Abstract
Understanding the diffusive transport behavior of volatile organic compounds (VOCs) in near-surface soils is important because soil VOC emissions affect atmospheric conditions and climate. Previous studies have suggested that temperature changes affect the transport behavior; however, the effect of these changes are poorly understood. Indeed, under dynamic temperature conditions, the change in VOC flux is much larger than that expected from the temperature dependency of the diffusion coefficient of VOCs in air. However, the mechanism is not well understood, although water in soil has been considered to play an important role. Here, we present the results of experiments for the upward vertical vapor-phase diffusive transport of two VOCs (benzene and tetrachloroethylene) in sandy soil under sinusoidal temperature variations of 20–30 °C, as well as its numerical representation. The results clarify that the unexpectedly large changes in emission flux can occur as a result of changes in the VOC concentration gradient due to VOC release (volatilization) from/trapping (dissolution) into water, and that such flux changes may occur in various environments. This study suggests the importance of a global evaluation of soil VOC emissions by continuous measurements in various soil environments and/or predictions through numerical simulations with thorough consideration of the role of water in dynamic soil environments.
Highlights
Understanding the diffusive transport behavior of volatile organic compounds (VOCs) in near-surface soils is important because soil VOC emissions affect atmospheric conditions and climate
These experiments were conducted in finer and coarser grained sandy soils, with water contents ranging from air-dry to 10 wt% during sinusoidal temperature variation between 20 and 30 °C
We initially describe a new experimental method for exploring the vertical vapor-phase diffusive transport of VOCs in the soil, which has been developed based on the experimental methods in our previous studies[30,31] and allows continuous measurements of VOC concentrations at multiple depths and of relative humidity (RH) in a soil-packed column under dynamic temperature conditions
Summary
Understanding the diffusive transport behavior of volatile organic compounds (VOCs) in near-surface soils is important because soil VOC emissions affect atmospheric conditions and climate. Enhanced flux changes were assessed based on experiments of upward vertical vapor-phase diffusive transport of benzene and T CE30,31 These experiments were conducted in finer and coarser grained sandy soils, with water contents ranging from air-dry to 10 wt% during sinusoidal temperature variation between 20 and 30 °C. The inverse relationship and enhanced flux changes were clearly observed in all conditions, except in the air-dry condition, in the presence of water condensation/evaporation, and their strengths did not depend on sand grain size but were greater for TCE with stronger temperature dependence of its volatility (e.g., Henry’s constant in Supplementary Figure S1b), higher water content, and/or larger intensity in the phase change of water It was, hypothesized that such flux changes are induced by changes in the vertical concentration gradient of VOCs resulting from concentration changes due to VOC release (volatilization) from and trapping (dissolution) into water, and/or water evaporation and condensation that contribute to VOC release and trapping, respectively. To the best of our knowledge, this hypothesis has not previously been examined
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