Deposition to a grid cell in CMAQ is calculated from grid average land use characteristics such as surface roughness, leaf area index, and canopy height.  The grid-average value is constructed in the Pleim-Xu Land surface Model contained in the Weather Research and Forecasting (WRF) meteorological model.  The grid averaged characteristics are used to calculate the aerodynamic resistance and the stomatal resistance which are key parameters in the formulation for surface fluxes, including momentum and moisture.  The grid-average resistances are passed from the meteorological model to the CMAQ chemical transport model for use in the deposition velocity calculations to obtain a deposition velocity for each chemical for each grid cell.

Dry deposition varies with the underlying land surface (see Figure 1) and ecological applications often require more detailed estimates rather than grid average values.  EPA has recently added the capability to CMAQ to calculate and output hourly land-use specific deposition values.

Figure 1. Example of hourly HNO3 deposition velocity by land use type for a grid cell in Oklahoma.