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 NO_x fluxes and the recycling and redistribution of reactive nitrogen across Antarctica and Greenland. Snow-sourced NO_x 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 NO_x 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 NO_x fluxes, these fluxes lead to smaller factor increases in mean austral summer boundary layer mixing ratios of total nitrate (HNO₃+ NO₃⁻), NO_x, 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 NO_x 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.