Atmospheric organophosphate esters in the urban environment: Seasonal trend, diurnal variation, and risk assessment

Organophosphate esters (OPEs) are widespread environmental contaminants used primarily as flame retardants and plasticizers, yet their fate remains poorly understood. This study comprehensively examined OPEs in indoor and outdoor air in urban environments, focusing on concentrations, seasonal patterns, and health risks. Seventeen OPEs, including aryl, alkyl, and chlorinated types, were analyzed in fine particulate matter (PM_2.5) and the gas phase over a year-long sampling campaign. Outdoor gas phase OPE concentrations ranged from 0.04 to 2.68 ng/m³, while particulate concentrations varied from 0.07 to 0.49 ng/m³. Indoor gas phase concentrations were higher, ranging from 0.13 to 7.84 ng/m³, and particulate concentrations from 0.08 to 1.26 ng/m³. Chlorinated OPEs dominated the gas phase, whereas alkyl OPEs were more prevalent in the particulate phase. Seasonal variations revealed elevated OPE levels in spring and summer, attributed primarily to increased volatilization and marine-derived air masses, whereas winter concentrations were the lowest and occasionally linked to long-range transport from northeastern China. Diurnal measurements indicated that indoor gas phase OPE concentrations were higher during daytime, coinciding with increased human activities and elevated indoor temperatures. Principal component analysis suggested local industrial emissions and regional transport from Southeast Asia and Northeast China as key sources. Estimated daily intake (EDI) values for children exceeded those for adults, indicating greater vulnerability. However, hazard quotient (HQ) values for both groups remained below unity, implying low non-carcinogenic inhalation risk. These findings highlight the importance of continued monitoring and source control of OPEs, especially in indoor environments where human exposure is most pronounced.