Environ. Sci. Technol. 2005, 39, 8812-8816
Abiotic Source of Reactive Organic Halogens in the Sub-Arctic Atmosphere? LUCY J. CARPENTER,* JAMES R. HOPKINS, CHARLOTTE E. JONES, ALASTAIR C. LEWIS, RAJENDRAN PARTHIPAN, AND DAVID J. WEVILL Department of Chemistry, University of York, York YO10 5DD, U.K. LAURIER POISSANT, MARTIN PILOTE, AND PHILIPPE CONSTANT Service Mete´orologique du Canada, Montre´al, Que´bec, H2Y 2E7, Canada
Recent theoretical studies indicate that reactive organic iodocarbons such as CH2I2 would be extremely effective agents for tropospheric Arctic ozone depletion and that iodine compounds added to a Br2/BrCl mixture have a significantly greater ozone (and mercury) depletion effect than additional Br2 and BrCl molecules. Here we report the first observations of CH2I2, CH2IBr, and CH2ICl in Arctic air, as well as other reactive halocarbons including CHBr3, during spring at Kuujjuarapik, Hudson Bay. The organoiodine compounds were present at the highest levels yet reported in air. The occurrence of the halocarbons was associated with northwesterly winds from the frozen bay, and, in the case of CHBr3, was anticorrelated with ozone and total gaseous mercury (TGM), suggesting a link between inorganic and organic halogens. The absence of local leads coupled with the extremely short atmospheric lifetime of CH2I2 indicates that production occurred in the surface of the sea-ice/overlying snowpack over the bay. We propose an abiotic mechanism for the production of polyhalogenated iodo- and bromocarbons, via reaction of HOI and/or HOBr with organic material on the quasiliquid layer above sea-ice/snowpack, and report laboratory data to support this mechanism. CH2I2, CH2IBr, and other organic iodine compounds may therefore be a ubiquitous component of air above sea ice where they will increase the efficiency of bromine-initiated ozone and mercury depletion.
Introduction The role played by bromine during polar sunrise ozone and mercury depletion events has been described in numerous studies (1-3 and references therein) and the suggested mechanism involves uptake of atmospheric HOBr into slightly acidic and concentrated brine on the sea-ice surface resulting in release of gaseous Br2 and BrCl, which are rapidly photolyzed to halogen atoms. Every HOBr molecule entering the liquid phase has the potential to release two bromine * Corresponding author e-mail:
[email protected]. 8812
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atoms to the gaseous phase, resulting in an exponential increase of Br, the so-called “bromine explosion”. In the marine environment, the presence of highly photolyzable iodine-containing CH2I2, CH2IBr, and CH2ICl molecules has been demonstrated (4 and references therein), which leads to rapid production of iodine atoms whose main fate is to react with ozone, forming the IO radical. Halogen-related ozone loss is controlled by the fraction of halogen oxide (XO, where X ) Cl, Br, or I) radicals that, rather than photolyzing to X + O, react to reform chain-carrying halogen atoms without releasing an oxygen atom. The presence of iodine dramatically increases this fraction, thus iodine compounds added to a Br2/BrCl mixture have a significantly greater ozone depletion effect than additional Br2 and BrCl molecules (5, 6). In a theoretical study, Calvert and Lindberg (5, 6) found that CH2I2 is potentially an extremely effective agent for tropospheric Arctic ozone and mercury depletion, only slightly less efficient than I2 per molecule and more efficient than IBr and ICl, and that relatively small inputs could cause a significant enhancement of ozone depletion relative to that from bromine and chlorine chemistry alone. Although there has been much speculation on the sources of the particulate iodine observed in spring and autumn (7), the source of iodine is unknown. Certainly, the seasonality of observed aerosol iodine is not consistent with the only known source of Arctic iodine, the under-ice spring bloom of ice algae that produce a wealth of organohalogens (8, 9). It is worth noting that, until now, there are no reported Arctic measurements of CH2I2 and CH2IBr which have proved to be the most effective organic I atom producers in coastal environments (4, 10). To investigate potential organic sources of Arctic iodine, we made in situ measurements of reactive halocarbons and collected supporting data in March 2004 at Kuujjuarapik on the east shore of Hudson Bay (55.30° N, 77.73° W), during which time the bay was frozen, with a covering of snow.
Experimental Section Halocarbon Measurements. Hourly in situ measurements of the reactive halocarbons were made using an automated GC-MS system. Ambient air was sampled through a 1/2-in. PFA Teflon line approximately 20 m long, using an Aerospace metal bellows pump at a flow rate of 20 L min-1. The inlet was approximately 10 m from any building and 1 m above ground level. The pump was housed in an aluminum box located outside the laboratory and connected via 3 m of 1/ -in. stainless steel tubing to the GC-MS instrument. 8 Instrument zeros were determined by flowing high purity N2 gas through the entire manifold with sub-sampling into the instrument. A full description of the instrument and calibration methods is given in Wevill and Carpenter (11). Mercury and O3 Measurements. Total gaseous mercury (TGM) was measured with an automatic analyzer (Tekran 2537A). The analytical train of this instrument is based on the amalgamation of mercury onto a pure gold surface followed by a thermodesorption and analysis by cold vapor atomic fluorescence spectrophotometry (CVAFS) (λ ) 253.7 nm) providing analysis of TGM in air at sub-ng m-3 levels. A dual cartridge design allowed alternate sampling and desorption, resulting in continuous measurement of mercury in the air stream. The analyzer was programmed to sample the air at a flow of 1.5 L min-1 at 5-min intervals. Particulate matter was removed by a 47-mm diameter Teflon filter (0.45 µm). Ozone concentration was measured with a TECO 49 analyzer. The ozone monitoring system was designed to continuously (5 s) measure the concentration with stateof-the-art instrumentation interfaced with a powerful data 10.1021/es050918w CCC: $30.25
2005 American Chemical Society Published on Web 10/14/2005
FIGURE 1. CH2I2 (red), CH2IBr (green), CH2ICl (blue), net solar radiation (pink - unavailable before March 7) and wind direction (black) during March 5-10. logger. The analyzer was calibrated with zero air and ozone of known concentration. Ambient sampling was through a 1/ -in. Teflon line at 3 m above ground level. 4 Laboratory Experiments with I2/HOI and Fulvic Acid. The formation of iodocarbons from reactions of I2/HOI with fulvic acid was investigated using 10 µmol L-1 of aqueous I2 solution (which exists in equilibrium with HOI in the presence of water), prepared by dissolving solid I2 (>99.8%, Riedel-de Hae¨n) into ultrahigh-purity deionized water. A 10-µL aliquot of fulvic acid (Premium Grade, General Hydroponics Europe) was injected into an airtight 500-mL round-bottomed flask containing the I2/HOI solution. The reaction vessel was covered in aluminum foil and reactions proceeded at ambient laboratory temperatures of between 20 and 25 °C. Samples (10 mL) were extracted periodically via a gastight syringe and analyzed for halocarbons by purge-and-trap GC-MS.
Results and Discussion Figure 1 shows the pronounced diurnal cycles observed in CH2I2, CH2IBr, and CH2ICl concentrations during the March 5-9 sampling period, when wind speeds were low ( Br > Cl (19, 27).
FIGURE 5. Reaction profile of CH3I (dashed line), CH2I2 (gray line), and CHI3 (black line) production following addition of 10 µL of fulvic acid to 0.5 L of 10 µmol L-1 I2/HOI solution. surface, may result from either atmospheric deposition or from reactions in the liquid phase
HOBr + I- f HOI + BrThe formation of trihalomethanes in natural waters via haloform reactions of HOI with dissolved organic material is well-known (e.g., 24). Halogenation of compounds containing hydrogens R- to keto groups is both acid and base catalyzed (25) and leads to production of CH3X, CH2X2, and CHX3, with relative amounts depending upon the reaction conditions. We carried out laboratory experiments using fulvic acid as a proxy for the organic matter content of sea ice (26), which behaves as an organic matter “tank” wherein the humification process proceeds as phytoplanktonic material is trapped during its formation. We observed that CHI3 and CH2I2 were the major products of reaction with HOI at pH 6.5 (Figure 5). The reason for the depletion of the halocarbons with time is currently being investigated. We did not monitor CHI3 during the Hudson Bay field campaign, but if the mechanism responsible for the production of CH2I2 and other polyhalomethanes observed at Kuujjuarapik follows our laboratory experiments, then it is likely that CHI3 and/or its decomposition product I2 were also present. The
A haloform-type reaction is widely believed to be the source of volatile polyhalogenated compounds from macroalgae (28) but so far has not been invoked to explain halocarbon production in sea-ice/snowpack. This mechanism links the release of both inorganic and organic bromine and iodine and is consistent with our and other published observations that CHBr3 is inversely correlated to O3 and positively correlated to inorganic Br (29, 30). Further, it might explain the so-far puzzling enrichment in CHBr3 observed in upper sea-ice (enhanced in salinity but low in chlorophyll) of the Canadian Arctic (31). We suggest that, under conditions where rapid release of Br2 and BrCl occurs from sea salt particles on the snowpack, sea-ice and frost flowers, release of organic bromine and iodine (and possibly inorganic iodine) may also occur. Our observed total organic reactive iodine mixing ratios of up to 5 pptv (sum of CH2I2 + CH2IBr + CH2ICl) are of the same order of magnitude as BrCl and Br2 mixing ratios of 0-35 pptv and 0-25 pptv, respectively, observed during polar sunrise at Alert, Canada (2, note many of the observations were below the detection limit). Since Calvert and Lindberg’s study (5, 6) using currently available kinetic data found that, per molecule, CH2I2 and other iodine compounds added to a Br2/BrCl mixture have a significantly greater ozone and mercury depletion effect than additional Br2 and BrCl molecules, halogen chemistry in the polar boundary layer may be more efficient than previously thought.
Acknowledgments We acknowledge the Natural Environment Research Council (grant NER/A/S/2001/01064) for funding this project. We are also grateful to Claude Tremblay (Centre d’ EÄ tudes Nordiques, Kuujjuarapik) for his assistance in organizing the field campaign, and to Christina Peters (University of Heidelberg) and Dr. Gerd Ho¨nninger (Meteorological Service of Canada, Toronto, now deceased) for useful discussions. We gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for providing the HYSPLIT transport and dispersion model. D.J.W. thanks the EPSRC for award of a studentship. VOL. 39, NO. 22, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Received for review May 13, 2005. Revised manuscript received August 5, 2005. Accepted September 15, 2005. ES050918W
