Investigating Long-term Trends in PCB Concentrations in Ambient Air: The TOMPs Network and the UK-Norway Transect

Polychlorinated Biphenyls (PCBs) were first synthesised in 1881, but their commercial production only began in 1929 in the USA. They were marketed under various trade names depending on the country where they were produced such as Aroclor (Monosanto, USA) and Clophen (Bayer, Europe). Because of high chemical and thermal stability, electrical resistance, and low or no flammability, PCBs had extensive
applications. They were used as dielectric fluids in capacitors and transformers, in plasticisers, adhesives,
inks, sealants and surface coatings. Production of PCBs peaked in the 1960s in Europe and USA; they were
ultimately banned in the late 1970s / early 1980s. Before the ban, PCBs entered the environment through point and diffusive sources such as landfill sites, accidental releases/spillages via leakage during commercial
use of electrical equipment and transformer and capacitor fires, or incineration of PCB waste. Current atmospheric levels of PCBs in the environment can be accounted for by ongoing primary anthropogenic emissions (e.g. accidental release of products or materials containing PCBs), volatilisation from environmental reservoirs (e.g. oceans and soils) which have previously received PCBs (secondary
emissions), or incidental formation during combustion processes.
The UK Toxic Organic Micro Pollutants (TOMPs) Network, which has operated since 1991, collects ambient air samples at six sites across England and Scotland, using high-volume active air samplers. The network provides long-term ambient air trend data for a range of Persistent Organic Pollutants (POPs), including PCBs, at both urban and rural locations, and thus allows detailed studies on atmospheric fate and
behaviour processes of persistent chemicals. It has played an important role in the successful understanding and modelling of their environmental sources and fate.
The continuous monitoring of PCBs for the TOMPs programme has demonstrated the constant decline in their UK air concentrations over the last decades. The concentrations of all congeners are generally significantly higher at urban compared to semi-rural and rural sites. The PCB fingerprint (relative concentrations) in TOMPs samples has not changed over time, which indicates that primary diffusive sources
in urban areas are still dominant and probably the controlling factor of trends in the UK ambient air PCB
concentrations.In 1994, a latitudinal sampling transect from the south of England to the north of Norway was established
with eleven sampling sites, mainly in remote locations, using Semi-Permeable Membrane Devices (SPMDs) as passive air samplers. The aim of this transect was to investigate spatial and temporal trends of POPs concentrations, and in particular to test the hypotheses of global distillation and fractionation. Since 1994, the UK/Norway SPMD transect has produced ten two-year samples (except 1996 – 1998) at each site. Results show that PCB concentrations have generally decreased from the south to the north of the transect, as
well as over time. They have, however, levelled off in recent years, which may be explained by increasing
importance of secondary emissions.