The importance of alkyl nitrates and sea ice emissions to atmospheric NO<sub><i>x</i></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 pre-industrial 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 to 70∘ S and across the northern edge of the Weddell Sea. The δ15N–NO3- decreased with latitude from −2.7 ‰ to −42.9 ‰. 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 isotope fractionation. Using air mass back trajectories and an isotope mixing model, we calculate that oceanic alkyl nitrate emissions have a δ15N 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 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.

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

“…1). The summer samples presented here are the same as those in Burger et al (2022). The winter and spring samples were collected and analysed as in Burger et al (2022), with any methodological modifications noted below.…”

“…The summer samples presented here are the same as those in Burger et al (2022). The winter and spring samples were collected and analysed as in Burger et al (2022), with any methodological modifications noted below. Briefly, all voyages departed from Cape Town (33.9° S, 18.4° E) and sailed southward along the Good Hope transect (0°E), until reaching Penguin Bukta (71.4° S, 2.5° W) in summer and the northern extent of the sea ice in winter (approximately 58.1° S) and spring (approximately 59.3° S).…”

“…Extracts were immediately sonicated for one hour and then stored at 4°C for at least 12 hours. Thereafter, extracts were filtered (0.2 μm) using an acid washed syringe into clean 30 mL HDPE bottles and stored at -20°C until analysis (Burger et al, 2022).…”

“…Savarino et al, (2007) derived an Antarctic stratospheric NOx source signature of 19 ± 3‰. Additionally, Burger et al (2022) estimated the δ 15 N signature of NOx produced by surface ocean RONO2 emissions over the mid-latitude Southern Ocean to be ~ -22‰.…”

Isotopic components and source analysis of inorganic nitrogen in coastal aerosols of the Yellow Sea

“…Surface seawater is considered to be the source of gaseous organic nitrogen and ammonia in the marine atmosphere (Altieri et al, 2013;Bertram et al, 2018;Altieri et al, 2021;Yu and Li, 2021). The marine emission of organic alkyl nitrates is converted into atmospheric NO x and eventually nitrate (Burger et al, 2022). As marine atmospheric environment is acidic, ammonium is formed right after ammonia is discharged (Altieri et al, 2014).…”