Water-Enhanced Flux Changes under Dynamic Temperatures in the Vertical Vapor-Phase Diffusive Transport of Volatile Organic Compounds in Near-Surface Soil Environments

Abstract

The quantitative understanding of the transport behavior of volatile organic compounds (VOCs) in near-surface soils is highly important in light of the potential impacts of soil VOC emissions on the air quality and climate. Previous studies have suggested that temperature changes affect the transport behavior; however, the effects are not well understood. Indeed, much larger changes in the VOC flux under in situ dynamic temperatures than those expected from the temperature dependence of the diffusion coefficients of VOCs in the air have been suggested but rarely investigated experimentally. Here, we present the results of a set of experiments on the upward vertical vapor-phase diffusive transport of benzene and trichloroethylene (TCE) in sandy soils with water contents ranging from an air-dried value to 10 wt% during sinusoidal temperature variation between 20 and 30 °C. In all experiments, the flux from the soil surface was correlated with the temperature, as expected. However, the changes in flux under wet conditions were unexpectedly large and increased with increasing water content; they were also larger for TCE, the volatility of which depended more strongly on the temperature. Additionally, the larger flux changes were accompanied by a recently discovered water-induced inverse correlation between temperature and flux into the overlying soil. These results demonstrated that the flux changes of VOCs under dynamic temperatures could be increased by volatilization-dissolution interactions of VOCs with water. Future extensive studies on this newly discovered phenomenon would contribute to a better understanding of the impacts of soil VOC emissions on the air quality and climate.

Role of water in unexpectedly large changes in emission flux of volatile organic compounds in soils under dynamic temperature conditions

“…Similar changes were observed in our previous studies using similar experimental systems, which involved concentration measurements only at two ports (i.e., P1 and P4 in the present study) using the same gas detectors 30 , 31 . Thus, the results of Run 1 confirmed the suitability of the experimental system, equipped with two additional measurement ports and gas detectors, to support the objective of the present study.…”

“…Consequently, only the mean values are presented in the results. Our previous studies that used similar experimental systems reported that temperature changes at the center of the sand-packed column were delayed only by approximately 5 min when compared to those in the bottom and top spaces 31 . In these previous studies, concentrations were measured at two ports (i.e., P1 and P4 in the present study) using the same gas detectors 30 , 31 .…”

“…The few laboratory experiments that have considered dynamic conditions in near-surface soils did not cover the unexpected or counterintuitive effects of temperature on VOC flux 28 , 29 . In this context, our recent studies 30 , 31 revealed the inverse relationship between flux and temperature changes and enhanced flux changes. Moreover, enhanced flux changes were assessed based on experiments of upward vertical vapor-phase diffusive transport of benzene and TCE 30 , 31 .…”

“…In this context, our recent studies 30 , 31 revealed the inverse relationship between flux and temperature changes and enhanced flux changes. Moreover, enhanced flux changes were assessed based on experiments of upward vertical vapor-phase diffusive transport of benzene and TCE 30 , 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.…”

“…Therefore, in the present study we examined this hypothesis using a novel laboratory experiment and a complementary numerical simulation of upward vertical vapor-phase diffusive transport of VOCs in soil under dynamic temperature. 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. Thereafter, an experiment using benzene at a static temperature (25 °C) was conducted to verify that no fluctuations in concentration or RH occurred because of the multiple-depth concentration measurement by gas detectors equipped with a pump.…”