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

Asma Akter Parlin,Monami Kondo,Noriaki Watanabe,Mizuki Yamada,Jiajie Wang,Takeshi Komai,Kengo Nakamura

doi:10.3390/su13126570

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.

Highlights

Introduction iationsVolatile organic compounds (VOCs) emitted into the atmosphere from both anthropogenic and biogenic sources are of great interest because they participate in atmospheric photochemical reactions that contribute to the formation of secondary compounds such as ozone and secondary organic aerosols [1,2,3]
The literature review provided here reveals that both the inverse correlation and unexpectedly large changes have been observed simultaneously, and the inverse correlation has been observed in wet soils. These facts suggest that larger flux changes under dynamic temperatures occur for many types of volatile organic compounds (VOCs) in wet soils and that the strength of this effect increases with increasing water content. We examine this hypothesis via laboratory experiments on the upward vertical vapor-phase diffusive transport of benzene and TCE in sandy soils with different water contents ranging from an air-dried condition to 10 wt% during sinusoidal temperature variations between 20 and 30 ◦ C, where moderately high effective diffusion coefficients for the sandy soils prevent too fast or too slow flux changes to observe
The VOC fluxes under static temperature showed no significant fluctuation with time, which confirmed that the experimental system was suitable for the investigation of the effects of temperature on VOC flux

Summary

Introduction

Volatile organic compounds (VOCs) emitted into the atmosphere from both anthropogenic and biogenic sources are of great interest because they participate in atmospheric photochemical reactions that contribute to the formation of secondary compounds such as ozone and secondary organic aerosols [1,2,3]. VOC emissions mitigate climate change through the formation of secondary organic aerosols and are involved in climate feedback processes; the effects are still very uncertain. Vegetation and soil are significant sources of emissions for various VOCs. In addition to the large impact of vegetation, the importance of soil for the global balance of VOCs, where soils can act as both sources and sinks for VOCs, has been highlighted [4,5]. A global assessment of VOC emissions should provide quantitative information on soil VOC emissions, for example, changes in the VOC flux owing to gas-phase vertical transport in dynamic near-surface.

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Conclusion