[1] Laboratory experiments were conducted to determine the wavelength and temperature dependence of NO and NO 2 release from Antarctic snow. This photochemically driven process has been observed during recent field measurements in polar regions. In this work the photochemical production of NO and NO 2 was reproduced under laboratory conditions with NO 2 dominating the production. The wavelength of incident light was varied over the range 295-385 nm. We observed a l dependence where NO and NO 2 release ceases when the snow was illuminated with l > 345 nm. Comparing these data with the aqueous absorption cross section of the nitrate ion (NO 3 À ) indicates that NO 3 À is the precursor N-oxide species, which is photolyzed in snow. The temperature of the experimental system was varied over the range 253-243 K with no effect on NO and NO 2 production. The occurrence of these photochemical processes followed by release to the atmosphere will impact the chemistry of the boundary layer over any snow-covered region. In addition, understanding these processes is essential for accurate ice core interpretation.

An investigation into the reactions between Cl atoms and acrolein (1), methacrolein (2) and methyl vinyl ketone (3)is presented. Values of the rate constants for the reactions have been determined by an absolute technique for the Ðrst time. At a pressure of 1.6 Torr, the rate constants obtained were :6 in units of 10~10 cm3 molecule~1 s~1. was also determined at atmospheric pressure using k 3 \ 0.99 ^0.20 k 1 a relative-rate technique. The rate constant obtained was (2.2 ^0.3) ] 10~10 cm3 molecule~1 s~1 ; the larger value compared with that for 1.6 Torr is thought to reÑect a true dependence of the reaction rate on pressure. The Ðnal products of the reactions performed under an atmosphere of synthetic air were investigated using FTIR spectroscopy. The only chlorinated organic species identiÐed as products of the reactions were chloroacetaldehyde in the case of reaction of Cl atoms with acrolein ; chloroacetone with methacrolein ; and chloroacetaldehyde with methyl vinyl ketone. Branching ratios for abstraction (the fraction of reactions occurring by abstraction relative to the sum of addition and abstraction processes) were found to be 0.22 ^0.13 for acrolein, 0.18 ^0.02 for methacrolein and \0.03 for methyl vinyl ketone. The reaction of Cl atoms with methacrolein proceeds via a mechanism that involves the decomposition of the methyl vinyl radical. The decomposition of this radical in synthetic air, and in the absence of NO, appears to lead to the formation of signiÐcantly more CO than previously thought. This observation is in agreement with the work of

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