Heterogeneous reactions of N2O5 with H2O and HCl on ice surfaces: Implications for Antarctic ozone depletion

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Abstract

Laboratory studies of heterogeneous reactions of N2O5 with H2O and HCl on ice surfaces have been performed in a fast flow reactor. A differentially pumped quadrupole mass spectrometer with an electron impact ionizer was used as a detector for the trace gas analysis. The reaction probability (or the heterogeneous reaction rate) of N2O5 on water ice was measured to be 0.028 (±0.011) at 195 K, while nitric acid was the sole product in the condensed phase. This reaction may effectively provide a sink for odd nitrogen species during the polar winter. In the presence of HCl with mole fractions between 0.015 and 0.04 in ice, the measured reaction probability of N2O5 was enhanced by about a factor of 2 at 195 K while nitryl chloride (ClNO2) or chlorine nitrite (ClONO) was found to be the major product in the gas phase. The quoted error bar represents the statistical error only. ClNO2 or ClONO can be readily photolyzed in the austral spring to form active chlorine which removes stratospheric ozone through several catalytic cycles. Another reaction product was nitric acid which remained in the solid phase. Since the polar stratospheric clouds are thought to consist of ice particles or possibly mixtures of HCl-ice on the surface, the large reaction probabilities measured in this work suggest that these reactions should be a major factor in producing the observed springtime ozone depletion in the Antarctic stratosphere. Finally, a new ozone destruction mechanism based on surface recombination of ClO radicals is also proposed.

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