The importance of alkyl nitrates and sea ice emissions to atmospheric NO<sub>x</sub> sources and cycling in the summertime Southern Ocean marine boundary layer

Abstract

Abstract. 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 preindustrial atmosphere. Here, we measured the isotopic composition (δ15N and δ18O) of atmospheric nitrate in coarse-mode (> 1 μm) aerosols collected in the summertime marine boundary layer of the Atlantic Southern Ocean from 34.5° S to 70° S, and across the northern edge of the Weddell Sea. The δ15N-NO3− decreased with latitude from −2.7 ‰ to −43.1 ‰. The decline in δ15N 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 isotopic fractionation. Using air mass back trajectories and an isotope mixing model, we calculate that oceanic alkyl nitrate emissions have a δ15N signature of −22.0 ‰ ± 7.5 ‰. 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 δ18O-NO3− was always less than 70 ‰, indicating that daytime processes involving OH are the dominant NOx oxidation pathway during summer. Unusually low δ18O-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 δ18O-NO3− is therefore attributed to increased exchange of NO with peroxy radicals, which have a low δ18O, relative to ozone, which has a high δ18O. 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.

Isotopic Evidence That Alkyl Nitrates Are Important to Aerosol Nitrate Formation in the Equatorial Pacific

“…Based on samples that contained ≥15% contribution (i.e., across both hemispheres), the average δ 15 N-RONO 2 is −27.8‰ ± 23.3‰. The similarity in the estimations between this work and Burger et al (2021) make us confident that (a) RONO 2 is contributing to aerosol NO 3 − in marine systems and (b) RONO 2 has a low isotopic signature. If the observed variability is real, then it is due to either changes in the NO 2 − source or fractionation that varies with latitude.…”

“…Remarkably, using a different isotopic mass balance approach, Burger et al (2021) recently estimated the δ 15 N-NO 3 − derived from oceanic RONO 2 in the Southern Ocean averages −22.0‰ ± 7.5‰ for samples collected between (41°-70°S), which is in agreement with our average δ 15 N-RONO 2 estimations (−27.1‰ ± 17.3‰ (NH) and −37.5‰ ± 32.8‰ (SH) for all samples). Based on transport, Burger et al (2021) determined the time spent in marine RONO 2 emission regions for each sample, and combined that knowledge with the δ 15 N-NO 3 − to carryout a two-member mixing model. We suspect that our larger estimated range can be explained by the more extensive latitudinal and longitudinal area of our study.…”

“…On the other hand, it is possible that the predicted variation in the δ 15 N-RONO 2 reflects variation in fractionations during formation/conversion controlled by changes in environmental conditions. Remarkably, using a different isotopic mass balance approach, Burger et al (2021) recently estimated the δ 15 N-NO 3 − derived from oceanic RONO 2 in the Southern Ocean averages −22.0‰ ± 7.5‰ for samples collected between (41°-70°S), which is in agreement with our average δ 15 N-RONO 2 estimations (−27.1‰ ± 17.3‰ (NH) and −37.5‰ ± 32.8‰ (SH) for all samples). Based on transport, Burger et al (2021) determined the time spent in marine RONO 2 emission regions for each sample, and combined that knowledge with the δ 15 N-NO 3 − to carryout a two-member mixing model.…”

“…We suspect that our larger estimated range can be explained by the more extensive latitudinal and longitudinal area of our study. However, Burger et al (2021) estimated the δ 15 N-NO 3 − only for samples with a ≥ 10% RONO 2 contribution. If we similarly include only samples with ≥15% RONO 2 contribution, the SH δ 15 N-RONO 2 average raises to −28.2 ± 14.5‰, which is within 1‰ of the NH average.…”