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Modeling Nitrate Concentrations and Deposition: Sensitivities to Cloud Treatment and Landuse

Alice Gilliland*, Shawn Roselle, Jon Pleim, and Prakash Bhave
NOAA Atmospheric Sciences Modeling Division
In partnership with USEPA
USEPA Mailroom E243-01
109 T.W. Alexander Drive
Research Triangle Park, NC 27711


Past evaluations of the Community Multiscale Air Quality (CMAQ) model have shown notable over predictions of nitrate aerosol and total nitrate concentrations under winter conditions. Much work has been done to refine heterogeneous chemical pathways, thermodynamic equilibrium models, and ammonia emission estimates; however, over prediction biases are still evident. Recent model tests for winter 2002 have identified key meteorological model sensitivities that are affecting the total nitrate concentrations and deposition of nitrate. Using two different microphysics schemes with the MM5 meteorological model, the CMAQ model predictions of wet deposition varies by approximately 25% while precipitation volume is only negligibly different. Additional analysis has shown large differences in the nitric acid dry deposition velocities from these two MM5 simulations, which appears to be related to differences in the landuse data and resulting surface roughness and aerodynamic resistance estimated for CMAQ. Sensitivities of total nitrate concentrations and deposition to these factors are quite large. More detailed evaluation of the total nitrate budget needs to be conducted using various meteorological model options with particular attention to the cloud and landuse impacts.