Temporal trends of polycyclic aromatic hydrocarbons in the U.K. atmosphere: 1991-2005.

Internationally there are few long-term air monitoring programs, which are necessary to assess the effectiveness of source abatement measures as required underthe UNECE POPs protocol. In the United Kingdom, the Toxic Organic Micropollutants (TOMPS) program, funded by the Department for Environment, Food, and Rural Affairs (Defra), was started in 1991 and includes regular monitoring of a range of compounds including polycyclic aromatic hydrocarbons (PAHs). In this study, the time series (1991-2005) of atmospheric concentrations of 15 PAHs at six U.K. monitoring sites were investigated. Most show a statistically significant decrease in PAH levels over time, broadly consistent with the reported decline in emissions. Higher levels of heavier PAHs were noted in winter than in summer at most sites. At one coastal site, higher levels of lighter PAHs were noted in summer, possibly due to temperature-driven outgassing of these compounds from seawater. Current annual averages of benzo[a]pyrene are below the recently introduced annual air quality standard of 0.25 ng m(-3) at all sites, although quarterly averages have exceeded 0.25 ng m(-3) in recent years but only at the urban sites in winter. The atmospheric signature of total PAHs closely mirrors the emission signature, which lends strength to the idea that levels of PAHs in air are still mostly influenced by direct/local sources.