Hi, hczuo-
For grid-resolved (rescld.F) and sub-grid (convcld_acm.F) clouds, CMAQ calculates aqueous-phase chemistry, wet scavenging, accumulated wet deposition amounts, and in the case of sub-grid clouds, vertical redistribution.
When liquid water content (LWC), represented as the sum of cloud water, rain water, and graupel, in a cell (or column average in the case of sub-grid clouds) exceeds a critical threshold of 0.01 gm-3, a call is made to the cloud chemistry module (aq_map.F, aqchem.F) where in-cloud scavenging and wet deposition are calculated in addition to aqueous-phase chemistry. Accumulation and coarse mode aerosols are assumed to be instantaneously activated (i.e., nucleation scavenging), and Aitken mode particles (i.e., interstitial aerosol) are scavenged by the cloud droplets for the duration of cloud processing (Binkowski and Roselle, 2003). Gas-phase species that participate in aqueous-phase chemistry are taken up into cloud water according to their Henry’s law coefficient, dissociation constants, and droplet pH. For each cloud chemistry time step, dissolved gas and aerosol species and associated ions are deposited out of the system according to a scavenging rate that is based on precipitation rate, cloud/layer thickness, and total water content (i.e., the sum of cloud water, rain water, graupel, ice, and snow).
When the liquid water content does not exceed the threshold to call the cloud chemistry module (or for all species that do not participate in cloud chemistry), the wet deposition is calculated similarly in scavwdep.F. Using the same expression for the washout coefficient as in the aqueous-phase chemistry module, aerosol species are subject to wet removal assuming they are incorporated into cloud/rain water as above; while the fraction of gas phase species’ concentrations subject to wet removal is a function of their effective Henry’s law coefficients at a prescribed droplet pH of 4.
Essentially what is represented in CMAQ is in-cloud scavenging (or “rainout”); though arguably some effects of below-cloud scavenging (or “washout”) may also be represented via the inclusion of rain water in the LWC considered in calling/calculating the cloud chemistry module and, in the case of sub-grid raining clouds, calculating aqueous-phase chemistry and scavenging for the column (extending from the cloud top to the ground). Explicit treatment of below-cloud scavenging (e.g., impaction scavenging of below-cloud aerosols by rain drops and snow) is not implemented at this time.
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 additional information about how wet scavenging and deposition is calculated.
Hope this is helpful!
Kathleen