Diurnal variation of diazinon volatilization: soil moisture effects.

Diurnal variations in diazinon volatilization were monitored in three field experiments conducted with differing soil moisture contents. The highest flux rates in all experiments were recorded just after diazinon application, but the magnitudes of those initial rates differed according to the soil moisture content, with wetter soil producing a higher rate: 5.6 × 10(-4) μg cm(-2) min(-1) for initial soil moisture above field capacity, 8.3 × 10(-5) μg cm(-2) min(-1) for initial soil moisture at field capacity, and 2.5 × 10(-5) μg cm(-2) min(-1) for initially very dry soil. Volatilization decreased during the first day in the two experiments with initially wet soils but remained relatively constant in the experiment with initially dry soil. The volatilization rate during the first night for the wettest soil remained about an order-of-magnitude higher than that observed for driest soil. When the surface dried in the experiment conducted at the intermediate water content, the volatilization rate and temporal pattern transitioned and became similar to that observed for the initially dry soil. Around noon of the second day, a daily maximum value was observed in the volatilization rate for wet soil, whereas a minimum value was observed for the dry soil, resulting in an order-of magnitude difference. This study demonstrates the importance of soil moisture on emissions of pesticides to the atmosphere.