Sub-grid Dry Deposition Estimates from the Community Multiscale Air Quality Model (CMAQ)

Donna Schwede1, Robin Dennis and Jonathan Pleim
U.S. Environmental Protection Agency
National Exposure Research Laboratory
Atmospheric Modeling and Analysis Division
Research Triangle Park, NC 27711, USA

Atmospheric deposition is an important contributor to total ecosystem loadings of pollutants such as sulfur and nitrogen. Quantification of the deposition is often challenging as direct measurements of dry deposition are seldom available for individual ecosystems. Monitoring networks provide point estimates of deposition, but spatial interpolation of the values to other sites is not appropriate. CMAQ, a regional scale air quality models is capable of providing the needed estimates of deposition at all sites within the modeling domain. However, CMAQ currently only calculates a single deposition velocity per grid cell for each chemical.

Dry deposition velocity varies with underlying vegetation type due to differences in leaf area index, canopy height, and plant characteristics such as minimum stomatal resistance. The deposition velocity calculation for CMAQ is a combination of processes modeled in the land-surface component of the meteorological model and the chemical transport model. Since CMAQ is a grid-based model, the influences of the different land covers that comprise a grid cell are averaged in the meteorological model for use in the land-surface model. These grid-average values are carried forth from the meteorological model to the chemical transport model where chemical specific deposition velocity calculations are done. Ecological applications require information regarding the amount of deposition to subgrid land cover types. To be able to provide this information without requiring modification of the meteorological model, an approach has been implemented in CMAQ that disaggregates these grid-average values within CMAQ to allow output of dry deposition estimates for each land cover type within a grid in a manner consistent with meteorological model flux calculations. These land cover specific deposition estimates can then be used in studies such as critical loads analyses. Comparisons between the grid-average values of deposition flux and the land cover specific fluxes are presented.

1 Corresponding author: Donna Schwede, U.S. Environmental Protection Agency, National Exposure Research Laboratory, Atmospheric Modeling and Analysis Division, Research Triangle Park, NC 27711, phone: 919-541-3255, email: