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Nitrogen oxides, collectively referred to as NOx (NO + NO2), 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 NOx is atmospheric nitrate, which has an influence on climate and the biogeochemical cycling of reactive nitrogen. NOx sources and NOx to NO3- 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 (δ15N, δ18O and Δ 17O) 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 δ15N-NO3 - were observed in summer and spring, suggesting similar NOx sources. Based on δ 15N-NO3 the primary NOx 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 NOx sources, likely transported from the lower latitude Atlantic contribute to the background level NO3- observed in winter, with the potential for a stratospheric NOx source evidenced by one sample of Antarctic origin. Low summertime δ18O-NO3 - (< ~70‰) are consistent with daytime processes involving oxidation by OH dominating nitrate formation, while higher winter and springtime δ18O-NO3 - (> ~60‰) indicate an increased influence of O₃ oxidation (i.e., N2O5, DMS, BrO). Significant linear relationships between δ18O and Δ 17O suggest isotopic mixing between H₂O(v) and O₃ in winter, with the addition of a third endmember (atmospheric O₂) becoming relevant in spring. The onset of sunlight in spring, coupled with large sea ice extent, can activate chlorine chemistry with the potential to increase peroxy radical concentrations, contributing to oxidant chemistry in the marine boundary layer. [ABSTRACT FROM AUTHOR]

Across the Southern Ocean in winter, nitrification is the dominant mixed-layer nitrogen cycle process, with some of the nitrate produced therefrom persisting to fuel productivity during the subsequent growing season, potentially weakening the spring/summer biological CO2 sink. To better understand the controls on Southern Ocean nitrification, we conducted nitrite oxidation kinetics experiments in surface waters across the western Indian sector in winter. While all experiments (seven in total) yielded a Michaelis-Menten relationship with substrate concentration, the nitrite oxidation rates only increased substantially once the nitrite concentration exceeded 115±2.3 to 245±18 nM, suggesting that nitrite oxidizing bacteria (NOB) require a minimum (i.e., "threshold") nitrite concentration to produce nitrate. The half-saturation constant ranged from 134±8 to 403±24 nM, indicating a relatively high affinity of Southern Ocean NOB for nitrite, in contrast to results from culture experiments. Despite the high affinity of NOB for nitrite, its concentration rarely declines below 150 nM in the Southern Ocean's mixed layer, regardless of season. In the upper mixed layer, we measured ammonium oxidation rates that were two-to seven-fold higher than the coincident rates of nitrite oxidation, indicating that nitrite oxidation is the rate-limiting step for nitrification in the winter Southern Ocean. The decoupling of ammonium and nitrite oxidation, combined with a possible nitrite concentration threshold for NOB, may explain the non-zero nitrite that persists throughout the Southern Ocean's mixed layer year-round. We hypothesize that the apparent threshold nitrite requirement of NOB indicates nitrite undersaturation of the heme-rich nitrite oxidoreductase enzyme, perhaps driven by the limited availability of iron in surface waters. [ABSTRACT FROM AUTHOR]

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 δ1^) of atmospheric nitrate in coarse-mode (> ^m) 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%o to -43.1%o. The decline in δ1^ 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 δ1^ 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 δ1^, relative to ozone, which has a high δ1^. 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. [ABSTRACT FROM AUTHOR]

The production and consumption of ammonium (NH4+) are essential upper-ocean nitrogen cycle pathways, yet in the Southern Ocean where NH4+ has been observed to accumulate in surface waters, its mixed-layer cycling remains poorly understood. For surface samples collected between Cape Town and the marginal ice zone (MIZ) in winter 2017, we found that NH4+ concentrations were five-fold higher than is typical for summer, and lower north than south of the Subantarctic Front (SAF; 0.01-0.26 µM versus 0.19-0.70 µM). Our observations confirm that NH4+ accumulates in the Southern Ocean's winter mixed layer, particularly in polar waters. NH4+ uptake rates were highest near the Polar Front (PF; 12.9 ± 0.4 nM day-1) and in the Subantarctic Zone (10.0 ± 1.5 nM day-1), decreasing towards the MIZ (3.0 ± 0.8 nM day-1) despite high ambient NH4+ concentrations, likely due to low sea surface temperatures and light availability. By contrast, rates of NH4+ oxidation were higher south than north of the PF (16.0 ± 0.8 versus 11.1 ± 0.5 nM day-1), perhaps due to the lower light and higher iron conditions characteristic of polar waters. Augmenting our dataset with NH4+ concentration measurements spanning the 2018/2019 annual cycle reveals that mixed-layer NH4+ accumulation south of the SAF likely derives from sustained heterotrophic NH4+ production in late summer through winter that outpaces NH4+ consumption by temperature-, light, and iron-limited microorganisms. Our observations thus imply that the Southern Ocean becomes a biological source of CO2 to the atmosphere for half the year not only because nitrate drawdown is weak, but also because the ambient conditions favour net heterotrophy and NH4+ accumulation. [ABSTRACT FROM AUTHOR]

Across the Southern Ocean, large (≥20 μm) diatoms are generally assumed to be the primary vector for carbon export, although this assumption derives mainly from summertime observations. Here, we investigated carbon production and export potential during the Atlantic Southern Ocean's spring bloom from size-fractionated measurements of net primary production (NPP), nitrogen (nitrate, ammonium, urea) and iron (labile inorganic iron, organically complexed iron) uptake, and a high-resolution characterization of phytoplankton community composition. The nanoplankton-sized (2.7 to 20 μm) diatom, Chaetoceros spp., dominated the biomass, NPP, and nitrate uptake across the basin (40°S to 56°S), which we attribute to their low iron requirement, rapid response to increased light, and ability to escape grazing when aggregated into chains. We estimate that the spring Chaetoceros bloom accounted for >25% of annual export production across the Atlantic Southern Ocean, a finding consistent with recent observations from other regions highlighting the central role of the phytoplankton "middle class" in carbon export.