Comment on acp-2022-704

Abstract

Nitrogen oxides, collectively referred to as NOx (NO + NO2), are an important component of atmospheric chemistry involved in the production and destruction of various oxidants that contribute to the oxidative capacity of the troposphere. The primary sink for NOx is atmospheric nitrate, which has an influence on climate and the biogeochemical cycling of reactive nitrogen. NOx sources and NOx to NO3- formation pathways remain poorly constrained in the remote marine boundary layer of the Southern Ocean (SO), particularly outside of the more frequently sampled summer months. This study presents seasonally resolved measurements of the isotopic composition (δ15N, δ18O and Δ17O) of atmospheric nitrate in coarse mode (> 1μm) aerosols, collected between South Africa and the sea ice edge in summer, winter and spring. Similar latitudinal trends in δ15N-NO3- were observed in summer and spring, suggesting similar NOx sources. Based on δ15N-NO3-, the primary NOx sources were lightning, oceanic alkyl nitrates and snowpack emissions at the low, mid and high latitudes, respectively. Snowpack emissions associated with photolysis were derived from both the Antarctic snowpack as well as from snow on sea ice. A combination of natural NOx sources, likely transported from the lower latitude Atlantic contribute to the background level NO3- observed in winter, with the potential for a stratospheric NOx source evidenced by one sample of Antarctic origin. Low summertime δ18O-NO3- (< ~70 ‰) are consistent with daytime processes involving oxidation by OH dominating nitrate formation, while higher winter and springtime δ18O-NO3- (> ~60 ‰) indicate an increased influence of O3 oxidation (i.e., N2O5, DMS, BrO). Significant linear relationships between δ18O and Δ17O suggest isotopic mixing between H2O(v) and O3 in winter, with the addition of a third endmember (atmospheric O2) becoming relevant in spring. The onset of sunlight in spring, coupled with large sea ice extent, can activate chlorine chemistry with the potential to increase peroxy radical concentrations, contributing to oxidant chemistry in the marine boundary layer.