In the TOXSWA (TOXic substances in Surface WAters) model volatilization of pesticides from surface water is computed because it may be an important component of the mass balance of pesticides in water bodies. Here, we briefly review the physics of air-water gas exchange relevant in this context. A new parameterization of volatilization is proposed. This new TOXSWA parameterization would take into account key physical aspects of the air-water gas exchange process, while still fitting into the TOXSWA philosophy. It is recommended to cast the new parameterization into a micrometeorological framework, based on two atmospheric resistances and one aqueous resistance against gas transport. In this context, the dimensionless Henry coefficient is a crucial characteristic of the pesticides and determines whether atmospheric properties as modelled by the atmospheric resistances exert the main dynamic control on air-water gas exchange or the aqueous properties and resistance. Most pesticides will be subject to atmospherically controlled volatilization.
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