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.
Highlights
Atmospheric nitrate (NO−3 ), hereafter defined as gas-phase nitric acid (HNO3) and particulate NO−3 (p-NO−3 ), impacts air quality and climate by contributing to atmospheric particulate matter (Park and Kim, 2005) and influencing the Earth’s radiative heat budget (IPCC, 2013)
Our observations reveal a latitudinal gradient in atmospheric NO−3 concentration and δ15N–NO−3, which we hypothesize may be attributed to the varying contribution of the dominant NOx sources present between Cape Town and coastal Antarctica
Similar to other studies in the region (Savarino et al, 2007; Morin et al, 2009), we suggest that photochemical NOx production on snow or ice accounts for the low aerosol δ15N–NO−3 in high-latitude air masses, where high-latitude air mass samples are defined as those exposed to the Antarctic continent or the surrounding sea ice
Summary
Atmospheric nitrate (NO−3 ), hereafter defined as gas-phase nitric acid (HNO3) and particulate NO−3 (p-NO−3 ), impacts air quality and climate by contributing to atmospheric particulate matter (Park and Kim, 2005) and influencing the Earth’s radiative heat budget (IPCC, 2013). It plays a major role in the biogeochemical cycling of reactive nitrogen (Altieri et al, 2021). Because of its pristine nature, the summertime Southern Ocean serves as a unique region in which to study atmospheric chemistry and is a useful pre-industrial reference point for comparing the magnitude of anthropogenic aerosol impacts on climate (Haywood and Boucher, 2000; Hamilton et al, 2014)
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