Hi, Lin-
Please checkout section 6.12 of the CMAQ users’ guide (CMAQ/DOCS/Users_Guide/CMAQ_UG_ch06_model_configuration_options.md at main · USEPA/CMAQ · GitHub) and Chapter 11 of the original CMAQ science document (CMAS: Community Modeling and Analysis System) for more information about how wet scavenging and deposition is calculated.
Below-cloud scavenging by snow is not explicitly calculated at this time. However, wet removal of gas and aerosol species occurs according to a parameterization that is based on precipitation rate, Henry’s law coefficient (for gases) and total water content (i.e., the sum of cloud water, rain water, graupel, ice, and snow).
Please let me know if you have any other questions.
Kathleen