Evaluation of the effectiveness of different indicator PCBs to estimating total PCB concentrations in environmental investigations.

Polychlorinated biphenyls (PCBs) are one of the most widely studied group of persistent organic pollutants (POPs). There are 209 different PCBs, however not all 209 can currently be individually quantified in one analytical run. This means that a subset of PCBs congeners are often determined and reported. Some of the most commonly reported subsets are the 7 indicator PCBs (28, 52, 101, 118, 138, 153 and 180) and the WHO 12 PCBs (77, 81, 105, 114, 118, 123, 126, 156, 157, 167, 169 and 189). The WHO 12 congeners are co-planar 'dioxin like' PCBs that are effective for establishing health risks. The 7 indicator PCBs were selected as some of the most common PCBs across the compositional range of the most common technical mixtures (such as Aroclors), and are used to give an indication of the total PCB concentrations. These groups of indicator PCBs were established several decades ago. However, in the environment commercial mixtures are subject to weathering and fractionation processes, and additional sources of non-Aroclor PCBs are also becoming more important. In this manuscript we use existing large scale comprehensive congener specific datasets to evaluate the effectiveness of indicator PCBs to predict total concentrations and establish if they are still fit for purpose. The results indicate that while these traditional indicators are a useful tool to estimate total concentrations in humans with background exposure there are many instances where they are not fit for purpose and can lead to significant under predictions in total PCB concentrations in environmental matrices.

Uptake and reaction of atmospheric organic vapours on organic films.

Films composed in whole or in part of organic compounds represent an important atmospheric interface. Urban surfaces are now known to be coated with a film ("grime") whose chemical composition somewhat resembles that of urban atmospheric aerosols. Such films may act as media in which atmospheric trace gases may be sequestered (leading to their removal from the gas phase); they may also act as reactive media, either as a "solvent" or as a source of reagents. Organic coatings on aqueous surfaces are also important, not just on ocean and lake surfaces ("biofilms") but also on the surfaces of fogwaters and atmospheric aerosol particles. We have initiated experimental uptake studies of trace gases into simple proxies for urban organic films using two techniques: a Knudsen cell effusion reactor and a laser-induced fluorescence method. We will discuss our first results on non-reactive uptake of organic compounds by organic films we use as proxies for urban grime coatings. In general, the measured uptake coefficients appear to track the octanol-air partition coefficients, at least qualitiatively. We have also measured the kinetics of reactions between gas-phase ozone and small polycyclic aromatic hydrocarbons (PAHs), when these are adsorbed at the air-aqueous interface or incorporated into an organic film. Reactions at the "clean" air-water interface and at a coated interface consisting of a monolayer of various amphiphilic organic compounds all follow a Langmuir-Hinshelwood mechanism, in which ozone first adsorbs to the air-aqueous interface, then reacts with already adsorbed PAH. By contrast, the reaction in the pure organic film occurs in the bulk phase. Under some circumstances, heterogeneous oxidation of PAHs by ozone may be as important in the atmosphere as their gas phase oxidation by OH.