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.