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Halogen atoms, particularly chlorine atoms, are well known to be highly reactive and to play a central role in the chemistry of the upper atmosphere. A large potential source of these halogens in the lower atmosphere (troposphere) exists in the form of sea salt particles. A variety of laboratory, ® eld and modelling studies strongly suggests that there are heterogeneous reactions of sea salt particles which generate photochemically active halogen species such as Cl # in marine areas. In addition, there is increasing evidence for a contribution of bromine atoms to tropospheric chemistry in marine regions at high latitudes. We review here brie¯y the potential importance of such halogen reactions and evidence for their role in the chemistry of the troposphere. Studies carried out in this laboratory to elucidate, at a molecular level, the mechanisms of reaction of synthetic sea salt and its components with gases of tropospheric interest are reviewed. Initial results obtained using a new aerosol apparatus recently constructed in this laboratory to study the reactions of aerosol particles above and below the deliquescence point of the salts are also discussed.

Abstract. The contribution of atomic chlorine to the chemistry of marine regions as well as the Arctic at ground level at polar sunrise is the subject of a number of recent studies. However, identifying the specific chlorine atom precursors has proven difficult. One potential approach is the measurement of definitive products of chlorine atom reactions, for example with biogenic hydrocarbons. We report here product studies of the chlorine atom reaction with isoprene using ppm concentrations at one atmosphere air and 298 K in a NOx-free system using atmospheric pressure ionization-mass spectrometry (API-MS) as well as GC-MS. 1-chloro-3-methyl-3-butene-2-one (CMBO) is identified as a unique product of this reaction, and there is evidence of the formation of three additional isomers of CMBO as well. Methyl vinyl ketone (MVK) is formed in small yields (9 + 5 %), consistent with earlier studies of this reaction in which an upper yield of 13% was reported. The stable product expected from allylic hydrogen atom abstraction (measured in earlier kinetic studies to be 15% of the total reaction), 2-methylene-3-butenal, is also tentatively identified using API-MS. Assuming that similar chemistry occurs at the ppb-ppt levels found in the atmosphere, identification of CMBO and/or its isomers in field studies could provide strong evidence of chlorine atom chemistry in low NO x environments where there are also sources of isoprene.

This note describes the recent development of a new technique, aerosol time-of-flight mass spectrometry, that allows for realtime analysis of single particles. We report here the first results demonstrating the utility of this technique for real-time analysis of polydisperse systems of aerosols, determining both size and composition. Future calibration studies will follow, enabling complete analysis of complex atmospheric systems, determining particle size distributions and, more importantly, the corresponding compositional variations for each particle size.Real-time aerosol analysis, determining the size and chemical composition of individual particles, has been an elusive goal for aerosol researchers since the early 1970s.1,2 We recently reported a method for accomplishing this goal using aerosol time-of-flight mass spectrometry (ATOFMS), a technique which combines aerodynamic particle sizing and time-of-flight mass spectrometry.1 23

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