Measurement and Modeling the Phase Partitioning of Organophosphate Esters Using Their Temperature-Dependent...

Atmospheric concentrations of 11 organophosphate esters (OPEs) were measured in an urban area in Izmir, Turkey to explore their phase partitioning. Octanol-air partition coefficients (K_OA) and vapor pressures (P_L) of the OPEs were also measured as a function of temperature. Average Σ₁₁OPE gas-phase concentrations were 1.77 ± 0.84 and 4.00 ± 1.77 ng/m³, while particle-phase concentrations were 1.95 ± 0.77 and 1.15 ± 0.36 ng/m³ during winter and summer, respectively. TCiPP1 dominated Σ₁₁OPEs, followed by TnBP and TEP. OPE concentrations generally increased and shifted to gas-phase in the summer probably due to higher temperatures that favor partitioning to the gas-phase. Distribution between two phases covered a wide range from being primarily in gas-phase (TEP, TnBP) or particle-phase (EHDPP, TEHP, T2iPPP). Phase partitioning was also examined via four widely used models (K_OA, Soot, Steady-State, and pp-LFER). All models underestimated the majority of particle-gas partition coefficients (K_P) especially for the compounds having higher volatilities. Estimations based on the recently reported molecular weight of organic matter in urban aerosols (MW_OM) and activity coefficients of OPEs in octanol (ξ_OCT) determined in the present study suggested that the basic assumptions of K_OA-based models (i.e., ξ_OCT/ξ_OM and MW_OCT/MW_OM = 1) are not valid.