Regional modeling of polycyclic aromatic hydrocarbons and other persistent organic pollutants : model development and applications

Polycyclic aromatic hydrocarbons (PAHs) and other persistent organic pollutants (POPs) are hazardous pollutants in the environment. Due to the relatively long environmental lifetime, they can often be distributed globally. POPs can accumulate along food chains and reach toxic levels for biota. Moreover, their physical and chemical processes in the Earth system are complex. In particular, (1) many substances are semi-volatile such that they partition between particulate and gas phase; (2) upon deposition to the soil, many substances which resist biodegradation in the soil, can re-volatilize into the atmosphere; (3) the degradation of the particulate phase in the air is not well accounted for in the ambient conditions, which is over-simplified or completely neglected. As these processes are not well described or missing in the current atmospheric models, the characterization of atmospheric cycling, fate, environmental exposure to and long-range transport potential of these pollutants is hindered. In this study, a regional atmospheric chemical transport model is updated by improving and including up-to-date terrestrial cycling processes of PAH/POP. In this PhD work, the regional chemical transport model WRF-Chem is extended to include all the up-to-date physical and chemical processes of PAH/POP, such as emission, transport, gas-particle partitioning, air-soil gas exchange, heterogeneous degradation, gas-phase reaction, cloud scavenging, dry deposition and wet deposition. The extended model is named WRF-Chem-PAH/POP. Predicted atmospheric concentrations and particulate mass fractions are evaluated against near-source and remote-outflow observation data in high resolutions. The predictions have been largely improved compared with previous modeling studies. Besides, sensitivity tests verify the necessity to include homogeneous reaction with NO3 and multiphase reaction with O3 in the model. The WRF-Chem-PAH/POP model is applied to study the multiphase degradation of benzo[a]pyrene (BaP), one of the most toxic PAHs. A new kinetic framework depending on environmental temperature and humidity is implemented. Temperature and humidity can change the phase state of BaP-absorbed organic particles, influence the chemical reactivity and thus long-range transport potential of BaP. Model results show that the new kinetic scheme can significantly improve model predictions at various kinds of sites systematically, implying a need to re-evaluate the underestimated long-range transport potential of BaP. Re-volatilization of soil accumulated POPs under the South Asian summer monsoon is also explored by the WRF-Chem-PAH/POP model. The onset of summer monsoon brings clean air masses from the southern hemisphere and corresponds with the observed reduction of atmospheric pollution levels over the southern parts of India. The enhanced difference between air and soil concentrations triggers the re-volatilization of POPs from soils, which are accumulated in soils as a result of multidecadal agricultural (e.g. pesticides) and industrial (e.g. polychlorinated biphenyls) activities. These pollutants are banned since decades but continue to cycle in the environment. The modeled results agree well with the observations and confirm this explanation of the phenomenon. In conclusion, WRF-Chem-PAH/POP is one of the most up-to-date large-scale PAH/POP models, which is unique with regard to temporal and spatial resolutions. It provides a powerful tool to study the fate of and environmental exposure to PAH/POP