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 n orthern England from the east. During this time, PCB air concentration s responded very closely to the changes in ambient temperature, follow ing a clear diurnal cycle. All PCB congeners correlated well with temp erature, but the correlation was strongest for lower chlorinated speci es. 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 interpr eted as providing evidence that rapid, temperature-controlled air-terr estrial surface exchange of PCBs influencing air concentrations and, h ence, 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 tem perature differences were small and the diurnal PCB congener pattern w as not discernible. Diurnal PAH concentration changes were not correla ted 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 hy pothesized that short-term PAH air concentrations are controlled by on going recent local/regional source inputs (rather than air-surface rec ycling), atmospheric reactions, and deposition processes. The air-surf ace exchange of PCBs is satisfactorily modeled by a sine curve describ ing ambient temperatures and controlled by a temperature-dependent air -terrestrial surface (K-TA) partition coefficient.