A quantitative study of atomic and molecular halogens in the lower atmosphere

Halogen chemistry has been implicated in the destruction of ground level ozone, the oxidation of non-methane hydrocarbons, and the oxidation of gaseous mercury in the Arctic troposphere. Quantitative determinations of halogen atom concentrations are important in understanding the extent of this chemistry, as are quantitative investigations of halogen atom precursors which are thought to include molecular halogen species. This work describes a new method, based on the measurements of the concentration increases of a suite of halogenated volatile organic compounds, for determining halogen atoms concentrations. The analytes of interest, brominated and chlorinated aldehydes and ketones, were determined by gas chromatography coupled with electron capture detection with a custom-built, integrated, sampling, separation, detection, and data acquisition system designed for automated analyses. The method was employed in Barrow, Alaska, and results indicate that halogen atom chemistry is active even in the absence of major ozone depletion events. Bromine atom concentration to atom chlorine concentration ratios were lower than previous observations, as were the absolute concentrations of bromine and chlorine atoms. Halogen atom concentrations, measured with this technique, were not significant enough to account for the observed ozone concentration decreases. Also described is a method, based on negative ion chemical ionization ion trap mass spectrometry, for the determination of the molecular halogens Br2, Cl2, and BrCl. The method used a cryogenic preconcentration to enhance molecular halogen limits of detection and a Teflon pinhole inlet for sample introduction into the mass spectrometer. Chemical ionization of the molecular halogens was performed by NO2 - generated by electron impact of nitromethane. Kinetics of the Br + NO2- ion-molecule reaction were explored and are reported. Laboratory tests indicated the method achieved limits of detection suitable for atmospheric detection of molecular halogens. The method was tested in Chicago, IL and found to be limited by ambient water vapor