Water‐Insoluble Organic Nitrogen in Wet and Dry Atmospheric Deposition Over Eastern China

Organic nitrogen (ON) is a substantial but incompletely constrained component of atmospheric nitrogen deposition, particularly the water‐insoluble fraction that is seldom quantified directly. Here we present one year (April 2023–March 2024) of simultaneous wet and dry deposition observations at four sites spanning major population and emission centers in eastern China (two urban sites and two suburban sites across the Beijing–Tianjin–Hebei, the Yangtze River Delta (YRD), and the Pearl River Delta (PRD) regions). Inorganic nitrogen (IN; NH 4 + , NO 3 − , and NO 2 − ) and water‐soluble total nitrogen were measured in the deposition solutions, and water‐insoluble organic nitrogen (WION) was quantified by direct elemental analysis of the particulate fraction retained on quartz filters. Annual total nitrogen deposition ranged from 18.62 to 32.85 kg N ha −1 yr −1 , with wet deposition contributing 56%–73%. IN was the largest contributor at all sites, while ON accounted for 26%–33% of wet and 32%–46% of dry nitrogen deposition. Water‐insoluble organic nitrogen contributed 5%–13% of total nitrogen deposition (annual WION flux 1.57–2.50 kg N ha −1 yr −1 ) and up to ∼24% of dry nitrogen deposition at the northern suburban site. Water‐insoluble organic nitrogen showed pronounced spatial and seasonal variability and generally covaried with calcium (Ca 2+ ) and episodically with potassium (K + ), consistent with contributions from mineral dust and biomass burning. These observations provide an updated constraint on ON speciation in deposition and demonstrate that explicitly quantifying WION can materially affect regional nitrogen deposition budgets.