Supplementary material to "The importance of alkyl nitrates and sea ice emissions to atmospheric NOx sources and cycling in the summertime Southern Ocean marine boundary layer"

Atmospheric nitrate originates from the oxidation of nitrogen oxides (NOx = NO+NO2) and impacts both tropospheric chemistry and climate. NOx sources, cycling and NOx to nitrate formation pathways are poorly constrained in remote marine regions, especially the Southern Ocean, where pristine conditions serve as a useful proxy for the pre-industrial atmosphere. Here, we measured the isotopic composition (delta N-15 and delta O-18) of atmospheric nitrate in coarse-mode (> 1 mu m) aerosols collected in the summertime marine boundary layer of the Atlantic Southern Ocean from 34.5 to 70 degrees S and across the northern edge of the Weddell Sea. The delta N-15-NO3- decreased with latitude from -2.7% to -42.9 %. The decline in delta N-15 with latitude is attributed to changes in the dominant NOx sources: lightning at the low latitudes, oceanic alkyl nitrates at the mid-latitudes and photolysis of nitrate in snow at the high latitudes. There is no evidence of any influence from anthropogenic NOx sources or equilibrium isotope fractionation. Using air mass back trajectories and an isotope mixing model, we calculate that oceanic alkyl nitrate emissions have a delta N-15 signature of -21.8 +/- 7.6 %. Given that measurements of alkyl nitrate contributions to remote nitrogen budgets are scarce, this may be a useful tracer for detecting their contribution in other oceanic regions. The delta O-18-NO3- was always less than 70 %, indicating that daytime processes involving OH are the dominant NOx oxidation pathway during summer. Unusually low delta O-18-NO3- values (less than 31%) were observed at the western edge of the Weddell Sea. The air mass history of these samples indicates extensive interaction with sea-ice-covered ocean, which is known to enhance peroxy radical production. The observed low delta O-18-NO3- is therefore attributed to increased exchange of NO with peroxy radicals, which have a low delta O-18, relative to ozone, which has a high delta O-18. This study reveals that the mid- and high-latitude surface ocean may serve as a more important NOx source than previously thought and that the ice-covered surface ocean impacts the reactive nitrogen budget as well as the oxidative capacity of the marine boundary layer.