OH RADICAL REACTIONS - THE MAJOR REMOVAL PATHWAY FOR POLYCHLORINATED-BIPHENYLS FROM THE ATMOSPHERE

Polychlorinated biphenyls (PCBs) are transported through the environme nt primarily in the atmosphere and may undergo chemical reactions, par ticularly with the OH radical, while in the vapor phase. Rate constant s for the gas-phase reaction of 14 PCB congeners With the OH radical o ver the temperature range of 323-363 K were measured. The calculated t emperature dependences of the reactions were used to estimate OH-PCB r eaction rate constants at 298 K; see Table 4. These 298 K rate constan ts agree well with literature rate constants for PCBs containing zero to two chlorines and with OH-PCB reaction rate constants estimated fro m a structure-activity method. Calculated atmospheric lifetimes of PCB s due to OH-initiated reactions varied from 2 days for biphenyl to 34 days for a pentachlorobiphenyl. A simple model for the vertical concen tration gradient of PCBs in the troposphere was developed and used to calculate the total global loss rate (8300 t yr(-1)) of PCBs from the atmosphere due to removal by OH, This pathway is very large in compari son to other known permanent PCB loss processes from the environment, such as deep ocean sediment burial (240 t yr(-1)).