Gas phase experiments were conducted to examine the heterogeneous oxidation of sulfur dioxide on methane soot in the presence of ozone and water vapor. The enhanced formation of sulfate at atmospheric pressure was confirmed by the use of a particle-into-liquid sampler (PILS) coupled with ion chromatography (IC). Due to the ozone oxidation, multilayer adsorption of sulfur dioxide on soot, as well as sulfate formation and physisorption on secondary surface layer sites was observed. The ozone exposure also caused the soot to become hydrophilic, due to the formation of sulfuric acid and also likely the formation of carboxyl groups on the surface. For the soot and sulfur dioxide concentrations used, the sulfate yield increased with ozone levels, but no increase was observed at ozone concentrations above 2600 ppm. (C) 2011 Elsevier Ltd. All rights reserved.