Temporal trends, temperature dependence, and relative reactivity of atmospheric polycyclic aromatic hydrocarbons.

Atmospheric polycyclic aromatic hydrocarbon (PAH) concentrations have declined appreciably between 1992 and 1997 at an urban background site in Birmingham, U.K. In contrast, no decline is evident in the city center between 1994 and 1997. Although most PAHs display statistically significant negative correlation with temperature, so does NOx, for which traffic (a nonseasonal activity) is the major source, and for which the negative correlation with temperature reflects seasonal boundary layer depth variations. When concentrations of PAHs divided by NOx were plotted against temperature, no significant relationship was detected for any PAH, except fluorene, phenanthrene, and fluoranthene. For these PAHs, the relationship was positive, suggesting volatilization from surfaces may be appreciable. For samples collected simultaneously at the city center and urban background sites, greater negative temperature-dependence was observed at the latter location. Although this may be partly due to the fact that the enhanced reactivity of PAHs at higher temperatures exerts a greater influence at sites more distant from emissions; the dichotomy in temperature-dependent behavior and temporal trends may also be due to city center concentrations being "buffered" by volatilization from surfaces to a greater extent than those at the urban background site.