Spatial variability of wind-induced air pressure fluctuations responsible for pressure pumping

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Small air pressure fluctuations in the atmosphere are responsible for the pressure pumping effect, which leads to an enhancement of soil gas transport. To investigate the spatial variability of these air pressure fluctuations, several high-precision differential pressure sensors were installed at the floor of a Scots pine forest. The alignment of the pressure sensors allowed for the determination of the propagation direction and speed of the observed air pressure fluctuations. Below-canopy and above-canopy airflow characteristics were monitored to find possible links between the air pressure fluctuations and the airflow. Results show that the propagation direction of the air pressure fluctuations observed at the forest floor correspond to the above-canopy and not to the below-canopy wind direction. Moreover, propagation speed of the air pressure fluctuations is higher than the below-canopy wind speed and corresponds to above-canopy wind speed. These findings indicate a connection between below-canopy air pressure fluctuations and above-canopy airflow. The air pressure fluctuations were found to be well correlated up to a distance of 15 m. With increasing distance, the correlation strongly decreases. However, the calculated pressure pumping coefficient, which quantifies the strength of the pressure pumping effect, yields similar values up to a distance of 90 m. This allowed specifying the range of influence of the air pressure fluctuations.