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

Wet 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 PM_2.5 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.