Impact of thermal convection on CO₂ flux across the earth-atmosphere boundary in high-permeability soils

Quantifying earth-atmosphere gas exchange is a challenging, yet important problem that is made more complicated by the large number of mechanisms that contribute to this process. This work investigates one mechanism controlling non-diffusive gas transport from high-permeability media that is driven by natural diurnal thermal gradients in the upper vadose zone. We quantified CO₂ migration through 1-m long columns packed with two different permeability values: sand and large soil aggregates - both dry to eliminate chemical reactions. The bottom ends of the columns were exposed to 2000ppm CO₂-enriched air and the CO₂ concentration profiles along the columns was continually monitored. The columns were exposed to two different thermal regimes: isothermal conditions and a range of typical nighttime thermal gradients that are known to lead to unstable gas density profiles. Under isothermal conditions, and regardless of the matrix air-permeability, diffusion was the major mechanism for surface-atmosphere gas exchange. Under nighttime conditions, the prevailing mechanism depended upon matrix air-permeability: diffusion controlled CO₂ transport in the low permeability matrix, whereas thermal convection dominated transport in the high permeability matrix. Venting by thermal convection caused a CO₂ flux of up to two orders of magnitude higher than the diffusive flux. Such a mechanism may be implicated in a number of environmental settings. In soil, thermally driven convection can contribute to soil aeration influencing root respiration and microbial activity, and is likely one of the mechanisms associated with rapid CO₂ exchange that is commonly noted to follow tillage. With respect to the global CO₂ output, thermal convective venting is shown to be a permeability-limited mechanism with high gas exchange potential and a continuous diurnal presence. Its characteristic spatial scale could include, geologic sources via fractured rock surfaces, soil cracks, mine tailings, and rock-fill embankments.