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 delta N-15-NO3- ranged from -14.5 parts per thousand-0.5 parts per thousand, with lowest values furthest from the coast, primarily reflecting a shift in sources. The delta O-18- and Delta O-17-NO3- were both relatively high (65.2 parts per thousand-85.4 parts per thousand and 21.4 parts per thousand-30.7 parts per thousand, 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 delta N-15, delta O-18 and Delta O-17 provided evidence for an important influence of marine-derived alkyl nitrates (RONO2) on aerosol NO3- formation. Based on the Delta O-17, 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 delta N-15-RONO2 of -27.8 parts per thousand +/- 23.3 parts per thousand.