3–4 July 2000.
Fig. 4. Mixing ratios of acetic and formic acids 85 cm above the snow (J) and 10 cm into the snow (K) at Summit on 20 June 2000. During the intervals indicated by the shaded bars a 2.4 m 2.4 m patch of shade, centered on the sampling inlet, was created on the snow surface.
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Fig. 5. Profiles of the mixing ratios of acetic and formic acids at different depths in the Summit snowpack. Arrows in upper left corners of the two panels indicate the average mixing ratios at 85 cm above the snow on the two different days. For each profile, measurements were made at successively deeper depth (see text).
ratios in firn air is not direct, in stark contrast to the case for the N oxides (Dibb et al., 2002). Acetic acid mixing ratios in firn air increased with depth (to our maximum depth of 35 cm) in all five firn air profiles measured at Summit (Fig. 5). Formic acid profiles were less consistent, but in four of the five cases mixing ratios were lower at 35 cm than at 25 cm. In three of these profiles maximum mixing ratios occurred at 25 cm. The variability of these depth profiles, especially those measured on the same day, is puzzling. The slow changes (Fig. 2), or nearly constant levels (Fig. 3), observed over periods of hours at a single depth when the probe was not moved suggest that we may not have sampled the exact same stratigraphic layers on successive profiles. It is hard to understand what could cause such large differences on relatively small vertical and horizontal scales, yet still create the consistent increase of HAc with depth. 3.2. South Pole Ambient mixing ratios of HAc and HFo averaged 310 and 159 pptv, respectively, lower than at Summit but still surprisingly high for so remote a site (Fig. 6). Talbot
et al. (1990) reported means of 300 pptv HAc and 400 pptv HFo in the Brazilian Amazon during the wet season, and, as noted above, the means in the summer Arctic troposphere were o300 pptv. Solar elevation changes extremely slowly at South Pole, making diel experiments useless for the investigation of response to varying light levels. We attempted manipulating light levels by shading (as described for Summit) but found very inconsistent results (one dark interval might show no response, yet the next could lead to large increases). Our current hypothesis is that attempting to block light also impacts air flow through the snow, with reduced airflow reducing air-to-snow fluxes, causing temporary increases in mixing ratios in the firn air. Such effects might be larger at South Pole than Summit due to the combination of higher wind speeds and more pronounced surface relief. We intend to explicitly investigate this hypothesis in future studies at Summit. Firn air mixing ratios of the monocarboxylic acids were elevated by at least a factor of 3 compared to ambient values. In the first depth profile both HAc and HFo steadily increased down to 30 cm, but a pair of profiles measured on 23 December showed even larger
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Fig. 6. Mixing ratios of acetic and formic acids 85 cm above the snow at South Pole in December 2000. Time is GMT.
relative variability than we observed at Summit (Fig. 7). Formic acid decreased with depth (though remaining above ambient) in both profiles, but showed up to 600 pptv differences at a common depth over about 5 h. Acetic acid displayed a pronounced minimum in the 15–20 cm range on the first profile that was no longer apparent in the second (due mainly to 400–600 pptv decreases in the deeper horizons between the two measurements). These variations over time are not understood, but they do not negate the primary observation that firn air mixing ratios of HAc and HFo in excess of 1 ppbv are common at the South Pole. Concentrations of Fo and Ac in South Pole snow ranged from 0.05 to 0.3 nmol g 1 (Fig. 8). The peak and baseline values are directly comparable to those characteristic of the upper meters of the snowpack at Summit (see Fig. 15 in Legrand and DeAngelis (1995)). Unlike Summit, where peak concentrations are preserved in winter snow, the peak in the South Pole pit is centered in the previous summer’s snow (starting in spring and extending well beyond the timing of the spring/summer NO3 peak (Whitlow et al., 1992)). In spring snow that accumulated in 2000 a very large enhancement of NO3 is clear (in the top 5 cm (Fig. 8)), but there is no parallel increase in the monocarboxylates.
4. Discussion The mechanisms releasing HAc and HFo into the pore air of snowpacks are not yet clear, but likely include significant production of these acids on the snow grains and/or in the firn air. The magnitude of firn air mixing ratios, and lack of correspondence between depth profiles of firn air and snow concentrations, suggests that simple degassing and desorption of HAc and HFo from the snow grains cannot be their major source in the firn air. On the other hand, possible precursors to HAc and HFo have been shown to be present in abundance in firn air. Release of HCHO from snow into firn air, and subsequently into the atmosphere, is well established at both Summit and Alert (Jacobi et al., 2002; Dassau et al., 2002; Hutterli et al., 1999a; Sumner and Shepson, 1999), and also in Antarctica (Hutterli et al., 1999b). Similarly, CH3CHO was shown to be nearly 3-fold higher in snow pore air than ambient air in northern Michigan (Couch et al., 2000), we expect the same is true at Summit, and perhaps South Pole, though no measurements have yet been made. Some fraction of the carbonyls released into snowpack pore space can be explained by thermal desorption of compounds incorporated into the snowflakes in cloud (e.g., Hutterli et al., 1999a, b), but it is also likely that they are produced by oxidation of
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Fig. 7. Profiles of the mixing ratios of acetic and formic acids at different depths in the South Pole snowpack. Arrows in upper left corners of the two panels indicate the average mixing ratios at 85 cm above the snow on the two different days. For each profile, measurements were made at successively deeper depth (see text).
Fig. 8. Concentrations of acetate and formate in the upper 30 cm of the snowpack at South Pole. Nitrate concentrations are included to indicate the depth of the summer 1999 layers (just above 20 cm).
organic compounds in the snow (e.g., Sumner and Shepson, 1999). Chemical production of carbonyls in the snowpack will be enhanced by elevated OH in firn air (Yang et al., 2002). Swanson et al. (2002) demonstrate that several short-lived alkenes are elevated in firn air relative to ambient air at Summit, with photooxidation of unknown organic compounds suggested as their source. Part of the O3 destruction in Summit firn
described by Peterson and Honrath (2001) might be linked to the formation of monocarboxylic acids from these very reactive hydrocarbons. Mixing ratios of HCHO and the alkenes in firn air respond to diel and fast variations in light, unlike HAc and HFo. We hypothesize that the monocarboxylic acids are produced from a number of organic compounds in firn air and snow, and that they build up in firn air due to inefficient
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removal to acidic ice surfaces (Legrand and DeAngelis, 1995). Higher mixing ratios in Summit firn air than at South Pole presumably reflect higher concentrations of organic matter in the snow.
5. Conclusions The elevated mixing ratios of monocarboxylic acids in firn air must be coming from the surrounding snow. We suspect that much of the HAc and HFo in firn air is produced there from reactive VOCs, which in turn are produced by oxidation of organics in the snow. Persistent gradients between firn air and the overlying atmosphere should sustain air-to-snow fluxes, maintaining higher mixing ratios of HAc and HFo in the lowermost atmosphere at South Pole and Summit than at many rural and remote sites that are much closer to biogenic and anthropogenic sources. Quantifying these fluxes, and their impact on the lower troposphere, is one of the primary objectives of planned future work. In situ production of monocarboxylic acids in snow presents an additional complication in the interpretation of Ac and Fo depth profiles in polar ice cores. It is hard to imagine how snow exposed to firn air with mixing ratios of HAc and HFo in excess of ppbv levels could retain a non-biased record of remote tropospheric mixing ratios an order of magnitude lower.
Acknowledgements This research was supported by the Arctic System Science, Arctic Natural Sciences and Antarctic Ocean and Climate programs of the Office of Polar Programs at NSF. Logistic support from PICO (in Greenland) and airlift by the NY ANG (both poles) is appreciated. We are grateful to the Danish Polar Board and the Greenlandic Home Rule government for granting us permission to work at Summit. Special thanks to Manuel Hutterli for sharing samples and pit digging at South Pole.
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Shouldn’t snowpacks be sources of monocarboxylic acids? Jack E. Dibb*, Matthew Arsenault Climate Change Research Center, Institute for the Study of Earth, Oceans and Space, University of New Hampshire, Durham, NH 03824, USA Received 4 June 2001; received in revised form 28 August 2001; accepted 11 January 2002
Abstract We report the first measurements of the mixing ratios of acetic (CH3COOH) and formic (HCOOH) acids in the air filling the pore spaces of the snowpacks (firn air) at Summit, Greenland and South Pole. Both monocarboxylic acids were present at levels well above 1 ppbv throughout the upper 35 cm of the snowpack at Summit. Maximum mixing ratios in Summit firn air reached nearly 8 ppbv CH3COOH and 6 ppbv HCOOH. At South Pole the mixing ratios of these acids in the top 35 cm of firn air were also generally >1 ppbv, though their maximums barely exceeded 2.5 ppbv of CH3COOH and 2.0 ppbv of HCOOH. Mixing ratios of the monocarboxylic acids in firn air did not consistently respond to diel and experimental (fast) variations in light intensity, unlike the case for N oxides in the same experiments. Air-to-snow fluxes of CH3COOH and HCOOH apparently support high mixing ratios (means of (CH3COOH/HCOOH) 445/460 and 310/159 pptv at Summit and South Pole, respectively) in air just above the snow during the summer sampling seasons at these sites. We hypothesize that oxidation of carbonyls and alkenes (that are produced by photo- and OH-oxidation of ubiquitous organic compounds) within the snowpack is the source of the monocarboxylic acids. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Snowpack chemistry; Acetic acid; Formic acid
1. Introduction The monocarboxylic acids, formic acid (HCOOH) and acetic acid (CH3COOH), (HFo and HAc, respectively, herein) are ubiquitous in the atmosphere and in precipitation. It has long been known that these acids can contribute a significant, or even dominant, fraction of the free acidity in precipitation falling in remote regions (Galloway et al., 1982; Keene and Galloway, 1988). In a recent review article, Chebbi and Carlier (1996) conclude that, despite over 20 years of active research, much still remains uncertain about the atmospheric chemistry of the carboxylic acids. In addition to being directly emitted by a variety of anthropogenic and biogenic sources, HAc and HFo are produced from volatile organic carbon (VOC) precursors in the atmo*Corresponding author. Tel.: +1-603-862-3063; fax: +1603-862-2124. E-mail address: [email protected] (J.E. Dibb).
sphere. Oxidation of alkenes by O3 and reactions of peroxy acyl radicals with other peroxy radicals can produce HAc and HFo (and additional carboxylic acids) but the exact mechanisms and their relative importance remain unclear (Chebbi and Carlier, 1996). Oxidation of HCHO in the aqueous phase (cloud water) has been proposed as a potential source of both monocarboxylic acids in the atmosphere. Production of HAc by the proposed mechanism has never been observed in the laboratory, arguing against its importance in the atmosphere. Formic acid is formed from HCHO in cloud drops, but the magnitude of this source is a point of contention (Chebbi and Carlier, 1996). Glasius et al. (2001) used 14C to determine that biogenic emissions of VOC precursors of the monocarboxylic acids dominate anthropogenic sources throughout Europe, even in urban areas with major pollution sources. Legrand and his collaborators pioneered investigations of the monocarboxylic acids in polar snow and ice. Stringent precautions must be taken to avoid
1352-2310/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 2 ) 0 0 1 3 1 - 0
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contamination of snow, and particularly firn core, samples by the sampling tools and storage containers (Legrand and Saigne, 1988). Concentrations of HAc and HFo are very low in polar snow, unlike rain in remote regions. These low concentrations appear to result from both poor preservation and inefficient incorporation of HAc and HFo within snowflakes. DeAngelis and Legrand (1995) demonstrated that the monocarboxylic acids can be lost from surface snow at Summit, Greenland in days to weeks after deposition. Similarly, Dibb et al. (1994) observed nearly complete disappearance of HAc and HFo within a period of 6 h after the deposition of ice fog had enriched their concentrations in the surface layer of snow by factors of 3–5. Cloud microphysics may also lead to more efficient scavenging of the monocarboxylic acids by rain than by snow (Legrand and DeAngelis, 1995). Legrand and DeAngelis (1996) pointed out that volcanic horizons in ice cores with low pH are depleted in Ac and, to a greater extent, Fo . They suggested that both the incorporation and preservation of Fo in snow increase with pH and hypothesized that enhanced concentrations of Fo in winter layers of the Greenland snowpack largely reflect the lower acidity of winter than summer snow. Because Antarctica is so far away from continental biogenic and anthropogenic sources of VOCs, Legrand and coworkers hypothesized that only long-lived precursors are important sources of monocarboxylic acids in the atmosphere above the snow. Specifically, they suggested that HCHO from the oxidation of CH4 must be the principal source of HFo in the Antarctic (Legrand and Saigne, 1988). Hydrolysis of peroxyacetyl nitrate in cloud water was suggested as a potentially important source of HAc at high northern latitudes, though emissions from boreal forest fires, and boreal vegetation more generally, are hypothesized as the dominant sources of monocarboxlic acids in Greenland snow (Legrand and DeAngelis, 1995, 1996). Osada and Langway (1993) inferred that biogenic emissions in Canada are the dominant source of Fo in preanthropogenic Greenland snow, based on their observation of decreasing concentrations in cores further north and east from Dye 2 (in the southwest). In contrast to the low concentrations in snow, mixing ratios of HAc and HFo in the atmosphere at Summit are significant. Dibb et al. (1994) pointed out that the average mixing ratios of HAc and HFo (715 and 1100 pptv, respectively) at Summit in summer 1993 were essentially the same as those in Schefferville, Ontario (in the middle of the boreal forest) during summer 1990. Although the average monocarboxylic acid mixing ratios at Summit in 1994 and 1995 (665 and 504 pptv HAc, 650 and 750 pptv HFo, respectively (Dibb et al., 1998)) were lower than in 1993, they remained far higher than average values at other Arctic sites (Dibb et al., 1994) and all of the free tropospheric and rural
boundary layer values compiled by Chebbi and Carlier (1996) and Glasius et al. (2001). (It should be noted that Talbot et al. (1995) reported median HFo and HAc mixing ratios (5.4 and 2.1 ppbv, respectively) from a mountain top site in rural Virginia that exceed values at Summit and all other rural sites tabulated in the recent compilations. The enhanced levels of these acids in Virginia compared to other rural sites did not appear to be due to strong anthropogenic or biomass burning sources during the study period, leaving mixed biogenic emissions as the likely dominant source (Talbot et al., 1995). High mixing ratios in Virginia do not negate the fact that HAc and HFo mixing ratios frequently >0.5 ppbv just above the snow at Summit are difficult to reconcile with a dominant biogenic source, given the great distance to vegetated regions.) In each of these previous studies, Dibb and coworkers tentatively suggested fluxes out of the snow to explain the high mixing ratios, but largely ruled out this possibility due to lack of a mechanism to sustain the necessarily large fluxes for periods of months. The discovery that NOx is apparently produced, in large amounts, in the snowpack at Summit (Honrath et al., 1999) suggested that active photochemistry in the snow could be producing a variety of reactive compounds. To explore this possibility we have conducted extensive sampling of the firn air at Summit in the summers of 1999 and 2000. In the latter campaign we included measurements of the moncarboxylic acids, using a firn air sampling probe developed to allow mistchamber sampling for HONO and HNO3 (Dibb et al., 2002). The techniques developed at Summit were also employed during a sampling campaign at South Pole in December 2000. This paper focuses on our measurements of HAc and HFo in polar firn air, with some speculation about their source and potential impact on the atmosphere above the snow.
2. Methods Gas phase sampling of the monocarboxylic acids was accomplished with the mistchamber technique, followed by ion chromatographic (IC) analysis in a field laboratory (Dibb et al., 1994, 1998). Keene et al. (1989) found this sampling technique to be the most reliable for gas phase collection of the monocarboxylic acids. We use IC columns and conditions developed by Talbot et al. (1995), and use aqueous standards provided by Talbot’s research group. Talbot et al. (1995) estimated that the overall uncertainty of mixing ratios during their Virginia campaign was 10–15% for 1-h integration times. At Summit and South Pole we generally sampled for 30 min at flow rates of 15–20 l min 1 (STP) and analyzed the samples within 1 h of collection (hence did not add biocide to preserve
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the samples). We estimate, by propagation of uncertainties in the IC analyses and measurement of air flow, that our overall uncertainty is no higher than 20%. Two samplers were operated simultaneously at all times. One sampler collected ambient air above the snow while the second employed a 2-m long heated inlet that could pull air from the pore spaces within the snowpack or at various heights ranging from 1 cm up to 2 m above the snow (Dibb et al., 2002). During the 2000 campaigns at Summit and South Pole the heated inlet was fitted with a firn probe constructed from PVC tubing during all experiments that sampled air in the pores between snow grains. This probe only allowed air to enter the inlet filter from the bottom of the hole in the snowpack into which it was inserted, and also insulated the surrounding snow from the heat applied to the tubing (the inlet was heated to facilitate passing of HNO3). During the Summit campaign 72 paired samples with both inlets B85 cm above the snow were collected to test the passing efficiency of the heated inlet/firn probe combination. At South Pole 26 such ‘‘null gradient’’ tests were performed. Average values of the ratio (mixing ratio through the heated inlet/mixing ratio through the standard configuration) for HAc and HFo were 0.99 and 0.95, respectively, at Summit and 0.84 and 0.97, respectively, at South Pole. These results indicate that the inlet and firn probe do not introduce a consistent bias in the measurement of the monocarboxylic acids. However, this ratio varied enough within the 98 null gradient tests to preclude confidence in small gradients of HAc and HFo we observed between 2 and 85 cm above the snow in both campaigns (unlike HONO and HNO3, see Honrath et al., 2002). To sample firn air from a given depth, a second length of PVC was used to ‘‘core’’ a cylindrical hole. In experiments examining the impact of changing light levels (both natural diel cycles and artificial shading studies) the firn probe was installed in such a hole and remained there for at least 15 h (up to several days in some instances). To measure depth profiles of the mixing ratios in firn air, a shallow hole was cored and samples were collected. The same hole was then made deeper and the probe repositioned, down to a maximum depth of 35 cm in the experiments discussed here. To repeat the depth profile the original hole was backfilled with loose surface snow and the process was started again at a new spot at least 50 cm away from the previous hole. Note that 35 cm represents roughly half the annual accumulation of snow at Summit, but is equivalent to almost 2 years of accumulation at South Pole. As discussed by Dibb et al. (2002), the snowpack is anisotropically permeable, so it is not well known which horizons contribute to the sampled flow. During the 2001 Summit field season we conducted extensive tracer tests to specifically examine this issue and will present the results and implications in a future publication.
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At South Pole three shallow snowpits were sampled to depths of 40 cm. Samples were collected into airtight glass bottles and then allowed to melt in the field laboratory that evening. Aliquots for the monocarboxylic acids were analyzed on the same ion chromatograph used for gas sampling as soon as the samples were completely liquid.
3. Results 3.1. Summit Mixing ratios of the monocarboxylic acids 85 cm above the snow were once again high for a remote site (Fig. 1). Both acids often exceeded 1 ppbv and averaged more than 0.4 ppbv (445 and 460 pptv of HAc and HFo, respectively). These means are lower than we measured in 1993–1995 (Dibb et al., 1994, 1998), but are still more than 2-fold higher than Talbot et al. (1992) found in the summer Arctic free troposphere during ABLE 3A. The 2000 Summit means are nearly equal to mean mixing ratios in ‘‘background’’ free tropospheric air above the Canadian boreal forest during ABLE 3B (Lefer et al., 1994). However, ambient mixing ratios of HAc and HFo at Summit are dwarfed by their mixing ratios in firn air. Both monocarboxylic acids were 5–7 times more abundant 10 cm below the air–snow interface than 85 cm above it (Fig. 2). In this experiment, and similar ones with the firn probe at 10 or 20 cm (not shown), mixing ratios in ambient and firn air decreased through the evening, perhaps reflecting decreasing sun elevation. It should be noted that the evening decreases of HAc and HFo in firn air shown in Fig. 2 are modest, especially compared to those of NOx and HONO which drop to nearly ambient levels around midnight (see Dibb et al., 2002). Deeper in the firn the monocarboxylic acid mixing ratios were even more elevated, and no sun synchronous behavior was apparent (Fig. 3). It is striking how constant HAc stayed through this 18-h interval, varying o10% from a mean of 5910 pptv. Formic acid did slowly increase nearly 2 ppbv between 9:00 and 16:00, but was then nearly constant for 9 h. During this experiment, ambient mixing ratios dropped to very low levels between 22:00 and about 2:00. Throughout the season most sampling was conducted between 8:00 and 22:00, but the few experiments that extended through the night are obvious in Fig. 1, showing up as the clusters of samples with mixing ratios near and below 100 pptv. When the patch of snow surrounding the firn probe was alternately shaded and exposed to full sun, the mixing ratios of HAc and HFo did not vary (Fig. 4). This combination of experiments suggests that the influence of sunlight on monocarboxylic acid mixing
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Fig. 1. Mixing ratios of acetic and formic acids 85 cm above the snow at Summit, Greenland in summer 2000.
Fig. 2. Mixing ratios of acetic and formic acids 85 cm above the snow (J) and 10 cm into the snow (K) at Summit on 1 July 2000. Times in this and all Summit plots are local time, 2 h behind GMT.
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Fig. 3. Mixing ratios of acetic and formic acids 85 cm above the snow (J) and 30 cm into the snow (K) at Summit on 3–4 July 2000.
Fig. 4. Mixing ratios of acetic and formic acids 85 cm above the snow (J) and 10 cm into the snow (K) at Summit on 20 June 2000. During the intervals indicated by the shaded bars a 2.4 m 2.4 m patch of shade, centered on the sampling inlet, was created on the snow surface.
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Fig. 5. Profiles of the mixing ratios of acetic and formic acids at different depths in the Summit snowpack. Arrows in upper left corners of the two panels indicate the average mixing ratios at 85 cm above the snow on the two different days. For each profile, measurements were made at successively deeper depth (see text).
ratios in firn air is not direct, in stark contrast to the case for the N oxides (Dibb et al., 2002). Acetic acid mixing ratios in firn air increased with depth (to our maximum depth of 35 cm) in all five firn air profiles measured at Summit (Fig. 5). Formic acid profiles were less consistent, but in four of the five cases mixing ratios were lower at 35 cm than at 25 cm. In three of these profiles maximum mixing ratios occurred at 25 cm. The variability of these depth profiles, especially those measured on the same day, is puzzling. The slow changes (Fig. 2), or nearly constant levels (Fig. 3), observed over periods of hours at a single depth when the probe was not moved suggest that we may not have sampled the exact same stratigraphic layers on successive profiles. It is hard to understand what could cause such large differences on relatively small vertical and horizontal scales, yet still create the consistent increase of HAc with depth. 3.2. South Pole Ambient mixing ratios of HAc and HFo averaged 310 and 159 pptv, respectively, lower than at Summit but still surprisingly high for so remote a site (Fig. 6). Talbot
et al. (1990) reported means of 300 pptv HAc and 400 pptv HFo in the Brazilian Amazon during the wet season, and, as noted above, the means in the summer Arctic troposphere were o300 pptv. Solar elevation changes extremely slowly at South Pole, making diel experiments useless for the investigation of response to varying light levels. We attempted manipulating light levels by shading (as described for Summit) but found very inconsistent results (one dark interval might show no response, yet the next could lead to large increases). Our current hypothesis is that attempting to block light also impacts air flow through the snow, with reduced airflow reducing air-to-snow fluxes, causing temporary increases in mixing ratios in the firn air. Such effects might be larger at South Pole than Summit due to the combination of higher wind speeds and more pronounced surface relief. We intend to explicitly investigate this hypothesis in future studies at Summit. Firn air mixing ratios of the monocarboxylic acids were elevated by at least a factor of 3 compared to ambient values. In the first depth profile both HAc and HFo steadily increased down to 30 cm, but a pair of profiles measured on 23 December showed even larger
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Fig. 6. Mixing ratios of acetic and formic acids 85 cm above the snow at South Pole in December 2000. Time is GMT.
relative variability than we observed at Summit (Fig. 7). Formic acid decreased with depth (though remaining above ambient) in both profiles, but showed up to 600 pptv differences at a common depth over about 5 h. Acetic acid displayed a pronounced minimum in the 15–20 cm range on the first profile that was no longer apparent in the second (due mainly to 400–600 pptv decreases in the deeper horizons between the two measurements). These variations over time are not understood, but they do not negate the primary observation that firn air mixing ratios of HAc and HFo in excess of 1 ppbv are common at the South Pole. Concentrations of Fo and Ac in South Pole snow ranged from 0.05 to 0.3 nmol g 1 (Fig. 8). The peak and baseline values are directly comparable to those characteristic of the upper meters of the snowpack at Summit (see Fig. 15 in Legrand and DeAngelis (1995)). Unlike Summit, where peak concentrations are preserved in winter snow, the peak in the South Pole pit is centered in the previous summer’s snow (starting in spring and extending well beyond the timing of the spring/summer NO3 peak (Whitlow et al., 1992)). In spring snow that accumulated in 2000 a very large enhancement of NO3 is clear (in the top 5 cm (Fig. 8)), but there is no parallel increase in the monocarboxylates.
4. Discussion The mechanisms releasing HAc and HFo into the pore air of snowpacks are not yet clear, but likely include significant production of these acids on the snow grains and/or in the firn air. The magnitude of firn air mixing ratios, and lack of correspondence between depth profiles of firn air and snow concentrations, suggests that simple degassing and desorption of HAc and HFo from the snow grains cannot be their major source in the firn air. On the other hand, possible precursors to HAc and HFo have been shown to be present in abundance in firn air. Release of HCHO from snow into firn air, and subsequently into the atmosphere, is well established at both Summit and Alert (Jacobi et al., 2002; Dassau et al., 2002; Hutterli et al., 1999a; Sumner and Shepson, 1999), and also in Antarctica (Hutterli et al., 1999b). Similarly, CH3CHO was shown to be nearly 3-fold higher in snow pore air than ambient air in northern Michigan (Couch et al., 2000), we expect the same is true at Summit, and perhaps South Pole, though no measurements have yet been made. Some fraction of the carbonyls released into snowpack pore space can be explained by thermal desorption of compounds incorporated into the snowflakes in cloud (e.g., Hutterli et al., 1999a, b), but it is also likely that they are produced by oxidation of
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Fig. 7. Profiles of the mixing ratios of acetic and formic acids at different depths in the South Pole snowpack. Arrows in upper left corners of the two panels indicate the average mixing ratios at 85 cm above the snow on the two different days. For each profile, measurements were made at successively deeper depth (see text).
Fig. 8. Concentrations of acetate and formate in the upper 30 cm of the snowpack at South Pole. Nitrate concentrations are included to indicate the depth of the summer 1999 layers (just above 20 cm).
organic compounds in the snow (e.g., Sumner and Shepson, 1999). Chemical production of carbonyls in the snowpack will be enhanced by elevated OH in firn air (Yang et al., 2002). Swanson et al. (2002) demonstrate that several short-lived alkenes are elevated in firn air relative to ambient air at Summit, with photooxidation of unknown organic compounds suggested as their source. Part of the O3 destruction in Summit firn
described by Peterson and Honrath (2001) might be linked to the formation of monocarboxylic acids from these very reactive hydrocarbons. Mixing ratios of HCHO and the alkenes in firn air respond to diel and fast variations in light, unlike HAc and HFo. We hypothesize that the monocarboxylic acids are produced from a number of organic compounds in firn air and snow, and that they build up in firn air due to inefficient
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removal to acidic ice surfaces (Legrand and DeAngelis, 1995). Higher mixing ratios in Summit firn air than at South Pole presumably reflect higher concentrations of organic matter in the snow.
5. Conclusions The elevated mixing ratios of monocarboxylic acids in firn air must be coming from the surrounding snow. We suspect that much of the HAc and HFo in firn air is produced there from reactive VOCs, which in turn are produced by oxidation of organics in the snow. Persistent gradients between firn air and the overlying atmosphere should sustain air-to-snow fluxes, maintaining higher mixing ratios of HAc and HFo in the lowermost atmosphere at South Pole and Summit than at many rural and remote sites that are much closer to biogenic and anthropogenic sources. Quantifying these fluxes, and their impact on the lower troposphere, is one of the primary objectives of planned future work. In situ production of monocarboxylic acids in snow presents an additional complication in the interpretation of Ac and Fo depth profiles in polar ice cores. It is hard to imagine how snow exposed to firn air with mixing ratios of HAc and HFo in excess of ppbv levels could retain a non-biased record of remote tropospheric mixing ratios an order of magnitude lower.
Acknowledgements This research was supported by the Arctic System Science, Arctic Natural Sciences and Antarctic Ocean and Climate programs of the Office of Polar Programs at NSF. Logistic support from PICO (in Greenland) and airlift by the NY ANG (both poles) is appreciated. We are grateful to the Danish Polar Board and the Greenlandic Home Rule government for granting us permission to work at Summit. Special thanks to Manuel Hutterli for sharing samples and pit digging at South Pole.
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