Quantifying Nitrate Formation Pathways in the Equatorial Pacific Atmosphere from the GEOTRACES Peru-Tahiti Transect

This study aims to better our collective understanding of the oxidative capacity and atmospheric chemistry over the equatorial Pacific. Bulk and size-segregated filter samples were collected during the GEOTRACES Eastern Tropical Pacific transect (4.1 degrees S, 81.9 degrees W to 10.5 degrees S, 152.0 degrees W; October-December 2013) and measured for aerosol concentration and complete isotopic composition of nitrate (delta N-15, delta O-18, Delta O-17 where Delta O-17 = delta O-17 - 0.52 x delta O-18). Combined size-segregated filters produced data similar to that found in bulk filter samples, and notably neither delta N-15 nor delta O-18 showed any trends based on aerosol size. Similar to other studies, NO3- is concentrated (>80%) in the coarse size fractions (>1.5 mu m). Bulk aerosol concentrations ranged from 6.6 to 89.8 nmol/m(3). The bulk delta N-15-, delta O-18-, and Delta O-17-nitrate ranged from -13.1 to -3.2%, 68.5 to 79.3%, and 23.5 to 28.4%, respectively. Higher delta N-15 values near the coast are best explained by the influence of continental sources; lower delta N-15 values far from the coast may be associated with chemical fractionation during long-range transport or an oceanic source. Both Delta O-17 and delta O-18 are interpreted using kinetic analysis and gas concentrations from a global atmospheric chemical transport model (GEOS-Chem), which showed that nitrate production in this environment is dominated by OH oxidation (60%) and RONO2 hydrolysis (15%). To best match the delta O-18 and Delta O-17 observations in this study, the terminal oxygen isotopic values for ozone must be higher than those suggested by available observations and/or halogen-mediated chemistry must be more important than the models currently suggest.