Isotopic evidence that alkyl nitrates are important to aerosol nitrate formation in the Equatorial Pacific

Atmospheric nitrogen (N) deposition to the surface ocean (i.e., via precipitation and aerosols) can have important implications for biogeochemistry. Recent work in the North Pacific has detected an excess amount of N along the coastlines and in the open ocean, which has been attributed to atmospheric deposition (AD) of anthropogenic N. Yet, observed nitrate (NO₃^-) within the free troposphere of the equatorial Pacific could not be attributed to anthropogenic sources. Globally, atmospheric NO₃^- is primarily derived from fuel combustion that produces precursor N oxides (NO_x = NO + NO₂), but the relatively short lifetimes of NO_x and NO₃^- result in significant variability that can be difficult to tie directly to source emissions. The ocean is not a major driver of NO_x; however, emission of alkyl nitrates (RONO₂) from the ocean can contribute directly to NO₃^- in the atmosphere or, via decomposition, alter the NO_x budget. We quantified concentrations and stable isotopes of NO₃^- (δ¹⁵N-, δ¹⁸O-, Δ¹⁷O-NO₃^-) in aerosols collected during two GEOTRACES cruises: 1) Alaska-Tahiti in Fall 2018 (GP15; 55.0 to -20.0°N, 152.0°W; n=22), and 2) Peru-Tahiti in Fall 2013 (GP16; 4.1°S, 81.9°W to 10.5°S, 152.0°W; n=17). The δ¹⁵N-NO₃^- ranges from -1.1 to -13.1‰, with the lowest values occurring in the Equatorial Pacific, away from the coasts. The very low δ¹⁵N-NO₃^- throughout the Pacific atmosphere represents an isotopically light source of N to the surface ocean, and the lowest values are consistent with RONO₂ being a source of aerosol NO₃^-. The δ¹⁸O and Δ¹⁷O of atmospheric NO₃^- are relatively high (65.2-85.4‰ and 21.4-30.7‰, respectively) reflecting the isotopic influence of ozone oxidation on NO_x. Both δ¹⁸O and Δ¹⁷O decrease with increasing distance from high [NO_x] regions (i.e. continental), likely reflecting a decrease in atmospheric ozone concentrations and an increase in NO₃^- produced via oxidants with much lower oxygen isotope values. The co-occurrence of low δ¹⁵N, δ¹⁸O and Δ¹⁷O provide evidence for an important influence of RONO₂ on aerosol NO₃^- in the Equatorial Pacific region. Based on the Δ¹⁷O, we quantify that the contribution of RONO₂ to aerosol NO₃^- can be as high as 45% (ranging 11-45%). This is also consistent with transport modeling, which showed that air masses associated with samples in this region originated over the open ocean.