Wind effects on soil-atmosphere gas exchange for gases of different density: A study with laboratory experiments and coupled subsurface – free flow modelling

Abstract:

The soil is one of the major sources of greenhouse gas emissions to the atmosphere. Significant contributions stem from sub-surface anthropogenic sources (landfills, gas leaks). Predicting gas transport from such sources and resulting emissions requires a thorough understanding of the transport mechanisms in the soil and at the soil-atmosphere-interface.

Near-surface winds can influence the transport considerably. To determine the underlying mechanisms, we investigate wind effects on sub-surface gas transport by combining controlled laboratory experiments and numerical modelling with a fully coupled porous medium - free flow model that accounts for wind turbulence. Experiments were carried out in a quasi-2d sand tank with overlying wind tunnel and a gas inlet placed at the bottom of the tank. Gas concentrations inside the tank were measured continuously for stead-state gas injection (steady-state transport) and after termination of gas supply (transient transport). Several conditions were tested: 4 wind velocities, dry and partially saturated sand and 3 different gases (He, N₂, CO₂). The present work is the first study to provide a comparative investigation on wind effects on sub-surface transport of light (He), heavy (CO₂) and quasi-density neutral gases (N₂).

Wind impact is stronger during transient transport than steady-state gas injection. Wind accelerates transport from the soil and introduces horizontal concentration gradients with an explicit dependency on wind velocity, indicating the presence of wind-induced horizontal advection. Wind effects differ depending on the transported gas. Light gases appear more sensitive to wind-induced advection. The coupled model is able to reproduce the main features of the measured concentration distributions. It shows that soil surface concentrations strongly depend on the wind velocity, which influences vertical concentration gradients and thereby impacts diffusive fluxes across the soil surface. Although wind-induced advective fluxes are relatively small in comparison, they cause relevant changes in the concentration distribution and thus indirectly influence the mass fluxes at the soil surface. The presence of density gradients adds additional complexity to the flow, showing that gases such as methane cannot be treated as inert tracers.