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

• Main Authors: Burger, Jessica, Joyce, Emily, Hastings, Meredith, Spence, Kurt, Altieri, Katye
• Format: Text
• Language: English
• Published: 2022
• Subjects: Antarctic; Southern Ocean; The Antarctic; Antarc*; Sea ice
• Online Access: https://doi.org/10.5194/acp-2022-704; https://acp.copernicus.org/preprints/acp-2022-704/

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 (SO), particularly outside of the more frequently sampled summer months. This study presents seasonally resolved measurements of the isotopic composition (δ 15 N, δ 18 O and Δ 17 O) 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 δ 15 N-NO 3 - were observed in summer and spring, suggesting similar NO x sources. Based on δ 15 N-NO 3 - , 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 NO 3 - observed in winter, with the potential for a stratospheric NO x source evidenced by one sample of Antarctic origin. Low summertime δ 18 O-NO 3 - (< ~70 ‰) are consistent with daytime processes involving oxidation by OH dominating nitrate formation, while higher winter and springtime δ 18 O-NO 3 - (> ~60 ‰) indicate an increased influence of O 3 oxidation (i.e., N 2 O 5 , DMS, BrO). Significant linear relationships between δ 18 O and Δ 17 O suggest isotopic mixing between H 2 O(v) and O 3 in winter, with the addition of a third endmember (atmospheric O 2 ) becoming relevant in spring. The onset of sunlight in spring, coupled with large sea ice extent, can activate ...