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Endosulfan Transport

II. Modeling Airborne Dispersal and Deposition by Spray and Vapor

^a CSIRO Land and Water, Canberra, GPO Box 1666, Canberra, ACT 2601, Australia
^b Australian Water Technologies, Sydney, NSW, Australia
^c NSW Agriculture, Locked Bag 21, Orange, NSW 2800, Australia

View larger version (25K): [in a new window]   Fig. 1. Results from particle dispersion model for four different particle size distributions (log-normal with median particle diameter = 80, 160, 240, and 320 µm and standard deviation ln(1.8) throughout). Conditions assumed throughout: canopy height hc = 0.50 m; leaf dimension dc = 0.01 m; field length xp = 1000 m; spray release height hs = 2 m; initial plume height Z0 = 1 m; atmospheric stability is neutral (D); wind speed ur = 2 m s-1. Panels: (a) depleting mass fraction s(x); (b) deposition -ds/dx = -s'(x) for a laterally uniform unit line source; (c) drift deposition fraction fdrift(x) for a plane source from Eq. [12].

View larger version (22K): [in a new window]   Fig. 2. Test of the single-layer model for the deposition velocity Wd, Eq. [15], against wind tunnel measurements of particle deposition to a sticky grass surface by Chamberlain (1967). Predictions are of Wd as a function of particle diameter d for vegetation of height 0.06 m and leaf area index 1, using the model of Raupach (1992)(1994) to calculate roughness length and other canopy aerodynamic properties. Data and predictions are for three wind speeds giving friction velocities of 0.35, 0.70, and 1.40 m s-1. The solid line is the predicted terminal velocity Wt as a function of d (Malcolm and Raupach, 1991).

View larger version (17K): [in a new window]   Fig. 3. Contributions of the three terms Wt (settling), Gimp (impaction), and Gbrow (Brownian diffusion) to the deposition velocity Wd. Conditions as in Figure 1 with u* = 1.40 m s-1.

View larger version (19K): [in a new window]   Fig. 4. The deposition velocity of endosulfan vapor to water, from the Deacon model, Eq. [18]. Assumed parameters: a = 1.2 kg m-3, w = 1000 kg m-3, a = 1.5 x 10-5 m2 s-1, w = 1 x 10-6 m2 s-1, a = 4.8 x 10-6 m2 s-1, w = 5.3 x 10-10 m2 s-1.

View larger version (35K): [in a new window]   Fig. 5. Layout of the field experiment at Auscott Warren, 1994–1995.

View larger version (29K): [in a new window]   Fig. 6. Predicted and measured endosulfan air concentrations (, ß, , total) at stations within Field 4 and at S200m outside Field 4.

View larger version (25K): [in a new window]   Fig. 7. Predicted and measured endosulfan water concentrations (, ß, , total) at four westward receptor locations for Sprays 1 and 2.

View larger version (15K): [in a new window]   Fig. 8. Time-averaged comparison (over 5 d) of measured and modeled total endosulfan concentrations at 13 receptor points, for Sprays 1 and 2.

View larger version (23K): [in a new window]   Fig. 9. Time-averaged comparison (over 5 d) of measured and modeled endosulfan species concentrations (, ß, , total) at 13 receptor points, for (a) Spray 1; (b) Spray 2.

View larger version (21K): [in a new window]   Fig. 10. Comparison of particle dispersion (spray drift) model with data from Bird et al. (1996)(standard case) for three wind speeds u. Parameters: canopy height hc = 0.15 m; leaf dimension dc = 0.005 m; field length xp = 274 m; spray release height hs = 2.5 m; initial plume height Z0 = 1 m; median particle diameter = 250 µm; particle size distribution is log-normal with standard deviation ln(1.7); atmospheric stability is neutral (D) or slightly unstable (C).