S. Langer, Brian T. McGovney, B. Finlayson‐Pitts
Jun 15, 1996
Citations
2
Influential Citations
12
Citations
Journal
Geophysical Research Letters
Abstract
GEOPHYSICAL RESEARCH LETTERS, VOL. 23, NO. 13, PAGES 1661-1664, JUNE 15, 1996 The dimethyl sulfide reaction with atomic chlorine and its implications for the budget of methyl chloride Sarka Langer, Brian T. McGovney, and Barbara J. Finlayson-Pitts University of California, Irvine, Department of Chemistry, Irvine, California Robert M. Moore Dalhousie University, Department of Oceanography, Halifax, Nova Scotia, Canada Abstract. DMS is produced in large quantities by natural also consistent with the production of CI atoms at these levels oceanic processes. Its chemistry and emissions have been [Jobson et al., 1994; Singh et al., 1996; Wingenter et al., 1996]. postulated to play a role in the radiative balance of the Keene and coworkers [Keene et al., 1996] also suggest that atmosphere. While its reactions with OH and NO3 radicals are contribution of chlorine atoms to the net loss of DMS in the well known, the reaction with chlorine atoms in the marine marine boundary layer may contribute to the observed diurnal boundary layer has also been suggested recently. If this produces variation of DMS. CH3CI, it would contribute to the global budget of this naturally In a kinetic study of the DMS-CI reaction, Wine and occuring halocarbon. Experiments were carried out to measure coworkers [Stickel et al., 1992] proposed a two-channel reaction the yield of CH3CI in the CI+DMS reaction. CH3CI was indeed mechanism: formed in this reaction in laboratory experiments at 1 atto. and CH3SCH 3 + CI 298 K, with a small yield of (1.34 n 0.07) x 10 '3. This yield is CH3SCH 2 + HCI sufficiently small that the contribution to the global CH3CI 3 + CI [CH3S(CI)CH3]* (2) budget is estimated to be _< 2% and it is unlikely to be responsible CH3SCH for the anomalously high CH3CI concentrations measured At one atmosphere pressure and room temperature, channel (1) recently over the Labrador Sea. accounted for 40-50% of the overall reaction. The final products of the chlorine addition path (2) are not known, but Stickel et al. (1992) proposed methyl chloride as one possibility: Introduction Dimethyl sulfide (DMS) is 13roduced by natural processes in [CH3S(CI)CH3] surface seawater in large quantities. DMS emissions from the M ocean have been estimated to r•ge from 12 to 35 x 10 •2 g S/yr Andreae, 1990; Spiro et al., 1992; Pham et al., 1995]. Atmospheric degradation of DMs initiated by OH and NO3 leads Cl2 or HOCI, in the marine boundary layer. Theft calculate that chlorine atom concentrations as high as 104 - 10 • cm '3 could be generated upon photolysis of these precursors. More recently, Spicer and coworkers (personal communication, 1996) identified Cl2 over the North Atlantic Ocean using atmospheric pressure ionization mass spectrometry. Several other recent studies are Copyright 1996 by the American Geophysical Union. Paper number 96GL01427 0094-853 4/96/96GL-01427505.00 (3a) CH3SCI + CH3 (3b) CH3S + CH3CI (3c) [see estimates by Bates et al., 1987; Erickson et al., 1990; to products which can contribute to acidic precipitation and influence global climate [Charlson et al., 1987]. There has also been recent interest in possible oxidation by CI atoms [Keene et al., 1996]. The production of atomic chlorine at the ocean surface is currently an area of active research. There are a variety of potential reactions producing photochemically labile chlorine atom precursors, [Finlayson-Pitts, 1993; Graedel and Keene, 1995]. Keene, Pszenny and coworkers [Keene et al., 1993; Pszenny et al., 1993] have used a mist chamber technique to identify chlorine-containing compounds other than HCI, possibly CH3S(CI)CH Methyl chloride, the most abundant halocarbon in the troposphere, has a major source in seawater [Singh et al., 1983]. It is also produced by microbial fermentation and biomass burning, and is of interest as a natural source of stratospheric chlorine. The total budget of CH3CI is estimated to be 3.5 x 10 •2 g/yr [Koppmann et al., 1993]. Measurements of methyl chloride in water and air samples were made during a cruise of CSS Hudson in the Labrador Sea in late May to early June 1994, and analyzed by oxygen-doped GC- ECD. While an average mixing ratio of methyl chloride in the troposphere of the northern hemisphere is around 530 pptv [Koppmann et al., 1993], it was found that occasionally much higher concentrations were measured in the atmosphere over the Labrador Sea, up to a maximum of 870 pptv. In contrast, surface waters were consistently undersaturated, the average saturation level being 83%. The local sea surface was therefore not the source of the anomalously high atmospheric signals. One possible source is the reaction (3c) of Cl atoms with DMS. We report here the results of an investigation of the DMS-CI reaction as a potential source of CH3CI and discuss the implications for chemistry in the marine boundary layer.