Temperature Dependence of the Characteristic Travel Distance

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Abstract
The effect of temperature variation on the environmental fate of organic chemicals can be evaluated in steady-state multimedia box models by expressing chemical partitioning data and reaction rate coefficients as functions of temperature. Using such a model the temperature dependence of the characteristic travel distance in air LA, which is a measure for the atmospheric long-range transport potential of organic chemicals, is calculated. Simulations are reported for a set of 40 chemicals of environmental interest. Increasing temperature is shown to have two opposing effects on LA. Rates of chemical transformations in the atmosphere (kair) and surface media are increased, which reduces LA. Rates of atmospheric deposition (kdep) are reduced leading to increased mobility and LA. Accordingly, LA can monotonically increase or decrease with increasing temperature, or it can have a maximum in the modeled temperature range, but it cannot have a minimum. For chemicals with a strong temperature dependence of kair relative to kdep, LA will increase with increasing temperature. Results for selected polychlorinated biphenyls are compared to monitoring data yielding qualitative agreement when chemical properties are adjusted to mean temperatures for the measurement period. The results demonstrate that the temperature dependence of the characteristic travel distance is highly dependent on chemical characteristics and can be counterintuitive. The use of mass balance models is thus essential. The difference between the LA values at 5 °C and 30 °C can be up to a factor of 6. Accordingly, chemical ranking with respect to LA can change significantly if performed at different temperatures. Implications of the different temperature dependencies on long-range transport to polar regions are discussed.