Improving Wet and Dry Deposition of Aerosols in WRF‐Chem: Updates to Below‐Cloud Scavenging and Coarse‐Particle Dry Deposition

Abstract

AbstractWet and dry depositions of aerosols in WRF‐Chem are revisited and updated based on recent observational findings. Traditionally, in‐cloud scavenging was thought to play a more dominant role in aerosol wet removal than below‐cloud scavenging. However, recent field measurements indicated a considerable contribution of below‐cloud scavenging of 50%–60% to total wet deposition. In contrast, the simulated contribution of in‐cloud scavenging in the previous version of WRF‐Chem was too large, exhibiting 88%–95%, likely due to the binary representation of cloud fraction. To reduce the model bias, this study adopts a continuous‐type cloud fraction and implements a semi‐empirical below‐cloud scavenging parameterization. Simulation results with the new scheme show that the contribution of below‐cloud (in‐cloud) scavenging is increased to 63%–66% (decreased to 34%–37%), well capturing the observational estimates. The magnitude of total wet deposition is increased by 18.2% for SO4, 7.16% for NO3, and 14.8% for NH4, showing better agreements with observations particularly for SO4 and NH4 deposition. The increased wet removal with the new scheme reduces and so better reproduces surface PM2.5 and PM10 concentrations, which is also partly attributed to the increased contribution of below‐cloud scavenging. It is found that dry deposition velocity in the previous version was too high for coarse mode particles when friction velocity is large, which underestimates surface PM10 concentration. The updated dry deposition scheme that is constrained by observations effectively improves PM10 performance by reducing the dry deposition velocity for coarse mode particles.