Abstract The relationships have been examined between the presence or absence of cumulus clouds and 3rd quarter fine particle sulfur concentrations in St Louis. An association between the presence of cumulus clouds with SO 2 conversions in droplets and incrementally higher fine particle sulfur concentrations can be demonstrated. However, diurnal patterns of fine particle sulfur concentrations in the presence of cumulus clouds are not consistent with local contributions from sulfate formation in cumulus clouds. Morning fog often occurs on the same days on which cumulus clouds form later. Reactions of SO 2 in fog droplets appear to make a contribution, but do not account for the major part of the increments in fine particle sulfur concentrations associated with cumulus clouds. The variations in fine particle sulfur concentrations observed can be explained if a substantial part of the sulfate formed in cumulus is transported upwards from the planetary boundary layer into the lower free troposphere. Subsequent multiday regional scale horizontal transport with concurrent gradual vertical transport of sulfate down to the surface would be consistent with the observed results.