Laboratory study of nitrate photolysis in Antarctic snow. II. Isotopic effects and wavelength dependence

Abstract

Atmospheric nitrate is preserved in Antarctic snow firn and ice. However, at low snow accumulation sites, post-depositional processes induced by sunlight obscure its interpretation. The goal of these studies (see also Paper I by Meusinger et al. ["Laboratory study of nitrate photolysis in Antarctic snow. I. Observed quantum yield, domain of photolysis, and secondary chemistry," J. Chem. Phys. 140, 244305 (2014)]) is to characterize nitrate photochemistry and improve the interpretation of the nitrate ice core record. Naturally occurring stable isotopes in nitrate ((15)N, (17)O, and (18)O) provide additional information concerning post-depositional processes. Here, we present results from studies of the wavelength-dependent isotope effects from photolysis of nitrate in a matrix of natural snow. Snow from Dome C, Antarctica was irradiated in selected wavelength regions using a Xe UV lamp and filters. The irradiated snow was sampled and analyzed for nitrate concentration and isotopic composition (δ(15)N, δ(18)O, and Δ(17)O). From these measurements an average photolytic isotopic fractionation of (15)ɛ = (-15 ± 1.2)‰ was found for broadband Xe lamp photolysis. These results are due in part to excitation of the intense absorption band of nitrate around 200 nm in addition to the weaker band centered at 305 nm followed by photodissociation. An experiment with a filter blocking wavelengths shorter than 320 nm, approximating the actinic flux spectrum at Dome C, yielded a photolytic isotopic fractionation of (15)ɛ = (-47.9 ± 6.8)‰, in good agreement with fractionations determined by previous studies for the East Antarctic Plateau which range from -40 to -74.3‰. We describe a new semi-empirical zero point energy shift model used to derive the absorption cross sections of (14)NO3 (-) and (15)NO3 (-) in snow at a chosen temperature. The nitrogen isotopic fractionations obtained by applying this model under the experimental temperature as well as considering the shift in width and center well reproduced the values obtained in the laboratory study. These cross sections can be used in isotopic models to reproduce the stable isotopic composition of nitrate found in Antarctic snow profiles.

Laboratory study of nitrate photolysis in Antarctic snow. I. Observed quantum yield, domain of photolysis, and secondary chemistry

“…All experiments were performed at −30 • C. Experiments 3, 7, 8, 9, and 10 correspond to experiments 6, 7, 8, 9, and 10 in Paper II. 41 Exp # Snow …”

“…These slices are weighed individually in clean bags and then divided into two vials, one for ion concentration and one for isotopic measurements. 41 Sample vials were kept in insulated boxes in the dark in the same cold room until measurement.…”

Investigation of post-depositional processing of nitrate in East Antarctic snow: isotopic constraints on photolytic loss, re-oxidation, and source inputs

“…Berhanu et al (2014) have recently proposed absorption cross sections derived from the measurements of Chu and Anastasio (2003) . At Dome C, where snow accumulation is typically less than 35 kg m −2 a −1 , close to the values of P4-P7 (Fig.…”

Surface-Enhanced Nitrate Photolysis on Ice

“…This may therefore be responsible for the highly variable amounts of photo-labile and photo-stable nitrates fractions which provide contributions to the photochemical NOx fluxes arising from the surface or the bulk of ice and snow, from liquid and solid domains, from micro-inclusions or grain boundaries, thereby complicating comparisons between observations from different locations. Therefore, laboratory investigations have been devised using natural samples 8,9 or nitrates-doped artificial snow/ice samples [10][11][12][13][14][15] to study the phenomenon at the molecular/mesoscopic levels under more controlled conditions. This is necessary in order to quantify the relevant physico-chemical parameters that could more appropriately describe the photolysis of snow-bound nitrates.…”