Optimized below-cloud scavenging scheme and its prominent performance of nitrogen wet deposition in polluted regions

Abstract. Simulations from Model Inter-Comparison Study for Asia (MICS-Asia) phase III generally underestimate regional nitrogen wet deposition, mainly because aerosol below-cloud scavenging processes are oversimplified or ignored in models. To address the problem, most existing studies adopt empirical corrections rather than optimizing physical mechanisms. This study develops an improved physically-based below-cloud wet scavenging parameterization scheme, which is implemented into the Nested Air Quality Prediction Model System (NAQPMS) for comparative simulations over East Asia in 2014. On the basis of conventional collision mechanisms, this scheme further incorporates three additional key mechanisms, including thermophoresis, diffusiophoresis and electrostatic interaction. The results show that the new scheme not only well captures the observed aerosols below-cloud scavenging coefficients, but also enables refined characterization of separate in-cloud and below-cloud wet deposition processes. Pronounced simulation improvements are found in heavily polluted regions including Beijing-Tianjin-Hebei region, Northeast China and Yangtze River Delta, while slight overestimation appears at several sites in Korea and Japan. Furthermore, the new scheme effectively elevates the contribution of below-cloud scavenging, especially for reduced nitrogen, achieving good agreement with observations in Beijing. Accurate simulations in both the in-cloud and below-cloud processes facilitate the reliable assessments of long-range transport and near-surface deposition fluxes of pollutants. This study demonstrates the necessity of fully resolving physically based wet scavenging mechanisms for high-precision nitrogen deposition modeling, which further advances our understanding of regional pollutant transport and the global nitrogen cycle.