The impact of snow nitrate photolysis on boundary layer chemistry and the recycling and redistribution of reactive nitrogen across Antarctica and Greenland in a global chemical transport model
Abstract. The formation and recycling of reactive nitrogen (NO, NO2, HONO) at the air–snow interface has implications for air quality and the oxidation capacity of the atmosphere in snow-covered regions. Nitrate (NO3−) photolysis in snow provides a source of oxidants (e.g., hydroxyl radical) and oxidant precursors (e.g., nitrogen oxides) to the overlying boundary layer, and alters the concentration and isotopic (e.g., δ15N) signature of NO3− preserved in ice cores. We have incorporated an idealized snowpack with a NO3− photolysis parameterization into a global chemical transport model (Goddard Earth Observing System (GEOS) Chemistry model, GEOS-Chem) to examine the implications of snow NO3− photolysis for boundary layer chemistry, the recycling and redistribution of reactive nitrogen, and the preservation of ice-core NO3− in ice cores across Antarctica and Greenland, where observations of these parameters over large spatial scales are difficult to obtain. A major goal of this study is to examine the influence of meteorological parameters and chemical, optical, and physical snow properties on the magnitudes and spatial patterns of snow-sourced NOx fluxes and the recycling and redistribution of reactive nitrogen across Antarctica and Greenland. Snow-sourced NOx fluxes are most influenced by temperature-dependent quantum yields of NO3− photolysis, photolabile NO3− concentrations in snow, and concentrations of light-absorbing impurities (LAIs) in snow. Despite very different assumptions about snowpack properties, the range of model-calculated snow-sourced NOx fluxes are similar in Greenland (0.5–11 × 108 molec cm−2 s−1) and Antarctica (0.01–6.4 × 108 molec cm−2 s−1) due to the opposing effects of higher concentrations of both photolabile NO3− and LAIs in Greenland compared to Antarctica. Despite the similarity in snow-sourced NOx fluxes, these fluxes lead to smaller factor increases in mean austral summer boundary layer mixing ratios of total nitrate (HNO3+ NO3−), NOx, OH, and O3 in Greenland compared to Antarctica because of Greenland's proximity to pollution sources. The degree of nitrogen recycling in the snow is dependent on the relative magnitudes of snow-sourced NOx fluxes versus primary NO3− deposition. Recycling of snow NO3− in Greenland is much less than in Antarctica Photolysis-driven loss of snow NO3− is largely dependent on the time that NO3− remains in the snow photic zone (up to 6.5 years in Antarctica and 7 months in Greenland), and wind patterns that redistribute snow-sourced reactive nitrogen across Antarctica and Greenland. The loss of snow NO3− is higher in Antarctica (up to 99 %) than in Greenland (up to 83 %) due to deeper snow photic zones and lower snow accumulation rates in Antarctica. Modeled enrichments in ice-core δ15N(NO3−) due to photolysis-driven loss of snow NO3− ranges from 0 to 363 ‰ in Antarctica and 0 to 90 ‰ in Greenland, with the highest fraction of NO3− loss and largest enrichments in ice-core δ15N(NO3−) at high elevations where snow accumulation rates are lowest. There is a strong relationship between the degree of photolysis-driven loss of snow NO3− and the degree of nitrogen recycling between the air and snow throughout all of Greenland and in Antarctica where snow accumulation rates are greater than 130 kg m−2 a−1 in the present day.