Spatial distribution of ammonium in the global marine boundary layer from Arctic to Antarctic

Abstract Ammonium (NH4+) in the marine boundary layer (MBL) was measured during the Chinese Antarctic (2015–2016) and Arctic (2021) Research Expedition. A significant latitudinal gradient in NH4+ concentration was observed, and NH4+ was mainly in the fine-mode (< 2.1 μm) in the MBL. NH4+ showed the highest concentration in the coastal East Asia (CEA), followed by the coastal Southeast Asia and Australia (CSAA). Low NH4+ levels were detected in the open oceans, i.e., the Arctic Ocean (AO), the Southern Ocean (SO), and the western Pacific Ocean (WPO). In the AO, WPO, and SO regions, the air masses arriving at the sampling sites mainly originated from the open oceans, and the significant correlation with methanesulfonate (MSA) which is considered as the marker of marine biogenic sources and NH4+ suggests that marine emissions associated with marine primary productivity are likely an important source of NH4+. However, in the two near-continental regions of CEA and CSAA, elevated NH4+ concentrations are mainly dominated by dense human activities associated to continental air masses. The Neutralization Ratio (NR), defined as the extent to which acidic sulfate and nitrate compounds are neutralized by NH₄⁺, was much smaller than 1.0 (mean=0.07) in all samples, indicating that the transformation and generation of NH4+ are mainly controlled by the source (i.e., NH3 levels). In addition, there was a significant correlation between non-sea-salt SO42- (nssSO42-) and NH4+, suggesting that NH3 preferentially reacted with H2SO4 over HNO3 in the MBL, especially in areas with excessive acidity.