Research is currently being conducted under the Defra-funded project PS2005 (known as BREAM) to improve the estimate of exposure of bystanders and residents to pesticides used in agriculture. Work to date has shown that levels of airborne pesticide, for a pesticide of low volatility, can be detected after application and these appear to exceed levels that would be expected to be caused by volatilisation. It is possible that these unexpected results may be as a result of particles (such as dust or pollen), which have a pesticide loading, becoming airborne. In terms of the exposure of bystanders to post-application pesticides, these particles could be an important component and, because of their small size, may behave similarly to vapours in their dispersion downwind and their ability to be inhaled. However, the factors influencing the rate of emission from the crop are likely to be very different and would need a separate source term if this is to be included.
A further volatilisation experiment is planned for the BREAM project in Spring 2008, extending the work conducted in spring/summer 2006 which measured airborne pesticides following an application, in order to estimate volatile emissions from the crop. It is proposed that an additional set of measurements, using a cascade impactor, is made at the same time in order to identify whether there is a significant component of airborne concentration that arises from particles. While this will not give sufficient information to develop an alternative source term for the BREAM model, it will identify whether this is an important exposure route for future consideration and will potentially give greater confidence in volatilisation models that are currently in doubt in part because of their apparent inability to predict concentrations from low-vapour-pressure chemicals.
Prior to conducting this experiment, it is necessary to undertake some preliminary work to establish as far as possible the most appropriate experimental techniques for using the cascade impactor and for subsequent analysis of the deposits. This will include:
- Using whatever information is available in published literature
- A preliminary experiment in the wind-tunnel using very fine droplets
- Evaluating the recovery efficiency of active ingredients from collecting surfaces of the cascade
The field experiment itself will then measure concentrations of two active ingredients (epoxiconazole and fenpropidin) following an application to a wheat crop. Alongside the suction samplers, a single cascade impactor will be operated, which collects only particles, not vapours. The collected particles will then be analysed to determine the quantity of active ingredient on them, and the approximate size of the particles.
Analysis of all the data from the field experiment will allow estimates to be made of the emission rate of each active ingredient from the crop for 24 hours following application, using a number of different techniques, and the proportion of this emission that might be attributable to particles such as pollen or dust.
This information will be used to determine appropriate methods of modelling or otherwise estimating post-application emissions from sprayed crops in order to improve exposure assessments for bystanders and residents.