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

Abstract

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‰.