On the Effect of Daytime Surface Evaporation on Pollution Dispersion

-The general effects of surface moisture availability on regional and local pollution dispersion during the day are examined through conceptual modeling and analytic analysis, when surface moisture availability is assumed to be homogeneous over regional scales. The results suggest that in low mid-latitudes and sub-tropical regions the change in Pasquill stability classes around noon, between extremely wet and very dry surfaces, is typically one stability class shift. In the morning and the afternoon, however, the corresponding change is more pronounced. Similar trends were suggested when the evaluations were made in terms of w*/U (where w* is the convective scaling velocity and U the mean wind speed within the daytime convective boundary layer). Also, numerical model simulations and analytical evaluations were carried out providing an estimation of the mixing layer depth as dependent on the surface moisture availability. The mixing layer depth over extremely dry surfaces was found to be ~ 1.5-2 higher compared to that computed over very wet surfaces. Observational aspects are discussed, suggesting complementary insight into the interpretation of the modeling and the analysis results. Key word index: Pollution dispersion, surface evaporation, convective boundary layer.