LOW-TEMPERATURE REACTION OF CHLORINE NITRATE WITH WATER ICE - FORMATION OF MOLECULAR NITRIC-ACID

We have examined the reaction of CIONO2 with water ice at 140 K and fo und evidence for the formation of molecular nitric acid under conditio ns of reduced surface water. This is very different from the behaviour of CIONO2 on ice at 180 K, where substrate-induced pre-reaction ionis ation of ClONO2 leads to a reaction mechanism involving the intermedia cy of [H2OCl](+) and nitrate ions. These two results are not inconsist ent with a single mechanism if a subtle change in the interplay betwee n the availability of water molecules at the surface and the variation of the substrate/adsorbate interactions of ice/ClONO2 with temperatur e are taken into account. This implies that reaction mechanisms and pr oduct branching ratios in the atmosphere may vary widely over a range of temperatures and reactant partial pressures.