Modelling of air–water exchange of PCBs in the Great Lakes

Volatilization from water may be an important emission or reemission process for PCBs. In previous work, we have expanded the community multi-scale air quality (CMAQ) model to simulate the transport, chemical transformation, gas/aerosol partitioning, deposition and air–water surface exchange of PCBs. The air–water surface exchange algorithm is based on a two-film model of the air–water interface. Using this expanded version of CMAQ, we simulated the air–water exchange flux of gas-phase PCBs in the Great Lakes and examined the concentrations and deposition patterns of PCBs in North America for 2002. For gas-phase PCBs, the volatilization from water surfaces is often greater than the absorption (dry deposition) to the water surfaces. For example, the net flux of PCBs from the Great Lakes to the atmosphere is much larger than the dry and wet deposition of particle-phase PCBs to the Great Lakes. Thus, we conclude that the Great Lakes are currently a source rather than a sink for PCBs. For remote areas such as Lake Superior, this air–water exchange appears to be the most important source of PCBs emission. Anthropogenic emission, however, is still the dominant source when averaged across the whole North American modeling domain. Transfer resistance calculations show that the transfer resistance at the water side of the interface is the biggest resistance, while the aerodynamic and air side resistances are approximately at the same level. The total air–water transfer resistance is more sensitive to wind speed than to temperature.