The effects of photochemical oxidation of reduced sulfur compounds (RSCs: H2S, CH3SH, DMS, and DMDS) on SO2 production were evaluated at high (HV) and low ventilation (LV) conditions, based on a CALPUFF dispersion model coupled with photo-chemical oxidation mechanisms for four RSCs. The RSC emission concentrations used in the modeling were measured in and around an urban valley during a field campaign held in October 2008. SO2 production with LV (up to 156 ppb at 0900 LST) was found to be significantly higher than that with HV (up to 30 ppb). SO2 produced by photochemical oxidation of RSCs with LV (78% of total SO2 concentrations) was much higher than that with HV (27%), while the predominant RSC species were similar: DMDS (>= 60% of the total contributions) with HV and LV when compared to three other RSCs (< 20%). The difference in SO2 concentration between HV and LV might be caused by the combined effects of photochemical oxidation of RSCs and ventilation condition.