We measured singlet oxygen (¹O₂*) and triplet excited states of organic matter (³C*) in illuminated aqueous extracts of PM_2.5 collected in different seasons at different sites in Hong Kong SAR, South China. In contrast to the locations, seasonality had significant effects on ³C* and ¹O₂* production due to seasonal variations in long-range air mass transport. The steady-state concentrations of ³C* and ¹O₂* correlated with the concentration and absorbance of water-soluble organic carbon.