A seasonal analysis of aerosol NO3- sources and NOx oxidation pathways in the Southern Ocean marine boundary layer.

Nitrogen oxides, collectively referred to as NO x (NO + NO 2), 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-NO 3- formation pathways remain poorly constrained in the remote marine boundary layer of the Southern Ocean, particularly outside of the more frequently sampled summer months. This study presents seasonally resolved measurements of the isotopic composition (δ15 N, δ18 O, and Δ17 O) of atmospheric nitrate in coarse-mode (> 1 µ m) aerosols, collected between South Africa and the sea ice edge in summer, winter, and spring. Similar latitudinal trends in δ15 N–NO 3- were observed in summer and spring, suggesting similar NO x sources. Based on δ15 N–NO 3- , the main NO x sources were likely a combination of lightning, biomass burning, and/or soil emissions at the low latitudes, as well as oceanic alkyl nitrates and snowpack emissions from continental Antarctica or the sea ice at the mid-latitudes and high latitudes, respectively. Snowpack emissions associated with photolysis were derived from both the Antarctic snowpack and snow on sea ice. A combination of natural NO x sources, likely transported from the lower-latitude Atlantic, contribute to the background-level NO 3- observed in winter, with the potential for a stratospheric NO 3- source evidenced by one sample of Antarctic origin. Greater values of δ18 O–NO 3- in spring and winter compared to summer suggest an increased influence of oxidation pathways that incorporate oxygen atoms from O 3 into the end product NO 3- (i.e. N 2 O 5 , DMS, and halogen oxides (XO)). Significant linear relationships between δ18 O and Δ17 O suggest isotopic mixing between H 2 O (v) and O 3 in winter and isotopic mixing between H 2 O (v) and O 3 /XO 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. As a result, isotopic mixing with an additional third end-member (atmospheric O 2) occurs in spring. [ABSTRACT FROM AUTHOR]