Impacts of snow nitrate photolysis and recycling on identification of oxidation information recorded in nitrate in Antarctica

The oxygen isotopes of nitrate (NO₃⁻), specifically δ¹⁸O(NO₃⁻) and Δ¹⁷O(NO₃⁻), provide insights into the oxidation mechanisms responsible for NO₃⁻ formation and reflect the atmospheric oxidation capacity. However, these isotopic signatures can be altered by post-depositional processes. This study collected data on NO₃⁻ nitrogen and oxygen isotopes in various samples, including snowpits, ice cores, snowfall, and skin layer snow, from different Antarctic regions. We observed a linear relationship between δ¹⁸O(NO₃⁻) and Δ¹⁷O(NO₃⁻) at each site, suggesting that NO₃⁻ production involves a mixture of oxidants such as O₃/XO, OH, and HO₂/RO₂, with O₃ identified as a key contributor. The δ¹⁸O values of other oxidants, termed as the derived δ¹⁸O, were obtained from this linear relationship at each site, excluding O₃, and showed a decreasing trend from the coast to inland region in Antarctica. We found that these derived δ¹⁸O values exhibited significant linear correlations with the reciprocal of the snow accumulation rate and δ¹⁸O(H₂O), indicating the differences in NO₃⁻ photolysis and recycling processes within snowpack across various sites. Additionally, we estimated the average photolysis-driven loss fraction of NO₃⁻ and its oxygen exchange fraction with H₂O, and found that both processes are more intense on the East Antarctic plateau and weaker in coastal regions, largely controlled by snow accumulation rate. At low accumulation sites, the derived δ¹⁸O values are more affected by local δ¹⁸O(H₂O), leading to modifications of the atmospheric oxidation signals recorded in the ice core NO₃⁻.