Aqueous secondary formation substantially contributes to organophosphate esters in aerosols

Chemicals of emerging concern (CECs), like organophosphate esters (OPEs), are toxic substances threatening human/wildlife health. Yet the atmospheric transformation of CECs remains poorly understood. Here we combine field measurements and partitioning models to test the hypothesis that the secondary formation of OPEs could be enhanced by heterogenous processes in aerosols. We found that hydrophobic OPEs are absorbed favorably into the organic phase, whereas hydrophilic OPEs preferably partition into the aqueous phase. We provide field evidence that enhanced aqueous secondary formation of OPEs occurs in winter, and the magnitude is strongly dependent on aerosol water content. We suggest that dissolved inorganic salts and transitional metals in aerosols impact positively on the formation of particle-bound hydrophilic OPEs, by facilitating their aqueous partitioning and/or oxidation. Our findings highlight the important role of aqueous oxidation chemistry for the fate of CECs in the atmosphere, urging for a better consideration of transformation products in future risk assessment and chemicals management.