The reactions of NO2+ in association with heterogeneous water clusters

Abstract

Within the context of stratospheric chemistry the reactions of NO ₂ ⁺ on ice particles represent an important loss mechanism for NO _x species via the formation and disproportionation of N ₂ O ₅ . Previous experiments involving NO ₂ ⁺ ·(H ₂ O) _n clusters have identified reaction pathways leading to the formation of nitric acid and demonstrated that a small cluster can reproduce some of the essential features of chemistry on an ice surface (J. Chem. Soc., Faraday Trans., 1994, 90, 3469). In the new experiments reported here the fragmentation products of heterogeneous clusters of the form NO ₂ ⁺ ·(H ₂ O) _n ·X, for n in the range 1–10, have been examined for evidence of chemical reactivity, and in particular, for the effect X may have on nitric acid formation. X is one of the following: CH ₃ OH, C ₂ H ₅ OH, CH ₃ CN, NH ₃ or CH ₃ COCH ₃ , each of which has been identified in the stratosphere in association with water molecules. Each molecule has a very individual effect on the chemistry of NO ₂ ⁺ , and to a large degree, patterns of behaviour appear to be influenced by relative proton affinities. However, the type of chemistry observed is very varied, ranging from the catalytic release of nitric acid which is promoted by the presence of acetone, through to the release of CH ₃ ONO ₂ , which results from the presence of methanol. Reactions of the type identified here could influence the chemical balance of the upper atmosphere and contribute to chemistry in polar stratospheric clouds.