Temperature dependence of PCBs in the UK atmosphere

A thermodynamic approach was taken to assess the state of equilibrium between air and the Earth’s surface for PCBs at a variety of sites located in urban and rural areas. The Clausius–Clapeyron equation was applied to atmospheric PCB data, relating PCB partial vapour pressure (ln P) to inverse temperature (1/K); essentially representing the temperature controlled transition between condensed phases and the atmospheric gas phase. The slopes of the resulting plots ranged from ?3100 to ?8272 for a range of congeners at two city sites, significantly steeper than those generated at two rural locations, where there was little or no correlation between ln P and temperature. It was inferred that advection and variable meteorological conditions mask any localised, temperature dependent, air–surface exchange at these rural locations when weekly or two weekly integrated samples were taken. At a third rural site, close to Lancaster University, an intensive highly time-resolved sampling regime, carried out during very stable meteorological conditions resulted in highly correlated plots (r²>0.6), with slopes ranging from ?7151 to ?14 148 for different congeners. By reducing meteorological variables in this manner localised temperature controlled air–surface exchange became evident. Enthalpies of phase change generated from the temperature coefficients were similar to literature values for the enthalpy of vapourisation and the enthalpy of phase change from octanol to air. This suggests that, under these stable conditions, equilibrium was achieved as a function of either vapour pressure (P^°_L) or the octanol–air partition coefficient (K_OA).