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Modeling Methyl Isothiocyanate Soil Flux and Emission Ratio from a Field following a Chemigation of Metam-Sodium

^a California Environmental Protection Agency, Department of Pesticide Regulation, Environmental Monitoring Branch, P.O. Box 4015, Sacramento, CA 95812-4015. L.Y. Li, current address: California Environmental Protection Agency, Air Resources Board, Planning and Technical Support Division, Sacramento, CA 95812-2815
^b California Environmental Protection Agency, Air Resources Board, Monitoring and Laboratory Division, P.O. Box 2815, Sacramento, CA 95812-2815

View larger version (45K): [in a new window]   Fig. 1. Field geometry, layout of drip lines, and location of receptors and the weather station.

View larger version (10K): [in a new window]   Fig. 2. Diagram of rows in the chemigated field. Each row consisted of raised bed, slope, and furrow, and their dimensions were illustrated respectively on the diagram. Drip lines were located in the middle of bed, which was tarped with the plastic film.

View larger version (17K): [in a new window]   Fig. 3. Linear regressions between measured air concentration and simulated air concentration using the nominal soil flux density (E0) of 0.0001 g m–2 s–1. As the simulated air concentration is directly proportional to the soil flux density, the simulated air concentration using the nominal soil flux density might be far off from the measured air concentration.

View larger version (16K): [in a new window]   Fig. 4. Measured air concentration vs. simulated air concentration using back-calculated soil flux density. The back-calculated soil flux density (E) was obtained by adjusting the nominal soil flux density (E0) with the slope of the regression model. The simulated air concentrations using the back-calculated soil flux density (E) show a good match to the measured air concentrations.

View larger version (19K): [in a new window]   Fig. 5. (a) Estimated average soil flux density, (b) measured average air concentration, and (c) measured maximum air concentration over various sampling periods. Estimated average soil flux density over daytime (Periods 1, 3, and 5) was significantly greater than that over nighttime (Periods 2 and 4). However, the average air concentration over nighttime was slightly higher than that over daytime. The measured maximum air concentration declined with time after the first night of chemigation.