Removal of sulfur dioxide and formation of sulfate aerosol in Tokyo

Abstract

Ground‐based in situ measurements of sulfur dioxide (SO2) and submicron sulfate aerosol (SO42−) together with carbon monoxide (CO) were conducted at an urban site in Tokyo, Japan from spring 2003 to winter 2004. The observed concentrations of SO2 were affected dominantly by anthropogenic emissions (for example, manufacturing industries) in source areas, while small fraction of the data (<30%) was affected by large point sources of SO2 (power plant and volcano). Although emission sources of CO in Tokyo are different from those of SO2, the major emission sources of CO and SO2 are colocated, indicating that CO can be used as a tracer of anthropogenic SO2 emissions in Tokyo. The ratio of SO42− to total sulfur compounds (SOx = SO2 + SO42−) and the remaining fraction of SOx, which is derived as the ratio of the linear regression slope of the SOx‐CO correlation, is used as measures for the formation of SO42− and removal of SOx, respectively. Using these parameters, the average formation efficiency of SO42− (i.e., amount of SO42− produced per SO2 emitted from emission sources) are estimated to be 0.18 and 0.03 in the summer and winter periods, respectively. A simple box model was developed to estimate the lifetime of SOx. The lifetime of SOx for the summer period (26 h) is estimated to be about two times longer than that for the winter period (14 h). The seasonal variations of the remaining fraction of SOx, estimated formation efficiency of SO42−, and lifetime of SOx are likely due to those of the boundary layer height and photochemical activity (i.e., hydroxyl radical). These results provide useful insights into the formation and removal processes of sulfur compounds exported from an urban area.