Incorporation of aerodynamic and boundary layer resistances in determining initial evaporation rates of pesticides from turf grass

Evaporative loss rates of pesticides, following application to turfgrass, were measured using the Backward-Time Lagrangian stochastic Dispersion model (Flesch, Wilson, Journal of Applied Meteorology, 34, pp. 1320-1332. All pesticides were applied to a 22.8 m x 22.8 m orchard ryegrass turf plot near Corvallis, Oregon. Solar radiation, ambient temperature, surface temperature, relative humidity, wind direction and wind speed at four heights were monitored continuously. Growth inhibitor was applied to the turf plot several days before pesticide application to maintain a constant mowed grass height and aerodynamic roughness length during the experiment. Pesticides were applied as mixtures to allow direct comparison of evaporative loss. Mixtures studied were chlorpyrifos + triadimefon, chlorpyrifos + triadimefon+\ud ethofumesate, tridopyr (acetic acid) + propiconazole + cyfluthurin. Airborne flux estimates were correlated with temperature, solar radiation, wind speed, time, and vapor pressure of the active ingredient. Over short time periods (2 hrs) volatile loss correlated most strongly with solar radiation, surface temperature and the vapor pressure of the active ingredient. A Clausius Clapeyron relationship (log vapor pressure vs. 1/Temperature (K)) was observed between flux and surface temperature for most pesticides. A fugacity-based model, which attempts to predict initial evaporative loss rates from turf grass, is introduced in this paper. Input parameters for the fugacity model include the vapor pressure of the active ingredient, surface temperature, wind profile Information, atmospheric stability, surface roughness, molecular diffusion coefficient of the pesticide and average upwind fetch distance to the center of the plot. Assumptions of the predictive model, which are thought to exist during the period immediately following application, are 1) the vapor pressure of the pesticide on the leaf surface is equivalent to the vapor pressure of the active ingredient at a given leaf temperature, 2) molecular diffusion of the pesticide vapor is the rate limiting step in the evaporative process, 3) an equation proposed by Shephard (Quart. J. R. Met. Soc., 84, pp. 205-224. 1953), which is in agreement with results from wind tunnel data for thorium-B, heat and water exchange between grass and an airstream (Chamberlain Proc. Roy. Soc. London A290, 236-265, 1966), is believed to be adequate in estimating the aerodynamic resistance to transfer between the region in the turf canopy where molecular diffusion occurs and a reference height in the equilibriated boundary layer of the airstream, 4) A height three times the thickness of the equilibriated boundary layer can be inserted into Shephard's equation as a fictitious height where the concentration of the evaporating substance is zero. A comparison between measured values of flux using Backward-Time Lagrangian stochastic Dispersion (BTLSD) model and flux values predicted by the fugacity method are generally within an order of magnitude apart, with the fugacity model consistently over-estimating the flux determined by the BTLSD model. This is thought to be due to errors in surface temperature measurements and the assumption of a saturated vapor occurring over the area of the treated surface. A comparison between measured BTLSD model flux values normalized for pesticide vapor pressure, calculated aerodynamic resistance (estimated by the fugacity model) and application spray density show a progressive decrease in difference indicating the theory underlying the fugacity model could have physical significance