Vapour. The emission of vapour from a treated field crop (the source) is described by the PEARL model. This model has been used since 2001 in the EU registration process for leaching to groundwater. The PEARL model has been tested in volatilisation studies against experimental data, so it is considered a suitable model for volatilisation exposure assessments under field conditions. The PEARL model has been coupled to a dispersion model, called OPS, which simulates the atmospheric process sequence of dispersion. The combination of PEARL and OPS is used to predict time-dependent air concentration at locations around and within the source field using real meteorological data from locations identified as worst case (90th percentile of average temperature across the growing season) within each EU regulatory zone.
Vapour. The BROWSE model for vapour exposure is the first attempt that has been made to model mechanistically the process involved in bystander and resident exposure to vapours released from a treated field for risk assessment purposes. The mechanistic approach is based on the PEARL model, already developed as part of the FOCUS package for the risk assessment of surface water , for determining volatile losses to air, combined with OPS, a plume-dispersion model which simulates the downwind dispersion of the emitted vapour and the resulting concentrations in air. The major challenge with this component of resident and bystander exposure has been defining the scenario(s) that are the basis for the vapour exposure models. Since there was no existing defined scenario (including, for example, meteorological conditions, field size and layout, location of bystander or resident, pattern of applications) there was a significant expenditure in time of developing ideas, and then attempting to put them into practice in the models. This is potentially the least ‘finished’ component of the resident and bystander exposure model because new ideas for improving the model were developed too late to be implemented. In particular, since this is a relatively complicated process, it may be appropriate to have a first tier simple approach, and then include a more sophisticated calculation, with additional input parameters, for higher tiers. A major restriction on developing more sophisticated scenarios was the time taken for the software to run. Stakeholder feedback was clear that the run time should be a few minutes at most, and this then limits the complexity of the model. The main variables that influence exposure in the vapour model are physico-chemical properties of the active ingredient, size of treated area, location of the application (defined by meteorological conditions) and whether the application is to crop or soil.
