1D Air Pressure Fluctuations Cannot Fully Explain the Natural Pressure‐Pumping Effect on Soil Gas Transport. Issue 4 (25th July 2019) - Laemmel, Thomas
Abstract : While gas transport in soil is commonly assumed to be driven by molecular diffusion, pressure‐pumping due to small air pressure fluctuations was found to enhance the gas transport rate during periods of high wind speed. A possible explanation is related to the vertical oscillating movement of gas in the air‐filled pore space due to these air pressure fluctuations. Exposing granular and artificial samples to vertical air pressure fluctuations with realistic amplitudes and frequencies in the laboratory, we show that they indeed can increase the effective gas diffusivity through the samples. The enhancement of gas diffusivity was higher for samples characterizing by a low bottleneck area and a low bottleneck density and it was lower for samples with small bottleneck length. However, there was a significant discrepancy between the gas transport enhancement observed in the field and under laboratory conditions by comparable soil air movement due to oscillating flow. The exclusive consideration of vertical air pressure fluctuations in the laboratory seems insufficient to explain effects of the pressure‐pumping in the field on soil gas transport rates. Core Ideas: 1D vertical air pressure fluctuations can enhance gas transport through soil samples. The effect increases by increasing amplitudes and frequencies of air pressure fluctuations. Bottlenecks in soil porous structure substantially contribute to the effect. The effect observed in the laboratory was not as strong as in the field observations. We hypothesize that the in situ pressure pumping effect is not exclusively driven by 1D vertical air pressure fluctuations.