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

Abstract

We quantify the NO_2 fluxes released into the gas phase during the continuous λ ∼ 300 nm photolysis of NO_3^- in submillimeter ice layers produced by freezing aqueous KNO_3 sprays on cold surfaces. Fluxes, F_(NO_2), increase weakly with [NO_3^-] between 5 ≤ [NO_3^-]/mM ≤ 50 and increase markedly with temperature in the range of 268 ≥ T/K ≥ 248. We found that F_(NO_2), the photostationary concentration of NO_2^- (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 NO_3^- is photodecomposed into NO_2 (+ OH) and NO_2^- (+ O), but that only the NO_2 produced on the uppermost region is able to escape into the gas phase. The remainder is trapped and further photolyzed into NO. We obtain φ_(NO_2^−) ∼ 4.8 × 10^(-3) at 263 K, i.e., about the quantum yield of nitrite formation in neutral NO_3^- aqueous solutions, and an apparent quantum yield of NO2 release φ'_(NO_2) ∼ 1.3 × 10^(-3) that is about a factor of 5 smaller than solution φ_(OH) data extrapolated to 263 K. These results suggest that NO_3^- photolysis in ice takes place in a liquidlike environment and that actual φ'_(NO_2) values may depend on the morphology of ice deposits. Present φ'_(NO_2) data, in conjunction with snow albedo and absorptivity data, lead to F_(NO_2) values in essential agreement with recent measurements in Antarctic snow under solar illumination.

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