Journal of Atmospheric Chemistry 14: 43-51, 1992. © 1992 KluwerAcademic Publishers. Printedin the Netherlands. THE TRANSPORT AND REDISTRIBUTION OF ATMOSPHERIC GASES IN REGIONS OF FRONTAL RAIN N. CHAUMERLIAC 1, R. ROSSET 1, M. RENARD and E.C. NICKERSON 2 1.LAMP/OPGC, 63000 Clermont-Ferrand, France 2.NOAA, Forecast Systems Laboratory, Boulder, Colorado 80303 ABSTRACT. Gases emitted in the planetary boundary layer can be transported very ef- ficiently to the free troposphere through vertical motion along a frontal surface. A mesoscale numerical model was used to simulate the vertical transport of a tracer by clouds during frontogenesis in a moist atmosphere (an evolving Eady wave) in order to illustrate such vertical transport conditions. It is shown that the efficient vertical transport of a tracer occurs only when clouds are present, either when a surface or an in-situ source is consid- ered. Insoluble, partially soluble, and soluble tracers are studied in order to determine the relative importance of vertical transport and scavenging on their redistribution. Keywords: Tracer vertical exchanges, cloud scavenging, mesoscale modeling. 1. Introduction Frontal systems are responsible for the transport of materials between the boundary layer and the upper troposphere and can transform a local air pollution problem into a regional or global problem. In global general circulation models, factors affecting the distribution of pollutants in frontal situations such as emissions, vertical exchanges and deposition mechanisms are poorly represented due to a coarse spatio-temporal resolution and a lack in subgrid scale parameterizations of the important microphysical and chemical processes. A mesoscale model, which includes detailed representations of boundary layer physics and microphysics can help to define an effective regional source and bridge the gap between a local problem and its global impact. This paper presents results from a series of sensitivity tests using a mesoscale meteoro- logical model to address successively the role of source effects and solubility effects on the vertical transport of a tracer both with and without clouds. Also shown and discussed are the mechanisms such as downdrafts created by evaporative cooling, and the filtering effects of clouds which scavenge the soluble tracer and hence limit its vertical transport.