Prediction of gas/particle partitioning of polybrominated diphenyl ethers (PBDEs) in global air: A theoretical study

Gas/particle (G/P) partitioning of semi-volatile organic compounds (SVOCs) is an important process that primarily governs their atmospheric fate, long-range atmospheric transport, and their routes of entering the human body. All previous studies on this issue are hypothetically based on equilibrium conditions, the results of which do not predict results from monitoring studies well in most cases. In this study, a steady-state model instead of an equilibrium-state model for the investigation of the G/P partitioning behavior of polybrominated diphenyl ethers (PB-DEs) was established, and an equation for calculating the partition coefficients under steady state (K-PS) of PB-DEs (log K-PS = log K-PE + log alpha) was developed in which an equilibrium term (log K-PE = log K-OA + log f(OM) -11.91 where f(OM) is organic matter content of the particles) and a non-equilibrium term (log alpha, caused by dry and wet depositions of particles), both being functions of log K-OA (octanol-air partition coefficient), are included. It was found that the equilibrium is a special case of steady state when the non-equilibrium term equals zero. A criterion to classify the equilibrium and non-equilibrium status of PBDEs was also established using two threshold values of log K-OA, log K-OA1, and log K-OA2, which divide the range of log K-OA into three domains: equilibrium, non-equilibrium, and maximum partition domain. Accordingly, two threshold values of temperature t, t(TH1) when log K-OA = log K-OA1 and t(TH2) when log K-OA = log K-OA2, were identified, which divide the range of temperature also into the same three domains for each PBDE congener. We predicted the existence of the maximum partition domain (the values of log K PS reach a maximum constant of -1.53) that every PBDE congener can reach when log K-OA >= log K-OA2, or t <= t(TH2). The novel equation developed in this study was applied to predict the G/P partition coefficients of PBDEs for our Chinese persistent organic pollutants (POPs) Soil and Air Monitoring Program, Phase 2 (China-SAMP-II) program and other monitoring programs worldwide, including in Asia, Europe, North America, and the Arctic, and the results matched well with all the monitoring data, except those obtained at e-waste sites due to the unpredictable PBDE emissions at these sites. This study provided evidence that the newly developed steady-state-based equation is superior to the equilibrium-state-based equation that has been used in describing the G/P partitioning behavior over decades. We suggest that the investigation on G/P partitioning behavior for PBDEs should be based on steady state, not equilibrium state, and equilibrium is just a special case of steady state when non-equilibrium factors can be ignored. We also believe that our new equation provides a useful tool for environmental scientists in both monitoring and modeling research on G/P partitioning of PBDEs and can be extended to predict G/P partitioning behavior for other SVOCs as well.