Distribution of natural halocarbons in marine boundary air over the Arctic Ocean

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 40, , doi: /grl.50734, 2013 Distribution of natural halocarbons in marine boundary air over the Arctic Ocean Yoko Yokouchi, 1 Jun Inoue, 2,3 and Desiree Toom-Sauntry 4 Received 27 May 2013; revised 3 July 2013; accepted 10 July 2013; published 30 July [1] Ongoing environmental changes in the Arctic will affect the exchange of natural volatile organic compounds between the atmosphere and the Arctic Ocean. Among these compounds, natural halocarbons play an important role in atmospheric ozone chemistry. We measured the distribution of five major natural halocarbons (methyl iodide, bromoform, dibromomethane, methyl chloride, and methyl bromide) together with dimethyl sulfide and tetrachloroethylene in the atmosphere over the Arctic Ocean (from the Bering Strait to 79 N) and along the cruise path to and from Japan. Methyl iodide, bromoform, and dibromomethane were most abundant near perennial sea ice in air masses derived from coastal regions and least abundant in the northernmost Arctic, where the air masses had passed over the ice pack, whereas methyl chloride and methyl bromide showed the opposite distribution pattern. Factors controlling those distributions and future prospects for natural halocarbons in the Arctic are discussed. Citation: Yokouchi, Y., J. Inoue, and D. Toom-Sauntry (2013), Distribution of natural halocarbons in marine boundary air over the Arctic Ocean, Geophys. Res. Lett., 40, , doi: /grl Introduction [2] Various volatile organic compounds (VOCs) that are emitted from the ocean are likely to be involved in several important atmospheric processes. Oceanic dimethyl sulfide (DMS) can be a source of cloud condensation nuclei [Charlson et al., 1987] and, thus, may influence climate. Short-lived (days to weeks) halocarbons such as bromoform (CHBr 3 ) and methyl iodide (CH 3 I), which are derived mostly from the ocean, influence tropospheric chemistry by supplying reactive halogen to the troposphere, and after convective transport to the stratosphere, they may also contribute to the destruction of stratospheric ozone [Montzka and Reimann, 2011]. The ocean is also a main source of methyl chloride (CH 3 Cl) and methyl bromide (CH 3 Br), which have a longer lifetime ( year) than CHBr 3 or CH 3 I. These two halocarbons are responsible for 17% and 34% of stratospheric Additional supporting information may be found in the online version of this article. 1 Center for Environmental Measurement and Analysis, National Institute for Environmental Studies, Tsukuba, Japan. 2 Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan. 3 National Institute of Polar Research, Tachikawa, Japan. 4 Science and Technology Branch, Environment Canada, Toronto, Ontario, Canada. Corresponding author: Y. Yokouchi, Center for Environmental Measurement and Analysis, National Institute for Environmental Studies, 16-2, Onogawa, Tsukuba, Ibaraki, Japan. (yokouchi@nies.go.jp) American Geophysical Union. All Rights Reserved /13/ /grl chlorine and bromine, respectively [Montzka and Reimann, 2011]. In some regions, however, the ocean acts as a major sink of CH 3 Cl and CH 3 Br [Tokarczyk et al., 2003]. For these reasons, these marine-derived VOCs have been measured in and over many of the world s oceans to determine their spatial variations and possible factors controlling the oceanatmosphere fluxes of these compounds. [3] Nevertheless, few measurements of marine-derived VOCs in the atmosphere over the Arctic Ocean have been reported, and most reported measurements are of DMS [Ferek et al., 1995; Leck et al., 1996]. Emissions of several species of natural halocarbons (CHBr 3, CH 2 Br 2, CH 2 I 2, etc.) by Arctic macroalgae have been reported [Laturnus, 1996; Sturges et al., 1992], but measurements of these species in the Arctic atmosphere have been carried out only at coastal sites. For example, campaign-based halocarbon measurements have been made at Alert, Canada, and at Spitzbergen, Norway [Bottenheim et al., 1990; Hopper et al., 1994; Schall and Heumann, 1993; Yokouchi et al., 1994], and long-term monitoring of selected halocarbons is being performed at Alert [Yokouchi et al., 1996, 2008, 2012]. At Alert, long-term measurements of atmospheric CH 3 I have recently shown an increasing trend, particularly in summertime, suggesting a possible relationship between the atmospheric CH 3 I concentration and global environmental change [Yokouchi et al., 2012]. [4] The ongoing global warming may cause unexpected changes in VOC fluxes between the atmosphere and the Arctic Ocean. Therefore, in this study, we investigated the current distribution patterns of natural halocarbons and of DMS in the atmospheric marine boundary layer over the Arctic Ocean, from the Bering Strait to the far north of the Alaska Peninsula (to 79 N), including the region close to sea ice. 2. Sampling and Analysis Methods [5] Air was sampled over the North Pacific, the Bering Sea, and the Arctic Ocean during Arctic Cruise MR09-03 of R/V Mirai from 28 August to 25 October 2009, the season of lowest sea ice coverage (Figure 1). During the cruise, air samples were collected on the outgoing cruise from offshore Japan to the Bering Strait (30 August to 9 September), over the Arctic Ocean (10 September to 12 October), and during the returning cruise from the Bering Strait to offshore Japan (12 21 October) (Figure 2). The air samples were collected in evacuated stainless steel canisters (6 L, SilicoCan, Restek Co., Ltd.) using a metal bellows pump at about 18 m above sea level on the compass deck. After the cruise, the VOCs in the samples were analyzed by preconcentration/capillary gas chromatography/mass spectroscopy in a laboratory at the National Institute for Environmental Studies. The

2 Figure 1. Cruise track of R/V Mirai over the Arctic Ocean with sea ice concentration (%) derived from the Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E) on 21 September A station at Alert is indicated by a red dot. sampling and analytical methods have been described in detail elsewhere [Li et al., 1999; Yokouchi et al., 1999]. Selected ion monitoring was employed to analyze CH 3 Cl, CH 3 Br, CH 3 I, CHBr 3, dibromomethane (CH 2 Br 2 ), DMS (CH 3 SCH 3 ), and tetrachloroethylene (C 2 Cl 4 ), as well as 20 other VOCs. Monitored ions for quantification were m/z (mass to charge ratio) 50 for CH 3 Cl, m/z 94 for CH 3 Br, m/z 142 for CH 3 I, m/z 173 for CHBr 3, m/z 174 for CH 2 Br 2, m/z 62 for DMS, and m/z 166 for C 2 Cl 4. A standard mixture gravimetrically prepared in a high-pressure 10 L aluminum cylinder containing 100 parts per trillion of each target compound (except 500 ppt of CH 3 Cl) (Taiyo Toyo Sanso Co., Ltd.) was used for calibration. Because such low levels of gases might decrease over time, the actual mixing ratios of the gases in the standard cylinder were periodically compared with a dynamically diluted standard made with a 1 ppm gravimetric standard gas; the mixing ratios were also confirmed by comparison with a new gravimetric working standard at replacement. [6] Previous testing of the stability of these compounds in the sampling canisters [Yokouchi et al., 1999] showed no significant change in their mixing ratios 6 months after sampling, except for that of DMS, which decreased by approximately 10% each month. All samples used in this study were analyzed within 3 months of collection. Therefore, the reported mixing ratios of DMS might be underestimated by up to 30%. 3. Results and Discussion 3.1. Latitudinal Variation [7] In this study, we focused on the observed mixing ratios of five natural halocarbons (CH 3 Cl, CH 3 Br, CH 3 I, CHBr 3, and CH 2 Br 2 ), DMS, and an anthropogenic halocarbon (C 2 Cl 4 ) and examined their latitudinal distributions together with that of the sea surface temperature (SST) (Figures 3a 3h). We also calculated the 7 day back trajectory at each sampling point with the NOAA Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model [Rolph, 2013] (supporting information). [8] The observed mixing ratios of C 2 Cl 4 (Figure 3g) were mostly close to the summertime lowest mixing ratio at Alert in the preceding 3 years ( ppt, unpublished) during the long-term halocarbon monitoring project there [Yokouchi et al., 2008]. The mixing ratio was consistently about 20% lower during the outgoing leg of the cruise (early September) than during the returning leg (mid-october). We attribute this difference to a higher photochemical loss of C 2 Cl 4 in early September than in mid-october. An unusually high C 2 Cl 4 mixing ratio was observed in October at around 42 N. The back trajectory analysis showed that this air mass came from Chinese industrial areas via mainland Japan (supporting information). In contrast, the two lowest mixing ratios were observed in late August at 41 N 44 N, when the air mass originated at lower latitudes over the open Pacific Ocean. [9] Among the natural VOCs, the mixing ratios showed rather different latitudinal variations. Those of the three methyl halides tended to decrease toward the north (higher latitudes) and were positively correlated with SST: CH 3 Cl, Figure 2. Air sampling points for VOCs during cruise MR09-03 (2009). (red) From offshore Japan to the Bering Strait (outgoing), 30 August to 9 September. (blue) In the Arctic Ocean, 10 September to 12 October. (green) From the Bering Strait to offshore Japan (returning), October. 4087

3 Figure 3. Latitudinal variations of mixing ratios of (a) CH 3 Cl, (b) CH 3 Br, (c) CH 3 I, (d) CHBr 3,(e)CH 2 Br 2, (f) DMS, and (g) C 2 Cl 4, and (h) SST measured at the VOC sampling points during cruise MR See Figure 2 for the explanation of the colors. R = 0.68; CH 3 Br, R = 0.62; and CH 3 I, R = 0.78 (for each, n = 53, P < 0.001). This result is consistent with previously reported latitudinal distributions of CH 3 Cl, CH 3 Br, and CH 3 I[Khalil and Rasmussen, 1999; Yokouchi et al., 2000a, 2000b, 2008]. The northward decreasing trend of CH 3 I continued over the Arctic Ocean (Figure 3c). [10] In contrast, mixing ratios of CHBr 3 and CH 2 Br 2 (Figures 3d and 3e) showed neither a directional trend nor a correlation with SST. They generally increased northward to around 60 N, and over the Arctic Ocean, they showed a slight decreasing trend with latitude, although CHBr 3 was highly variable there. [11] DMS showed much higher variability than any of the halocarbons (Figure 3f). South of the Bering Strait, its mixing ratios were greatly different between the outgoing and returning cruises, although they collectively were correlated with SST (R = 0.60, n = 53, P < 0.001). This high variability of DMS can be explained by drastic seasonal and spatial changes in its emission related to biological activity [Leck and Persson, 1996] Distributions Over the Arctic Ocean [12] Over the Arctic Ocean, 38 samples were collected between 10 September and 12 October 2009, but one (A-23) was lost because of leakage (Figure 4). We compared mean VOC mixing ratios over the Arctic Ocean with mixing ratios measured during the same period at Alert (82.5 N, 62.5 W), which is situated on the northeastern tip of Ellesmere Island, on the opposite side of the Arctic Ocean to our study area (Table 1). All data from Alert were in the range of our measurements over the Arctic Ocean and close to our mean values, except those for the highly variable DMS. This result suggests that measurements made at the Alert monitoring site are representative of halocarbon mixing ratios over the Arctic Ocean. [13] The spatial distribution of DMS (Figure 4f) showed that mixing ratios higher than 100 ppt were detected only in the early samples (A-4, A-8, and A-10); those in the samples collected after 18 September were all lower than 40 ppt. This high temporal variability, which is consistent with the exponential decrease of DMS over the central Arctic Ocean in 4088

4 (a) (b) (c) (d) (e) (f) (g) Figure 4. Distributions of (a) CH 3 Cl, (b) CH 3 Br, (c) CH 3 I, (d) CHBr 3, (e) CH 2 Br 2, and (f) DMS over the Arctic Ocean. (g) Sampling sites of the numbered Arctic samples (A-1 to A-38; A-23 is missing because of sample leakage). early autumn reported by Leck et al. [1996], makes it difficult to interpret its spatial distribution pattern. Therefore, we use the DMS distribution only as reference data with which to compare the natural halocarbon distributions Methyl Iodide [14] The mixing ratio of CH 3 I over the Arctic Ocean generally decreased northward, except for the measurements at sites A-7, A-9, and A-10 (Figure 4c, corresponding to the circled outlying data points in Figure 3c). Back trajectory analyses showed that the air masses at sites A-9 and A-10 were from the south, around the Alaskan coast, suggesting that they might have been affected by seawater warmer than the cold water at the collection site. The air mass at A-7 had also come from the south, but from the East Siberian coast. The lowest mixing ratio of CH 3 I, 0.32 ppt, was observed at the northernmost site (A-15), where the sea surface was covered with new ice [Inoue et al., 2011]. Moreover, that air mass had passed over the polar ice cap. [15] Long-term monitoring of atmospheric CH 3 I at Alert [Yokouchi et al., 2012] has revealed that summertime CH 3 I mixing ratios recently increased, from approximately 0.2 ppt during to 0.4 ppt during

5 Table 1. Mean, Minimum, Maximum, and Relative Standard Deviation of VOC Mixing Ratios of Samples From Over the Arctic Ocean Collected During Cruise MR09-03 and Mixing Ratios Measured at Alert a Over Arctic Ocean (10 September to 12 October, 37 samples) Alert Compound Mean Lowest Highest RSD (%) 11 September 23 September 7 October CH 3 Cl CH 3 Br CH 3 I CHBr CH 2 Br DMS 24.3 N.D. b C 2 Cl a The data from Alert are unpublished (Yokouchi et al.), except for CH 3 I[Yokouchi et al., 2012]. b N.D. denotes not detected (less than 0.5 ppt for DMS). Yokouchi et al. [2012] explained this temporal change as an effect of the considerable decline of Arctic sea ice coverage and as an effect of global-scale, SST-related, decadal anomalies. The spatial distribution of CH 3 I over the Arctic Ocean found in this study is consistent with this explanation Bromoform and Dibromomethane [16] The highest CHBr 3 mixing ratios were detected at A-7 (3.4 ppt), A-8 (3.0 ppt), and A-27 (2.9 ppt). Sites A-7 and A-8 are close to the regions of perennial sea ice and were affected by air masses from the coastal region (see supporting information). Site A-27 is near the Alaskan coast. These high mixing ratios may be due to emissions by ice algae growing on the perennial sea ice and by macroalgae growing on the Alaskan coast, which are important sources of CHBr 3 [Hughes et al., 2009; Sturges et al., 1992]. Although CH 2 Br 2 is also emitted by algae, its mixing ratios showed little variation, ranging from 0.9 to 1.9 ppt with a relative standard deviation (RSD) of 14%. The different distribution patterns of these two bromine-containing compounds can be explained by the longer atmospheric lifetime of CH 2 Br 2 (123 days) compared with that of CHBr 3 (23 days) [Montzka and Reimann, 2011]; the abundance of the short-lived CHBr 3 is more likely to show the effects of emissions by local sources. As with CH 3 I, the lowest concentrations of these halocarbons, 0.7 ppt for CHBr3 and 1.0 ppt for CH 2 Br 2, were also found in a northernmost sample (A-14). [17] The mixing ratios of CHBr 3 and CH 2 Br 2 showed a small positive correlation with each other (R CHBr3-CH2Br2 =0.40, n =37,P < 0.05), and they were also correlated with the CH 3 I mixing ratio (R CHBr3-CH3I =0.41 and R CH2Br2-CH3I =0.53; for both, n =37,P < 0.05). Furthermore, all three of these marinederived VOCs were correlated with DMS (R CH2Br2- DMS = 0.39, R CH3I-DMS = 0.39, and R CHBr3-DMS = 0.38; for all, n =37,P < 0.05) Methyl Chloride and Methyl Bromide [18] The mixing ratios of CH 3 Cl and CH 3 Br over the Arctic Ocean had the smallest variability, within 4% 5%, among the halocarbon compounds and DMS (Table 1), and their spatial distributions were different from those of the other marine-derived VOCs. At the northernmost sites (A-13, A-14, and A-15), where the lowest mixing ratios of CH 3 I, CHBr 3,CH 2 Br 2, and DMS were detected, those of CH 3 Cl and CH 3 Br were average or even high; the mixing ratios of CH 3 Cl (504 ppt) and CH 3 Br (8.3 ppt) were higher at A-14 than at all other sites, except those in the Bering Strait. The difference between these two methyl halides and the other compounds can be explained by the cold ocean being undersaturated with respect to CH 3 Cl and CH 3 Br and thus acting as a sink of atmospheric CH 3 Cl and CH 3 Br [Groszko and Moore, 1998; MacDonald and Moore, 2007; Moore et al., 1996]. Because the polar ice cap prevents oceanic uptake of CH 3 Cl and CH 3 Br, their mixing ratios might be higher in an air mass that has traveled over the ice cap than in one that has passed only over ocean water. Thus, by preventing any air-seawater interaction, sea ice might cause oceanic uptake as well as oceanic emission of gases to decrease. [19] CH 3 Cl and CH 3 Br mixing ratios were lowest at A-9, where that of CH 3 I was highest and those of CHBr 3 and DMS were also high. Although as mentioned, this site is adjacent to perennial sea ice, and the air mass originated over the Alaskan coast; these site characteristics cannot explain the low mixing ratios of CH 3 Cl and CH 3 Br. One possible explanation is that the air mass experienced more air-seawater interactionasittraveledovertheocean,resultingindecreased CH 3 Cl and CH 3 Br (for which the ocean is a sink) and increased CH 3 I and CHBr 3 (for which the ocean is a source). [20] Thus, future declines of Arctic sea ice might decrease the concentrations of these two stratospheric ozone-depleting gases. The related oceanic warming, however, might change the Arctic Ocean from a sink to a source of CH 3 Cl and CH 3 Br [MacDonald and Moore, 2007]. 4. Conclusion [21] Five natural halocarbons together with DMS and C 2 Cl 4 in the atmosphere were measured over the Arctic Ocean and along the paths of the outgoing and returning cruise of R/V Mirai from Japan. Their average mixing ratios over the Arctic Ocean were 472 ppt for CH 3 Cl, 7.6 ppt for CH 3 Br, 0.52 ppt for CH 3 I, 1.9 ppt for CHBr 3, and 1.2 ppt for CH 2 Br 2. These values are similar to the mixing ratios of these VOCs measured at Alert. The spatial distributions of CH 3 I, CHBr 3, and CH 2 Br 2, however, showed important differences from those of CH 3 Cl and CH 3 Br. The atmospheric concentrations of the former group were highest near perennial sea ice and in air masses that had originated over the Alaskan coast, and they were lowest at the northernmost sites, in air masses that had traveled over the polar ice cap. The latter group showed largely the opposite pattern. This difference can be explained by the Arctic Ocean being a source of CH 3 I, CHBr 3, and CH 2 Br 2, but a sink of CH 3 Cl and CH 3 Br. Thus, air-sea interaction is likely one of the most important factors controlling the spatial distributions of these 4090

6 atmospheric natural halocarbons. Therefore, sea ice retreat can be expected to strongly affect not only the marine-derived compounds CH 3 I and CHBr 3 but also CH 3 Cl and CH 3 Br through its effect on air-seawater interactions. [22] More intensive observations are required to fully understand the interactions between natural halocarbons and the Arctic Ocean and to evaluate the impact of the changing Arctic environment on these interactions. [23] Acknowledgments. We thank the chief scientists, captain, and crew of R/V Mirai (cruise MR09-03) for their cooperation with the sample collection. We gratefully acknowledge Satoshi Okumura, Souichiro Sueyoshi, Norio Nagahama, and Ryo Kimura of Global Ocean Development, Inc., for their help with the sampling. We also thank Yoko Inuzuka for her help in laboratory work and the staff of the Meteorological Service of Canada at the Alert Monitoring Station for the sampling at the station. 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