Evidence of current pesticide use from temporal and Clausius-Clapeyron plots: A case study from the Integrated Atmospheric Deposition Network

We found evidence of recent agricultural pesticide use in atmospheric samples collected at Point Petre, Ontario, one of the sampling sites of the Integrated Atmospheric Deposition Network. The evidence consists of a residual cycle of gamma-HCH concentrations that remains after correction for temperature effects. While the recent use of gamma-HCH has been observed by other researchers, the length and resolution of this study allow for a more quantitative assessment. To characterize the residual cycle, we compared the fit of three periodic functions and included the function with the best fit in a model with temperature and time. The estimated parameters were used to identify agricultural use as the source of the residual cycle. We also examined the effect of agricultural use on the information typically extracted from Clausius-Clapeyron plots. We found that the presence of an agricultural cycle maximizing near the temperature cycle results in an increased enthalpy of phase transition. In addition, the phase offset of the two cycles results in an increased apparent scatter in the data. Finally, the shape of the agricultural cycle imparts curvature to the normally linear relationship observed in Clausius-Clapeyron plots. These findings suggest that recent interpretations of Clausius-Clapeyron plots in terms of a compound's environmental behavior may be incomplete.