TEMPERATURE-DEPENDENCE OF ATMOSPHERIC CONCENTRATIONS OF SEMIVOLATILE ORGANIC-COMPOUNDS

Reported data on the temperature dependence of atmospheric concentrati ons of semivolatile organic compounds (SOCs) are compiled and expresse d as linear regressions of the logarithm of the partial pressure in ai r versus reciprocal temperature: In p(A) = m/T + b. Two simple models are introduced to explain the dependence of these air concentrations o n temperature. The first assumes equilibrium between the atmosphere an d the earth's surface. In the second, air concentrations are establish ed as a result of chemical inflow and outflow in advected air and reve rsible exchange with a soil or water surface. The model equations are rearranged to express the partial pressure of the chemical as a functi on of temperature. On the basis of these models, it is shown that only under selected circumstances, namely, if surface contamination is hig h and atmospheric background concentration low, does the slope m of th e In p vs 1/T relationship reflect the thermodynamics of air-surface p artitioning. Generally, however, m is a measure of the extent to which air concentrations are controlled by evaporation from surfaces close to the sampling location and by advection of air masses with global ba ckground concentrations. A shallow slope or low temperature dependence indicates that long-range transport controls atmospheric levels at a sampling site. Steeper slopes indicate high surface concentrations in the vicinity of the site. This hypothesis is applied to the observed t emperature dependence of the compiled atmospheric concentration data a nd is found to be capable of explaining differences in slope m (i) bet ween chemicals, (ii) between sampling sites, and (iii) at different se asons. Research efforts should be directed toward quantifying by measu rements and predicting by models the kinetics of exchange of SOCs betw een the atmosphere and various surfaces.