Technical Abstract: Field volatility of 5 pesticides (chlorpyrifos, metolachlor, atrazine, trifluralin, and endosulfan) were determined after application to freshly tilled soil. Factors such as timing of rain events, soil moisture levels, and temperature were found to be important at setting the magnitude of the volatile losses. Air/water partition coefficients provided the best predictive measure for determining relative air flux losses. The alpha isomer of endosulfan was lost at much greater levels than the beta isomer. A site-specific volatility model was developed using soil release parameters established by Jury and a 3-dimensional atmospheric dispersion model coupled to hourly meterological inputs from a mesoscale model. Using this model, the gaseous release amounts for atrazine were accurately predicted, however, the releases of metolachlor were over-estimated. Individual field volatilities were combined by adapting the model for multiple site emissions inventories for atrazine and metolachlor over the Chesapeake Bay watershed. The relative loadings of these pesticides to the Bay were estimated; atmospheric deposition was almost equal to runoff for metolachlor; however, atmospheric processes were found to account for only 1/8th of runoff loadings for atrazine. Laboratory studies were carried out to examine the effect of temperature on the volatile exchange at air water interfaces. The study showed that the melt point of trifluralin will affect its environmental partitioning.