The impact of snow nitrate photolysis on boundary layer chemistry and the recycling and redistribution of reactive nitrogen across Antarctica in a global chemical transport model

Abstract

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, ozone) and oxidant precursors (e.g., nitrogen oxides) to the overlying boundary layer, and disturbs the preservation of NO3- in ice cores. We have incorporated the photolysis of Antarctic snow NO3- into a global chemical transport model (GEOS-Chem) to examine the implications of snow NO3- photolysis for boundary layer chemistry, the recycling and redistribution of reactive nitrogen across the Antarctic continent, and the preservation of ice-core NO3- in Antarctic ice cores. The calculated potential flux of snow-sourced NOx in Antarctica (0.5–7.8 × 108 molec cm-2 s-1) and calculated e-folding depths of UV actinic flux in snowpack (24–69 cm) are comparable to observations. Snow-sourced NOx increases mean austral summer boundary layer mixing ratios of total nitrate (HNO3 + NO3-), NOx, OH, and O3 in Antarctica by a factor of up to 32, 38, 7, and 2, respectively, in the model. Model results also suggest that NO3- can be recycled between the air and snow multiple times and that NO3- can remain in the snow photic zone for at least 7.5 years on the East Antarctic plateau. The fraction of photolysis-driven loss of NO3- from the snow is ∼ 0.99 on the East Antarctic plateau, while areas of wind convergence (e.g., over the Ronne Ice Shelf) have a net gain of NO3- due to redistribution of snow-sourced reactive nitrogen across the Antarctic continent. The modeled enrichment in ice-core δ 15N(NO3-) due to photolysis-driven loss of snow NO3- ranges from 0 to 363 ‰ and the magnitudes of the spatial trends are consistent with δ 15N(NO3-) observations, suggesting that the spatial variability in snow δ 15N(NO3-) across the Antarctic continent is determined mainly by the degree of photolysis-driven loss of snow NO3-. Further, 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, suggesting that ice-core δ 15N(NO3-) observations can be used to assess the degree of nitrogen recycling and loss over much of Antarctica and aid in the interpretation of ice-core NO3- in terms of past atmospheric variability of reactive nitrogen.