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

Concentrations and the stable isotopic composition of bulk aerosol nitrate (NO3−) were quantified from two GEOTRACES cruises: (a) Alaska–Tahiti (GP15; n = 22) and (b) Peru–Tahiti (GP16; n = 17) to explore the hypothesis that a marine source influences aerosol NO3− in the equatorial Pacific. The δ15N‐NO3− ranged from −14.5‰–0.5‰, with lowest values furthest from the coast, primarily reflecting a shift in sources. The δ18O‐ and Δ17O‐NO3− were both relatively high (65.2‰–85.4‰ and 21.4‰–30.7‰, respectively) and decreased away from continental regions, reflecting a shift in the oxidants that influence the formation of NO3−. Transport modeling and co‐occurrence of low δ15N, δ18O and Δ17O provided evidence for an important influence of marine‐derived alkyl nitrates (RONO2) on aerosol NO3− formation. Based on the Δ17O, we quantified that the contribution of RONO2 to aerosol NO3− can be as high as 47.5% (range 7.5%–47.5%). We also estimate an average δ15N‐RONO2 of −27.8‰ ± 23.3‰. Plain Language Summary: The availability of nutrients in surface waters, including inorganic nitrogen (i.e., nitrate, nitrite, and ammonium), regulates oceanic primary production. In the Pacific, the amount of nitrogen entering the ocean from the atmosphere (via precipitation and aerosols) has increased since the industrial revolution and is predicted to continue to grow with anthropogenic nitrogen emissions. The isotopic composition of atmospheric nitrate provides a novel tool for exploring the formation of nitrate in different environments. Here, we detail whether nitrate is primarily derived from external anthropogenic sources or internal oceanic sources. The nitrogen and oxygen isotopic compositions allow us to quantify that an important portion of the nitrogen entering the ocean from the atmosphere is recycled oceanic emissions, which means that excess nitrogen detected in the North Pacific may not be the result of external processes. Key Points: Low δ15N, δ18O, and Δ17O in aerosol nitrate from the Equatorial Pacific are consistent with alkyl nitrates being a sourceWe quantify that alkyl nitrates contribute between 7.5% and 47.5% to aerosol nitrate in the Equatorial PacificWe estimate that alkyl nitrates contribute a δ15N signature of −27.8‰ ± 23.3‰, with a latitudinal trend [ABSTRACT FROM AUTHOR]