Air Transport of Pesticides

Exponent scientists have extensive experience modeling the air transport and dispersion of numerous pesticides, including multiple modeling studies of incidents involving droplet drift from aerial or ground-based applications and subsequent downwind deposition on non-target crops or nearby communities and evaluations of impacts from pesticides’ and fumigants’ volatilization.

Exponent scientists are the developers of several air dispersion computer models with features customized to the agricultural industry. Our in-depth knowledge of the physics behind these air transport models allows us to design and perform the best evaluation study possible. We can also develop and implement new and unique modeling or air sampling techniques where appropriate.

CALPUFF is a Lagrangian air dispersion model developed and maintained by Exponent scientists that has seen broad application in both regulatory and research contexts. CALPUFF is a non-steady state model that can track pollutant transport and dispersion through a realistic three-dimensional atmosphere; simulate transport time, stagnation, recirculation, and the impacts of terrain features; and model both airborne concentrations and the mass of gas or droplets deposited on the ground. These capabilities make CALPUFF especially useful for modeling pesticides and their transport away from agricultural fields where they have been applied.

Our experience as developers of this advanced model gives us excellent insight into how the model functions and how it is best applied. Exponent has also worked with the U.S. Forest Service to develop modules in CALPUFF that link it with spray drift droplet distribution and height data produced by the AGDISP model. This module allows for modeling longer-range transport of droplets and fully accounts for non-steady state behavior in the atmosphere. These more refined techniques allow for a more robust and reliable assessment of spray drift transport.


Exponent scientists developed the Probabilistic Exposure and Risk Model for FUMigants (PERFUM) to evaluate the transport of fumigants that may volatilize following application and be transported downwind, potentially causing exposure to people around the application site. PERFUM uses probabilistic modeling to address the regulatory need to establish buffer zones around applied fields. It has more recently been used for estimating air exposures for non-fumigant, semi-volatile pesticides.


The Soil Fumigant Exposure Assessment (SOFEA) system is a probabilistic numerical modeling tool that uses Monte Carlo techniques to sample fluxes and distributions of multiple agricultural fields in a diverse airshed and calculate air concentrations of the pesticide based on the ISCST3, AERMOD, or CALPUFF air dispersion models. Probabilistic selections for the treated fields include randomization of field sizes and their locations, application date and time, application rate type, depth, degradation rates and tarp presence which will vary the flux rate. Long-term meteorological data from the closest observational site to the airshed is used in the model, as well as the GIS database for determining land use.