When studying and modelling the volatilisation of pesticides from crops, their vapour pressure is an essential property. In the critical evaluation of vapour pressures stated by various sources, problems were encountered. Therefore, an inventory was made of readily-usable methods for estimating vapour pressures of organic chemicals, for checking the stated values. When applied to some of the five example pesticides, the estimates were at the same level as the stated values. However, for other pesticides there were substantial differences. Unfortunately, there is a wide range of uncertainty in both measurement and estimation of comparatively low vapour pressures. The solid (crystalline) and sub-cooled-liquid states of the chemical have to be distinguished when specifying and using vapour pressures, because the latter can be much higher. The influence of the uncertainty in vapour pressure on the source strength of volatilisation from plants is illustrated in a few computations. Only data for pesticides with accurately known vapour pressure are usable in the development and testing of computation models on the volatilisation from crops.
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