Tropospheric Lifetimes of PCBs, PCDDs and PCDFs

The little information available (see above) suggests that photolysis of gas-phase PCBs in the troposphere will be negligible for those PCBs with 4 chlorine atoms, and this may also be the case for the more chlorinated PCBs. However, it appears that the PCDDs and PCDFs will absorb radiation in the actinic region in the troposphere, and hence the importance of photolysis will depend on the photodissociation quantum yields in the gas phase, which are, however, not presently known. 

The measured rate constants, or upper limits thereof, for the gas-phase reactions of OH radicals, N03 radicals and 03 can be combined with the estimated ambient tropospheric concentrations of OH radicals, N03 radicals and 03 to calculate the tropospheric lifetimes of the gas-phase PCBs, PCDDs and PCDFs due to each of these reactions. The lifetime, tx, for reaction with species X is given by tx = (fcx[X]) Ambient concentrations (molecule cm- 3) of OH radicals, a 24 h, seasonal, annual and global tropospheric average of 9.7 x 105 91 N03 radicals, a 12h nighttime average of 5 x 108 92 and 03, a 24h averageof7 x 1011(30x 10 9 mixing ratio)8 7 are used to calculate lifetimes due to these gas-phase reactions. 

The upper limits to the 03 reaction rate constants given in Table 1, and the expected upper limit to the rate constants for the gas-phase reactions of 03 with PCB, PCDD and PCDF congeners of 2 x 10-2Ocm3 molecule-1 s-1 (see above), lead to a calculated lower limit to the tropospheric lifetimes of PCB, PCDD and PCDF congeners due to gas-phase reaction with 03 of 2 years. Similarly, the upper limit to the measured bimolecular rate constants kabs for reactions of the N03 radical with PCBs, PCDDs and PCDFs under tropospheric conditions, of kabs 1 x 10-15 cm3 molecule-1 s-1, leads to a lower limit to the tropospheric lifetimes of PCB, PCDD and PCDF congeners due to gas-phase reaction with the N03 radical of 45 days. 

In contrast, reactions of the PCBs, PCDDs and PCDFs with the OH radical 

There are clearly several areas of significant uncertainty, including the role of photolysis in both the gas and particle phases and the identities and formation yields of the products of the gas-phase OH radical reactions. 

---