Characterizing the Global Tropospheric Budget of Oxidized Nitrogen (NO_y)

Abstract. Nitrogen oxides (NO_x = NO + NO₂) in the troposphere form an array of secondary pollutants that are detrimental to air quality, ecosystems, and climate. The family of reactive oxidized nitrogen (NO_y) in the atmosphere consists of NO_x and its reservoir species (e.g. HNO₃, PAN). Our understanding of the processes underlying the transformation of NO_y has advanced considerably over recent decades, however, the relative importance of NO_y partitioning and loss pathways remain uncertain. In this study, we use the GEOS-Chem global chemical transport model and observations from the ATom flight campaign to assess the simulated global budget of tropospheric NO_y, and the production and loss fluxes between key NO_y species. Our simulation indicates that the mean global chemical lifetime of NO_x is ~21 hours and the mean global deposition lifetime of NO_y is 5.5 days. The global mean NO_x:NO_y ratio at the surface is 0.23 (over continents it is 0.34) and at 500 hPa is 0.10. In addition to the four most prevalent gas-phase species (NO, NO₂, HNO₃, PAN) that have been central to previous descriptions of tropospheric NO_y chemistry, we find that other species play key roles in driving overall chemical cycling. The model representation of organic nitrate chemistry is highly simplified and likely overestimates the importance of hydrolysis as a sink while underestimating deposition. Finally, the photolytic loss of particulate nitrate (pNO₃^-) to form NO₂ and HONO, as represented in our simulations, is comparable to its depositional loss, indicating the importance of further constraining this photolysis sink.