Nitrogen Dioxide Release in the 302 nm Band Photolysis of Spray-Frozen Aqueous Nitrate Solutions. Atmospheric Implications

Abstract

We quantify the NO2 fluxes released into the gas phase during the continuous λ ∼ 300 nm photolysis of NO3- in submillimeter ice layers produced by freezing aqueous KNO3 sprays on cold surfaces. Fluxes, FNO2, increase weakly with [NO3-] between 5 ≤ [NO3-]/mM ≤ 50 and increase markedly with temperature in the range of 268 ≥ T/K ≥ 248. We found that FNO2, the photostationary concentration of NO2- (another primary photoproduct), and the quantum yield of 2-nitrobenzaldehyde in situ photoisomerization are nearly independent of ice layer thickness d within 80 ≤ d/μm ≤ 400. We infer that radiation is uniformly 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 uppermost region is able to escape into the gas phase. The remainder is trapped and further photolyzed into NO. We obtain φNO2− ∼ 4.8 × 10-3 at 263 K, i.e., about the quantum yield of nitrite formation in neutral NO3- aqueous solutions, and an apparent quantum yield of NO2 release φ‘NO2 ∼ 1.3 × 10-3 that is about a factor of 5 smaller than solution φOH data extrapolated to 263 K. These results suggest that NO3- photolysis in ice takes place in a liquidlike environment and that actual φ‘NO2 values may depend on the morphology of ice deposits. Present φ‘NO2 data, in conjunction with snow albedo and absorptivity data, lead to FNO2 values in essential agreement with recent measurements in Antarctic snow under solar illumination.