Measurement and modeling of the diurnal cycling of atmospheric PCBs and PAHs

PCBs a nd PAHs were quantified in a ir samples ta ken every 6 h over a 7 day period in August 1995 at a rural site in northwest England. For the first 5 days, a stable high-pressure system moved slowly across northern England from the east. During this time, PCB air concentrations responded very closely to the changes in ambient temperature, following a clear diurnal cycle. All PCB congeners correlated well with temperature, but the correlation was strongest for lower chlorinated species. Daily PCB concentration maxima exceeded minima by a factor of 2.1 -3.7 for different congeners; in contrast, daily maxima:minima ratios for different PAHs were in the range 0.47-1.44. These data are interpreted as providing evidence that rapid, temperature-controlled air-terrestrial surface exchange of PCBs influencing air concentrations and, hence, regional/global scale cycling of these compounds occurs. Toward the end of the study period,the stable air conditions were replaced by a turbulent (windy), unstable low-pressure system, when day/night temperature differences were small and the diurnal PCB congener pattern was not discernible. Diurnal PAH concentration changes were not correlated with temperature, but groups of compounds were strongly correlated with each other(e.g., phenanthrene, fluoranthene, and pyrene; benzo(k)fluoranthene, benzo(a)pyrene, benzo(gh,)perylene). It is therefore hypothesized that short-term PAH air concentrations are controlled by ongoing recent local/regional source inputs (rather than air-surface recycling), atmospheric reactions, and deposition processes. The air-surface exchange of PCBs is satisfactorily modeled by a sine curve describing ambient temperatures and controlled by a temperature-dependent air-terrestrial surface (K-TA) partition coefficient.