Reply on RC1

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Nitrogen oxides, collectively referred to as NO_x (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 NO_x is atmospheric nitrate, which has an influence on climate and the biogeochemical cycling of reactive nitrogen. NO_x sources and NO_x 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 (&delta;¹⁵N, &delta;¹⁸O and &Delta;¹⁷O) of atmospheric nitrate in coarse mode (&gt; 1&mu;m) aerosols, collected between South Africa and the sea ice edge in summer, winter and spring. Similar latitudinal trends in &delta;¹⁵N-NO₃^- were observed in summer and spring, suggesting similar NO_x sources. Based on &delta;¹⁵N-NO₃^-, the primary NO_x 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 NO_x sources, likely transported from the lower latitude Atlantic contribute to the background level NO₃^- observed in winter, with the potential for a stratospheric NO_x source evidenced by one sample of Antarctic origin. Low summertime &delta;¹⁸O-NO₃^- (&lt; ~70 &permil;) are consistent with daytime processes involving oxidation by OH dominating nitrate formation, while higher winter and springtime &delta;¹⁸O-NO₃^- (&gt; ~60 &permil;) indicate an increased influence of O₃ oxidation (i.e., N₂O₅, DMS, BrO). Significant linear relationships between &delta;¹⁸O and &Delta;¹⁷O 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.