Investigation of the photochemical decomposition of nitrate, hydrogen peroxide, and formaldehyde in artificial snow

Abstract

Species like nitrate (NO 3 − ), hydrogen peroxide (H 2 O 2 ), and formaldehyde (HCHO) are ubiquitous trace compounds in snow. Photochemical reactions of these compounds in the snow can have important implications for the composition of the atmospheric boundary layer in snow-covered regions and for the interpretation of concentration profiles in snow and ice regarding the composition of the past atmosphere. Therefore, we performed laboratory experiments to investigate such reactions in artificially produced snow samples. Artificial snow samples allow to execute experiments under defined and reproducible conditions and to investigate single reactions. All reactions were carried out under comparable experimental conditions and indicated that the photolysis of H 2 O 2 and NO 3 − occurred equally fast, while the photolysis of HCHO was considerably slower. Moreover, the photolysis of HCHO was only observed if initial concentrations were much higher than found in natural snow samples. These results indicate that the H 2 O 2 and NO 3 − reactions are possibly equally important in natural snow covers regarding the formation of OH radicals, while the photolysis of HCHO is probably negligible. Nitrite (NO 2 − ) was observed as one of the products of the NO 3 − photolysis; however, it was itself photolyzed at a higher rate than NO 3 − . After a certain photolysis period (≥8 h) the NO 3 − and NO 2 − concentrations in the snow remained constant at a level of 10% of the initial nitrogen content. This is probably due to a recycling of the anions from nitrogen oxides in the gas phase of the reaction cells indicating that the chemical reactions occur in or near the surface layer of the snow crystals.

Photolysis imprint in the nitrate stable isotope signal in snow and atmosphere of East Antarctica and implications for reactive nitrogen cycling

“…Surface snow samples from the DC-DDU traverse indicate to a first order that fractionation increases as annual accumulation rates decrease (accumulation data from Pettre et al, (Jacobi et al, 2006). 1986). δ 15 N(NO − 3 ) are negatively correlated with annual accumulation rates (r=−0.67, p<0.01) in agreement with previous findings from Antarctic ice cores (Freyer et al, 1996), whereas δ 18 O(NO is depleted by 4-5‰ similar to our finding at DC (McCabe et al, 2007).…”

“…However, limitations of the ZPE-shift model are that it does not account for changes in shape and intensity of absorption spectra upon isotopic substitution (Miller and Yung, 2000). Furthermore, the underlying assumptions of direct photo-dissociation and unit quantum yield (Miller and Yung, 2000) are likely not met by NO (Jacobi et al, 2006). Photolysis rates j for both NO − 3 isotopologues allow to calculate the photolytic fractionation constant 15 ε (Eqs.…”

“…In these experiments artificial snow made from dilute solutions of NaNO 3 was used (Jacobi et al, 2006), but additional experiments replacing NaNO 3 with HNO 3 did not indicate any significant changes in the observed decrease of NO − 3 concentrations upon irradiation (H. W. Jacobi, personal communication, 2008). Since fractionation is related to mass loss the same should apply also to 15 ε.…”

“…First, irradiated snow samples were kept in closed cells and therefore photolysis products NO 2 and OH not flushed from the cell likely participated in reformation (Monse et al, 1969). and deposition of NO − 3 , thereby reducing isotopic fractionation (Jacobi et al, 2006). NO − 3 reformation is consistent with NO − 3 concentrations never dropping below 10% of the starting levels, even after prolonged irradiation (Jacobi et al, 2006).…”

“…The photolytic 15 ε is process-specific and therefore applies to any snow covered location. Previously published 15 ε values are not representative for conditions at the Earth surface, but apply only to the UV lamp used in the reported experiment (Blunier et al, 2005;Jacobi et al, 2006). Depletion of oxygen stable isotopes is attributed to photolysis followed by isotopic exchange with water and hydroxyl radicals.…”