Diffusion and uptake of chemical vapour volatilising from a sprayed target area

Estimates are made of the concentrations of vapour near the surface at different distances down‐wind from sprayed areas of various sizes from which vapour is volatilising at a known rate. Deposition velocities are also calculated for vapour transfer to an underlying crop, the leaves of which either act as a perfect sink, or provide stomatal pathways through which vapour may be absorbed. Typical values of the deposition velocities calculated on these bases are found to be around 5 × 10−3m s−1 in the case of vapours with molecular weights around 300 (applicable to many spray chemicals). These results are combined and applied to cases in which the vapour source is a few hundred metres long in the direction of the wind, and very broad. It is found that the calculated rate of uptake of vapour by the down‐wind underlying surface may exceed 1% of the rate of production of vapour by the source (in each case, per unit area), even at distances more than 500 m from the source area. The reasons why field values of uptake may fall short of these theoretical estimates are briefly discussed.