Low temperature reaction of chlorine nitrate with water ice Formation of molecular nitric acid

Abstract

We have examined the reaction of ClONO₂ with water ice at 140 K and found evidence for the formation of molecular nitric acid under conditions of reduced surface water. This is very different from the behaviour of ClONO₂ on ice at 180 K, where substrate-induced pre-reaction ionisation of ClONO₂ leads to a reaction mechanism involving the intermediacy of [H₂OCl]⁺ and nitrate ions. These two results are not inconsistent with a single mechanism if a subtle change in the interplay between the availability of water molecules at the surface and the variation of the substrate/adsorbate interactions of ice/ClONO₂ with temperature are taken into account. This implies that reaction mechanisms and product branching ratios in the atmosphere may vary widely over a range of temperatures and reactant partial pressures.

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