Atmospheric Source Apportionment of PCBs Using Chiral Signatures

Abstract

Polychlorinated biphenyls (PCBs) are organochlorine chemicals that found widespread use in a diverse range of applications, with around 1.2 million tones produced worldwide. Although, the production of PCBs was banned in western nations in order to health concerns in the late 1970s, concentrations in the environment remain elevated, particularly in urban areas. There are two principal sources for atmospheric PCBs; primary source (ventilation of indoor air contaminated via leakage from old electrical equipment, building materials) and secondary source (volatilisation from soil contaminated as a result of past releases). Recent literature suggests that while PCB volatilisation from soil is likely to constitute a significant source of atmospheric PCB, new sources (primary) such as the ventilation of contaminated indoor air may constitute the most important contributions in urban areas. Aim: to evaluate the relative significance of primary and secondary sources of PCBs to the atmosphere at a number of locations in the West Midlands. Chiral analysis was employed to monitor enantiomeric fractions of selected chiral PCBs in topsoil and outdoor air over a one-year period. Overall 220 air and soil samples were collected from 10 different sites. Sampling locations were located on a southwest (upwind) to northeast (downwind) transect at intervals of between 3 and 15 km across the conurbation. In this way, a mix of rural, suburban, and urban sampling locations was studied. Samples were extracted, purified, and subjected to chiral GC/MS as previously described. Result: EFs in air for PCBs 136 and 149 are essentially racemic (EF= 0.5± 0.005) at all locations, while the EFs for PCB 95 in air are less obviously racemic, but display appreciable divergence from the EFs found in the corresponding soil samples. Volatilisation from soil appears to make a minimal contribution to atmospheric PCB concentrations at these locations. EFs of PCBs 95 and 136 are essentially racemic in all samples taken at the most urban location, which we tentatively hypothesised, was due to impaired microbial activity. The racemic EFs at Centenary Square may also reflect the fact that at this highly urbanised location, the supply of atmospheric PCB inputs (for which EFs are essentially racemic) may exceed the rate at which enantioselective degradation occurs. Further tentative support for this inverse relationship between degree of urbanisation and extent of enantioselective degradation comes from the fact that the greatest enantioselective degradation is observed at the most rural locations. An ANOVA test of chiral signatures in soil samples at each location in this study reveals no statistically significant seasonal or temporal (month - to - month) difference in edaphic chiral signatures. Furthermore, there are congener-specific variations in the extent of enantioselective degradation, with PCB 95 being degraded more than PCB 136 and 149. The direction of this enantioselective degradation is also broadly in line with our previous findings, although there appears to be less enantioselective degradation of 136 than previously observed. These results have potentially important implications for public health and environmental protection, as they imply that destruction of PCB stocks remaining in use in indoor environments are likely to result in a significant reduction in atmospheric concentrations. As the atmosphere is the principal point of entry of PCBs into the food chain, and is also the principal vector via which PCBs are transported from their – largely urban - source regions, such action is likely to reduce human exposure and limit the future spread of these compounds.