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

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 between methanesulfonate (MSA), which is considered a 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 with continental air masses. The Neutralization Ratio, 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 sulfate (nssSO₄²⁻) and NH₄⁺, suggesting that NH₃ preferentially reacted with H₂SO₄ over HNO₃ in the MBL, especially in areas with excessive acidity.