The natural chemistry of inorganic chlorine in the lower atmosphere: A potential source for organochlorine compounds

08/2008

William C Keene

William Keene is a Research Associate Professor of Environmental Sciences at the University of Virginia, Charlottesville, Virginia, USA. He has published approximately 60 refereed scientific papers including many assessing the chemistry of reactive chlorine compounds in the troposphere. Much of his current research focuses on multi-phase chemical transformations involving inorganic halogens in marine air and associated impacts on tropospheric oxidizing capacity, the chemical cycling of other important atmospheric constituents, and climate. Professor Keene directs the International Global Atmospheric Chemistry Program's Reactive Chlorine Emissions Inventory; convened the special sessions "Halogens in the Troposphere" and "Atmospheric Chemistry Inventories" during the 1993 and 1997, respectively, Fall Meetings of the American Geophysical Union; and served on the Scientific Committee for the International Conference on Naturally Produced Organohalogens (proceedings volume published by Kluwer, 1995)
The injection into marine air of sea-salt aerosol generated by wind stress at the ocean surface is the major global source of atmospheric chlorine; production rates simulated by global circulation models are about 2 billion metric tonnes Cl per year. Most of this chlorine remains in the aerosol and is returned to the ocean surface via wet and dry deposition, but important fractions ranging from a few percent in remote marine regions to greater than 90 percent in heavily polluted coastal zones are released from the aerosol as inorganic chlorine vapor. The chemical processes responsible for dechlorinating sea-salt aerosol are not well understood but are believed to involve both acidic and oxidizing species naturally present in the background atmosphere; anthropogenic pollutants enhance rates of volatilization. The released gaseous compounds consist of hydrogen chloride (HCl) and probably other inorganic Cl species including molecular chlorine (Cl2), hypochlorous acid (HOCl), nitryl chloride (ClNO2), and bromine chloride (BrCl). The latter four compounds undergo rapid photolysis during the daytime to produce chlorine atoms. The reaction of hydrogen chloride with the hydroxyl radical also produces chlorine atoms. These processes are believed to be the major sources of atomic chlorine in the lower atmosphere.

Large quantities of volatile organic compounds (VOC) are produced by terrestrial and marine biota and subsequently released to the atmosphere. Most of the chlorine atoms in the lower atmosphere react with these organic compounds to produce hydrogen chloride. Indeed, measured loss rates of some organic species (in particular, the butanes, propane, dimethyl sulfide, and alkyl nitrates) have been used to estimate ambient concentrations of chlorine atoms in marine air. In addition to the hydrogen-abstraction reactions that produce hydrogen chloride, some organic species degrade via chlorine-addition reactions to form chlorinated products. Although such reactions have not been conclusively demonstrated to proceed in ambient air, various pathways are likely based on laboratory experiments. For instance, recent work demonstrates that isoprene, a ubiquitous VOC emitted by both terrestrial and marine biota, reacts with atomic chlorine via Cl addition to produce 1-chloro-3-methyl-3-butene-2-one (C5H7OCl) and three of its isomers. In the future, this compound may serve as a useful diagnostic of Cl-atom reactions in ambient air. Similar pathways may contribute to the large wet-deposition fluxes of chloroacetates and other organochlorine compounds reported by A. Grimvall and co-workers .

Other investigations raise the interesting possibility that Cl-addition reactions may be important global sources for methyl chloride, the most abundant chlorinated organic compound in the lower atmosphere and a major natural source of stratospheric chlorine. The principal known sources for methyl chloride are direct emissions from biomass burning, the surface ocean, and wood-rotting fungi. However, a recent, detailed analysis of the methyl chloride budget in the lower atmosphere reveals substantial imbalances between known sources and sinks; these results suggest the presence of a large (roughly equivalent to the combined sum of all known sources) and as yet unidentified source for methyl chloride in the lower atmosphere. Since most direct natural and anthropogenic emission sources have been assessed with reasonable confidence, attention is now focusing on the possible generation of methyl chloride from chemical reactions in the atmosphere. Laboratory studies demonstrate that methyl chloride is produced in relatively low yields from the oxidation of dimethyl sulfide via chlorine-addition reactions. Although this pathway cannot account for the large discrepancy between known sources and sinks, it does point to the possibility that Cl-atom reactions with other VOCs may be important sources for methyl chloride. Ongoing research is attempting to identify the inferred missing source(s) and, in particular, whether significant methyl chloride is produced from atmospheric transformations involving chlorine-addition reactions.

Available evidence suggests that reactions of chlorine atoms with VOCs via Cl addition produce chlorinated organic compounds in the lower atmosphere. At present, however, the relative importance of these pathways on a global scale cannot be determined with confidence and remains the focus of ongoing research.