Photochemistry Associated with Ice

In the following paragraphs we will summarize the recent developments in understanding direct and indirect photochemical processes in the light of this 

Structural picture of ice in the environment [367]. The examples include organic and inorganic chromophores as far as they are implicated in the chemistry of atmospherically relevant gases. 

Almost all laboratory studies of ice photochemistry have used illuminated bulk ice samples, with reagents frozen in solution. Often it is assumed that the reagents are excluded together and uniformly to the ice surface region in contact with the overlying atmosphere. Various thermodynamic formulations have been used to estimate the concentrations of the excluded reagents [272, 273], but such approaches seem to be deficient in some cases [274]. Nevertheless, photolytic kinetics experiments have generally, but not always, found similar loss rates for species frozen from solution as in the liquid phase [192, 251, 275-277]. 

One possible reason for this rate enhancement, at least in some instances, is a change in absorption cross sections and/or photolysis quantum yields due to selfassociation at the interface. This effect has been documented for aromatic compounds both spectroscopically and by simulations [278, 282, 283], and is a consequence of the different hydrogen bonding environment present at the air-ice interface compared to the liquid surface. In the case of benzene in particular, the self association gives rise to a significant red-shift in the absorption spectrum [279], such that benzene present at the air-ice interface may absorb available solar radiation in the lower atmosphere. This opens the possibility of a previously unconsidered fate for several aromatic pollutants present in snow- and ice-covered regions. 

Of similar photochemical importance as H2O2 is nitrate, which absorbs light above 290 nm. The ubiquitous presence of nitrate in environmental ices is well documented for cirrus ice particles [296, 297] as well as permanent and perennial snow packs [298-301]. hi aqueous solution, photolysis of nitrate ion leads to either OH and NO2 or 0( P) and nitrite ion, with typically significantly higher quantum yields for the first pathway [197, 200]. In the upper troposphere, it is currently thought that uptake of HNO3 to ice makes it ineffective as a photolytic source of 

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