Aqueous secondary formation substantially contributes to hydrophilic organophosphate esters in aerosols.

Chemicals of emerging concern (CECs), like organophosphate esters (OPEs), are toxic substances threatening human and wildlife health. Yet the atmospheric transformation of CECs remains poorly understood. Here we combine field measurements and partitioning models to reveal that OPEs could be enhanced by aqueous-phase processes in aerosols. We show 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 its magnitude is strongly dependent on aerosol water content. We suggest that dissolved inorganic salts and transition metals in aerosols positively impact the formation of particle-bound hydrophilic OPEs, by facilitating aqueous partitioning and/or oxidation. Our findings highlight the important role of aqueous oxidation chemistry for the fate of CECs in the atmosphere, urging better consideration of transformation products in future risk assessment and chemical management.