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.

Automated summary

This study investigated the oxidative capacity and atmospheric chemistry over the equatorial Pacific, finding that nitrate production is dominated by OH oxidation and RONO2 hydrolysis. Aerosols were mainly concentrated in coarse size fractions, with no trends in delta N-15 and delta O-18 based on aerosol size.