Hemispherical scale mechanisms of nitrate formation in global marine aerosols

Abstract Nitrogen oxides (NO x ) in the atmosphere directly affect air quality; however, their oxidation products (e.g., nitrate) are essential nutrients in terrestrial and marine ecosystems. To date, the mechanism and rate of nitrate formation in the global-scale marine boundary layer remains uncertain. Herein, aerosol nitrate isotopes covering the global ocean were analysed and observations indicated that nitrate formation was dominated by the proportion between the hydroxyl radical (daytime) and ozone (nighttime) pathways in the Northern Hemisphere (NH), whereas it changed to the BrO (Antarctic iceberg emission) pathway in the Southern Hemisphere (SH). These differences in the pathways suggested that the NO x removal (nitrate formation) efficiency was higher in the NH, which could be responsible for the much higher nitrate concentrations in the NH than in the SH (over twice). This study can assist in formulating effective policies to mitigate global NO x pollution and improve our understanding of the impact of increasing NO x concentrations on global ocean productivity.