Nonlinearity in the Slopes of Clausius−Clapeyron Plots for SVOCs

Plots of partial pressures of semivolatile organic compounds versus inverse temperature at the time of measurement (Clausius−Clapeyron or CC plots) have been used to infer local air exchange with surfaces. Recent studies have shown that the slopes of such plots are smaller than would be expected from the known heats of vaporization, of air−water exchange, and probably also of air−vegetation exchange. Using data from the Point Petre Master Station of the Integrated Atmospheric Deposition Network, CC plots for trans-chlordane are examined for various wind directions approaching the measurement site. The slopes of the CC plots are not linear over the entire range of temperatures measured. It is shown that such behavior can be expected from the combination of exchange and transport processes that affect the air concentration at a remote site. The positive curvature of the CC plots can be detected in the usually noisy CC plots by use of regressions, which include an increasing number of data from lower temperatures. The degree of such curvature is postulated to indicate the degree of importance of long-range transport (LRT) versus local exchange. A conceptual model is presented in which the variation in CC slopes can be ascribed to the relative amount of LRT versus local exchange.