Design Reiteration of a Chimney Gas Flowmeter for Natural CO2 Emissions from Mofettes: Differential Pressure Measurement Increases Resolution and Accuracy

Abstract

In this thesis, an established device for in situ gas measurements of the natural CO2 emissions from a mofetta is being improved in design and measurement principles. A cyclically erupting mofetta that is continuously submerged under the surrounding water table is observed. For the measurement of the volumetric flow rate, the previously utilized cup anemometer is discarded and instead, a self-made and self-calibrated differential pressure flowmeter is introduced. During field performance, it is validated as an accurate and high-resolution approach on flow rate quantification that is highly adapted to the conditions of the mofetta, therefore presenting a considerable improvement to the previous installment. The devices gas-channelling and sensor-carrying chimney has been redesigned completely based on a highly modular approach. This change makes field maintenance much more efficient, improving the devices flexibility by a considerable amount. Measurements on the 9th of August 2023 verified the systems ability to perform under field conditions and lead to the observation of an anomaly where the flow rate increased by ∼ 40 % over the course of 1 minute coupled with a temperature rise of 0.8 K. Similarities and differences between this anomaly and anomalies detected with previous iterations of the device are discussed. High resolution pressure data is obtained which leads to a temporal quantification of the exhaust dynamics of the mofetta on a small time scale. Compared to previous measurements conduced in winter of 2022, the main frequency window in which the mofetta erupts has shifted from ∼ 4 seconds to ∼ 3 seconds. Furthermore, the flow rate has increased by ∼ 36 - 41 % during calibration experiments and by ∼ 14 - 24 % during operation. These observations suggest a seasonal dependency of the mofettas exhaust. It is discussed that this dependency may be caused by an increase in evaporation coupled with a decrease in precipitation during the summer which induces a drop in water table height. As a consequence, the hydrostatic pressure the upstreaming gas needs to overcome to reach the atmosphere decreases, resulting in a rise in frequency and consequentially an increase in flow rate.