Model analysis of the influence of gas diffusivity in soil on CO and H₂ uptake

Abstract

CO and H 2 uptake by soil was studied as a diffusion process. A diffusion model was used to determine how the surface fluxes (net deposition velocities) were controlled by in-situ microbial uptake rates and soil gas diffusivity calculated from the 3-phase system (solid, liquid, gas) in the soil. Analytical solutions of the diffusion model assuming vertical uniformity of soil properties showed that physical properties such as air-filled porosity and soil gas diffusivity were more important in the uptake process than in the emission process. To incorporate the distribution of in-situ microbial uptake, we used a 2-layer model incorporating ''a microbiologically inactive layer and an active layer'' as suggested from experimental results. By numerical simulation using the 2-layer model, we estimated the effect of several factors on deposition velocities. The variations in soil gas diffusivity due to physical properties, i.e., soil moisture and air-filled porosity, as well as to the depth of the inactive layer and in-situ microbial uptake, were found to be important in controlling deposition velocities. This result shows that the diffusion process in soil is critically important for CO and H 2 uptake by soil, at least in soils with higher in-situ uptake rates and/or with large variation in soil moisture. Similar uptake rates and the difference in deposition velocity between CO and H 2 may be attributable to differences in CO and H 2 molecular diffusivity. The inactive layer is resistant to diffusion and creates uptake limits in CO and H 2 by soil. The coupling of high temperature and a thick inactive layer, common in arid soils, markedly lowers net CO deposition velocity. The temperature for maximum uptake of CO changes with depth of the inactive layer.

The impact of soil uptake on the global distribution of molecular hydrogen: chemical transport model simulation

“…Their model was able to capture the observed vertical profiles of H 2 in the soil and showed that the thickness of the inactive layer is important for a realistic simulation of the flux strength. In this study, we incorporate the same type of 1-D diffusion model described in Yonemura et al (2000a) to the deposition scheme of the CHASER model.…”

“…The inactive layer acts as a diffusion barrier, and is probably caused by the dryness and high temperature near the ground surface. Yonemura et al (2000a) calculated the deposition velocity of H 2 using a 2-layer diffusion model that incorporated the biologically inactive and active layer. Their model was able to capture the observed vertical profiles of H 2 in the soil and showed that the thickness of the inactive layer is important for a realistic simulation of the flux strength.…”

“…Previous studies have shown that most of this absorption is accomplished within several centimeters of soil below the surface (Yonemura et al, 1999). Soil temperature and moisture control much of the biological activity, while snow, litter function and heated soil layer can act as a diffusion barrier near the surface (Yonemura et al, 1999(Yonemura et al, , 2000aSmith-Downey et al, 2006bSchmitt et al, 2009). …”

Dry deposition of molecular hydrogen in the presence of H₂ production

“…Thus, eq. (1) can be used to describe the dependence of v d on U w and T. This description is more complete and more realistic than that derived from the onelayer model used so far (Yonemura et al, 2000;SmithDowney et al, 2008;Morfopoulos et al, 2012). Yet, eq.…”

“…The two-layer model was first suggested by Yonemura et al (2000), and it assumes uniform conditions in the respective layers. d is the depth of the dry top layer, D S is the diffusivity of H 2 in the soil (D S,I in the dry top layer, D S,II in the moist, deeper soil layer), k s is the rate constant for removal of H 2 from soil air, and U a is the fraction of soil volume filled with air.…”