Nitrogen dioxide release in the 302 nm band photolysis of spray-frozen aqueous nitrate solutions. Atmospheric implications

We quantify the NO2 fluxes released into the gas phase during the continuou s lambda similar to 300 nm photolysis of NO3- in submillimeter ice layers p roduced by freezing aqueous KNO3 sprays on cold-surfaces. Fluxes, F-NO2, in crease weakly with [NO3-] between 5 less than or equal to [NO3-]/mM less th an or equal to 50 and increase markedly with temperature in the range of 26 8 greater than or equal to T/K greater than or equal to 248. We found that F-NO2, the photostationary concentration of NO2- (another primary photoprod uct), and the quantum yield of 2-nitrobenzaldehyde in situ photoisomerizati on are nearly independent of ice layer thickness d within 80 less than or e qual to d/mum less than or equal to 400. We infer that radiation is uniform ly absorbed over the depth of the ice layers, where NO3- is photodecomposed into NO2 (+ OH) and NO2- (+ O), but that only the NO2 produced on the uppe rmost region is able to escape into the gas phase. The remainder is trapped and further photolyzed into NO. We obtain phi (-)(NO2) similar to 4.8 x 10 (-3) at 263 K, i.e., about the quantum yield of nitrite formation in neutra l NO3- aqueous solutions, and an apparent quantum yield of NO2 release phi ' (NO2), similar to 1.3 x 10-3 that is about a factor of 5 smaller than sol ution OK data extrapolated to 263 K. These results suggest that NO3- photol ysis in ice takes place in a liquidlike environment and that actual phi ' ( NO2) values may depend on the morphology of ice deposits. Present phi ' (NO 2) data, in conjunction with snow albedo and absorptivity data, lead to F-N O2 values in essential agreement with recent measurements in Antarctic snow under solar illumination.