The experimental results obtained can be summarized as follows:

a) The main gaseous products found were Cl2, Cl2O.

Cl2 was almost no obtained by Hg lamp irradiation because it photolyses at wavelength longer than 300 nm.

b) The concentration and ratio of products formed in (a) depends on the surface temperature, photolysis time and chlorine nitrate concentration adsorbed on ice.

c) The heterogeneous photochemistry of chlorine nitrate adsorbed on ice crystal depends on the surface conditions

According to our preliminary results, the gaseous products found in the heterogeneous photochemistry of chloride nitrate adsorbed on ice crystal, can be explained in terms of:

Direct photolysis of the intermediate species which result from ClONO2 interaction with ice^3-4.

[H₂OClO⁺....NO₃^-]

And recombination in gas phase

2 Cl ¾® Cl₂

or

2 ClO ¾® Cl₂ + O₂

Rossi et al³ suggested an S_N2-type nucleophilic displacement for the formation of Cl2O at high flow rates of chloride nitrate:

ClO^- + ClONO₂ ¾® Cl₂O + NO₃^-

ClO^- + HOCl ¾® Cl₂O + HO^-

Under our experimental conditions, where a high chloride nitrate concentration on ice is present, the radiation could start nucleophilic radical reactions (SR_N2 ?) and therefore favour the Cl2O formation. In addition, in a recent study of the heterogeneous photochemistry of Cl2O adsorbed on ice⁵, it was found that Cl2O can be converted to chloryl chloride on water-ice by photolysis with both near-UV and visible light. This compound released into the gas phase readily decomposed to Cl and OClO which contributes to ozone destruction. The postulated mechanism requires cluster of Cl2O or at least neighboring molecules for conversion to ClClO2. In our study we found Cl2O in the gas phase. It is reasonable to think in a high concentration of Cl2O on ice and therefore it could produce a higher conversion to ClClO2.

INTRODUCTION EXPERIMENTAL RESULTS DISCUSSION ATMOSPHERIC IMPLICATIONS REFERENCES