Chambers are commonly used to measure the emission of many trace gases and chemicals from soil. An aerodynamic (flow through) chamber was designed and fabricated to accurately measure the surface flux of trace gases. Flow through the chamber was controlled with a small vacuum at the outlet. Due to the design using fans, a partition plate, and aerodynamic ends, air is forced to sweep parallel and uniform over the entire soil surface. A fraction of the air flowing inside the chamber is sampled in the outlet. The air velocity inside the chamber is controlled by fan speed and outlet suction flow rate. The chamber design resulted in a uniform distribution of air velocity at the soil surface. Steady state flux was attained within 5 min when the outlet air suction rate was 20 L/min or higher. For expected flux rates, the presence of the chamber did not affect the measured fluxes at outlet suction rates of around 20 L/min, except that the chamber caused some cooling of the surface in field experiments. Sensitive measurements of the pressure deficit across the soil layer in conjunction with measured fluxes in the source box and chamber outlet show that the outflow rate must be controlled carefully to minimize errors in the flux measurements. Both over- and underestimation of the fluxes are possible if the outlet flow rate is not controlled carefully. For this design, the chamber accurately measured steady flux at outlet air suction rates of approximately 20 L/min when the pressure deficit within the chamber with respect to the ambient atmosphere ranged between 0.46 and 0.79 Pa.

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.

Vapor intrusion (VI) is well-known to be difficult to characterize because indoor air (IA) concentrations exhibit considerable temporal and spatial variability in homes throughout impacted communities. To overcome this and other limitations, most VI science has focused on subsurface processes; however there is a need to understand the role of aboveground processes, especially building operation, in the context of VI exposure risks. This tutorial review focuses on building air exchange rates (AERs) and provides a review of literature related building AERs to inform decision making at VI sites. Commonly referenced AER values used by VI regulators and practitioners do not account for the variability in AER values that have been published in indoor air quality studies. The information presented herein highlights that seasonal differences, short-term weather conditions, home age and air conditioning status, which are well known to influence AERs, are also likely to influence IA concentrations at VI sites. Results of a 3D VI model in combination with relevant AER values reveal that IA concentrations can vary more than one order of magnitude due to air conditioning status and one order of magnitude due to house age. Collectively, the data presented strongly support the need to consider AERs when making decisions at VI sites.

Simulations using the one-dimensional nonisothermal model (Reichman et al. Environ Sci. Technol. 2000, 34, 1313−1320). this issue) for pesticides that have been applied to the soil surface revealed that diurnal variation of climatic factors, soil type (silty clay and sand), initial soil-moisture distribution, and pesticide type (parathion, dieldrin, and trifluralin) potentially have major influences on pesticide volatilization rates and persistence. The diurnal variation of water evaporation, which is closely related to the diurnal variation in solar radiation, has potentially significant effect on the diurnal variation of parathion volatility and mobility (low volatility and high mobility). However, the volatilization rates of low-water-soluble pesticides (e.g., trifluralin and dieldrin) are only slightly affected by water evaporation rate. Their migration downward via vapor-phase diffusion reduces their concentration in the soil surface layer and decreases their actual volatilization rates. The limiting factor for these pesticides is their movement downward in the soil. The volatilized masses of parathion and dieldrin from a silty clay soil are higher than from sandy soil. However, trifluralin's volatilization rate, volatilized mass, and penetration depth are lower for the silty clay than for the sandy soil due to adsorption. Higher volatilization rates were obtained from initially dry soils for trifluralin and from initially wet soils for parathion and dieldrin. Comparison between isothermal and nonisothermal models revealed that use of the former might lead to erroneous conclusions with respect to the relative volatility of pesticides especially those with a low Henry constant and high water solubility.

Volatilization of soil-applied pesticides to the atmosphere and their vapor transport in the air are principal processes leading to their widespread dispersion in the environment. A detailed one-dimensional nonisothermal model was developed to accommodate the impact of outdoors-dynamic behavior on the fate of surface-applied pesticides. The model includes the effects of temporal changes in weather conditions on the distribution of soil temperature and moisture in the soil profile and the latter two parameters' role on the volatilization and persistence of the applied pesticides. The model is solved numerically by the finite differences method. The performance of the numerical model was tested in two ways:  by comparing its results with an analytic solution for a simplified case and by comparing predicted volatilization rates and soil residues of three pesticides (lindane, dieldrin, and trifluralin) with measured data. A good agreement was found between measured and simulated results. Hence, the model might be a useful tool for designing pesticide application and to explore their behavior under various soil and meteorological conditions. A sensitivity analysis of the model showed that volatilization of volatile pesticides such as lindane is most sensitive to the estimation of vapor pressure as a function of temperature, less sensible to adsorption coefficient, and insensitive to water solubility (within the range of published data) and decomposition coefficient.