SENSITIVITY OF URBAN AIRSHED MODEL (UAM-IV) CALCULATED AIR POLLUTANT CONCENTRATIONS TO THE VERTICAL DIFFUSION PARAMETERIZATION DURING CONVECTIVE METEOROLOGICAL SITUATIONS

It is shown that Urban Airshed Model (UAM-IV) calculated air pollutant concentrations during photochemical smog episodes in Atlanta, Georgia , depend strongly on the numerical parameterization of the daytime ver tical diffusivity. Results found suggest that vertical mixing is overe stimated by the UAM-IV during unstable daytime conditions, as calculat ed vertical diffusivity values exceed measured and comparable literatu re values. Although deviations between measured and UAM-IV calculated air pollutant concentrations may only in part be due to the UAM-IV dif fusivity parameterization, results indicate the large error potential in vertical diffusivity parameterization. Easily implemented enhanceme nts to UAM-IV algorithms are proposed, thus improving UAM-IV modeling performance during unstable stratification.