Passive Breathing of Earth‐Air: “Fingerprint” Evidence From Moisture Records
Hongshou Li, Shunren Wang, Fei Li, Yipu Gong, Xiaowei Wang
Abstract
Abstract “Earth‐air” refers to air in the vadose zone. Studies have long suggested that earth‐air undergoes a passive breathing effect due to atmospheric pumping. The earth‐air can be modeled as an ideal gas in a closed system, so that the volume of earth‐air varies as the atmospheric pressure (AP) fluctuates. When AP rises, earth‐air is compressed and external air is drawn into the vadose zone. Conversely, when AP drops, earth‐air expands and moist earth‐air enters the atmosphere. In arid regions, there is a moisture gradient in soil due to dryness of the climate: the humidity is relatively low in the upper soil layer, and relatively high in the subsoil. Therefore, if earth‐air undergoes passive breathing, the humidity of earth‐air will fluctuate with AP. This means that passive breathing of the earth‐air should leave traces in the recorded moisture which exactly match AP fluctuations. In this work, clear evidence is presented for the existence of earth‐air passive breathing. This is accomplished by using an innovatively‐designed trap to monitor humidity in 30 cm gravelly sand and showing that these moisture/temperature records contain fingerprints which are characteristic of passive breathing. It is found that relative humidity (RH) in soil is negatively correlated with the atmospheric fluctuations. The correlation between 10‐min average AP and RH timeseries reaches −0.75 for the time‐periods of 20 days. This study provides a new understanding in land‐atmosphere heat/mass exchange from the perspective of vertical fluid dynamics.