Endosulfan transport: II. Modeling airborne dispersal and deposition by spray and vapor

Endosulfan (C9H6O3Cl6S; 6,7,8,9,10,10-hexachloro-1,5,5a,6,9,9a-hexahydro-6,9-methano-2,4,3-benzodioxathiepin 3-oxide) and other agricultural chemicals can be transported from farms to rivers by several airborne pathways including spray drift and vapor transport. This paper describes a modeling framework for quantifying both of these airborne pathways, consisting of components describing the source, dispersion, and deposition phases of each pathway. Throughout, the framework uses economical descriptions consistent with the need to capture the major physical processes. The dispersion of spray and vapor is described by similarity and mass-conservation principles approximated by Gaussian solutions. Deposition of particles to vegetation is described by a single-layer model incorporating contributions from settling, impaction, and Brownian diffusion. Vapor deposition to water surfaces is described by a simple kinetic formulation dependent on an exchange velocity. All model components are tested against available field and laboratory data. The models, and the measurements used for comparisons, both demonstrate that spray drift and vapor transport are significant pathways. The broader context, described in another paper, is an integrative assessment of all transport pathways (both airborne and waterborne) contributing to endosulfan transport from farms to rivers.