Observations on PAH, PCB, and PCDD/F trends in UK urban air, 1991-1995

Ambient air concentrations at London and Manchester are reported for PCBs, PAHs, and PCDD/Fs from 1991 to 1995. This data set suggests that urban PAH concentrations are trending downward in the U.K. Sigma PCB concentrations (Sigma = congeners 28, 52, 101, 118, 138, 153, and 180) were usually similar to 0.2-2 ng m(-3) over the study period, with concentrations of the seven PCB congeners (Sigma PCB) in London generally 2-3 times higher than in Manchester. PCB concentrations have remained quite stable year-on-year over the study period. PCDD/F trends at the two sites were similar, with a decline through 1991-1994, followed by an upturn in 1995. Summer:winter seasonality of PCDD/Fs at both sites has been highly erratic year-on-year and compound-by-compound, far more so than for PAHs and PCBs. This is perhaps best interpreted as a sign that various sources (presumably combustion-derived) have contributed different mixtures of PCDD/Fs of different source strengths at different times to the air at these urban sites. Possible causes are discussed.