The Washington aerial spray drift study: assessment of off-target organophosphorus insecticide atmospheric movement by plant surface volatilization

Post-application pesticide emissions from wetted leaf surfaces and soil may present a significant pathway of exposure to humans in nearby residential communities. In this study, high volume air sampling was performed to measure airborne concentrations of the pesticide methamidophos in a residential community in close proximity to aerial spraying. Sampling occurred before, during and 24 h post-application. To evaluate whether predictive models could reliably estimate residential exposure to methamidophos, an emission factor was used for estimating fluxes of volatilized material over the sprayed area for a 1-day post-application period. These flux estimates were then incorporated into a fugitive dust gaussian dispersion model (FDM) for assessing distribution of mass around the sprayed area. The predictive model output was compared with the field air sampler measurements. In our comparison, 1-day flux estimates from the model were found to be associated to observed field measurement data, with an r2=0.63 the day of the spray and r2=0.67 the day after the spray. The volatilization model however appears to underestimate surface emission flux immediately after the spray and overestimate the emission the next day.