## 4 Discussion  
This study experimentally investigated and demonstrated the occurrence of an inverse correlation between the changes in temperature and the vertical vapor-phase VOC diffusive flux in soils, which was previously observed in the field study [8], but has not yet been investigated or demonstrated. Herein, we discuss the possible mechanisms for  
Figure 7: Changes in RH and corresponding water flux, top and bottom benzene concentrations, and corresponding top and bottom benzene fluxes in the dynamic temperature experiments Run 3 and Run 5, respectively, using finer-grained silica sand (**a**) at 10 wt% water content and (**b**) under air-dried conditions. the inverse correlation. Based on [PERSON]'s first law, the flux increases with an increase in the effective diffusion coefficient and/or vertical concentration gradient. Changes in the effective diffusion coefficient can be assumed to be correlated with temperature changes, considering that a higher molecular diffusion rate is observed at higher temperatures. Therefore, changes in the vertical concentration gradient should be inversely correlated with temperature changes. One possible process that causes the inverse correlation between temperature and concentration gradient changes is VOC desorption and sorption within the soil. In other words, rising and dropping temperatures induce VOC desorption and sorption, respectively [37; 38; 39], which in turn increases and decreases the VOC concentration in the soil gas, respectively, such that they may change the vertical VOC concentration gradient. However, the influence of the sorption and desorption of VOCs may not be significant, based on the present experimental results. The VOC sorption capacity is proportional to the available area within the sand [37] and is, therefore, higher for finer-grained silica sand and/or under air-dried conditions [16]. However, this trend is inconsistent with the fact that the intensity of the inverse correlation appears independent of grain size and larger at 10 wt% water content. Moreover, the sand employed in this study contained only a trace amount of organic material, which also suggests that desorption and sorption have a small influence [15]. Another possible process is the VOC volatilization from and dissolution into pore water [40; 41]. Rising temperature induces the volatilization of VOCs from water, where the volatilization of water also occurs, which further induces the volatilization of the VOCs. In contrast, a declining temperature induces the dissolution of VOCs in water, where the condensation of water also occurs and further induces the dissolution of the VOCs. In this case, VOC volatilization and dissolution, enhanced by the accompanying volatilization and condensation of water, respectively, may have relatively larger impacts on the vapor-phase VOC concentration at the parts with initially lower concentrations (i.e., shallower parts); thus, the vertical concentration gradient would ultimately be reduced and increased, respectively. This hypothesis is consistent with the fact that the inverse correlation in the present study appeared to be much more significant at 10 wt% water content and was linked to the occurrences of negative and positive water vapor fluxes (i.e., condensation and evaporation of water). This hypothesis should, therefore, be examined further in future studies, in which the transport phenomena of water and water-VOC interactions need to be addressed with thorough consideration of the reality of near-surface soil environments, including variations occurring in temporal and spatial scales because the temperature changes daily and seasonally as well as with depth. In this study, the employed experimental conditions are limited, and the suggested mechanism of the inverse correlation was not examined. However, this study suggests that, in many cases, temperature changes may have an inverse correlation with changes in vertical gas-phase VOC flux in relatively deeper parts of near-surface soils.