A gas-flow funnel system to quantify advective gas emission rates from the subsurface

Abstract

The estimation of greenhouse gas emission rates from the subsurface into the atmosphere is an important part of climate-related research activities and associated efforts concerning the global carbon cycle. For the direct quantification of gas emission rates from the subsurface to the atmosphere a large variety of gas detection and flux quantification techniques exists. With the goal of measuring advective CO2 gas exhalations circumventing limitations of available systems such as e.g. accumulation-chamber systems or eddy-flux covariance methods, we developed a simple, robust, and low-cost gas-flow funnel system. The device allows for the continuous measurement of mass flow rates with a free, unrestricted gas flow from advectively dominated gas exhalation spots. For the design of the gas-flow funnel we used custom-made, though easy-to-produce components, and sensors that are typically already available when working at such sites. Our general design can easily be applied at sites with focused, advectively driven gas exhalation like volcanic areas, shale-gas seeps, landfills, and open boreholes. For the proof-of-concept we tested the system during three field campaigns at a site with natural CO2-bound emissions associated with a geologic fault in southwestern Germany. The measurements showed to be comparable and repeatable throughout the three campaigns, and are consistent with findings from other field sites with comparable CO2 exhalations.

A portable low-cost device to quantify advective gas fluxes from mofettes into the lower atmosphere: First application to Starzach mofettes (Germany)

“…( 2022 ). Note: This is a different mofette than the one examined by Lübben and Leven ( 2022 ). (b) Gas sensor unit mounted laterally in the chimney consisting of Sensirion STC31 CO 2 sensor, Sensirion SHTC3 temperature and humidity sensor and Bosch BME280 absolute atmospheric pressure sensor.…”

“…Chimney-based designs to measure advective gas fluxes from mofettes were already introduced by Rogie et al. ( 2000 ) and Lübben and Leven ( 2022 ). However, those setups are not suitable for prolonged continuous monitoring.…”

“…This indicates a small-scale shift in the underground gas flow, a change contributing to the temporal and spatial heterogeneity of atmospheric CO 2 concentration at the site. Lübben and Leven ( 2022 ) presented a custom funnel flow meter with which they determined flow rate magnitudes in the order of a few liters per second from specific mofettes such as mofette “R” at the site in 2015.…”

“…The establishment of these inventories requires an array of methods, techniques and instruments to quantify gas fluxes over a variety of spatial scales. These range from in-situ point source estimation (Chiodini et al., 1998 ; Carapezza & Granieri, 2004 ; Lübben & Leven, 2022 ) to satellite remote sensing (Chevallier et al., 2005 ; Pan et al., 2021 ) and (global) inverse gas transport modelling for emission source back-tracing and budgeting (Pickett-Heaps et al., 2011 ; Gaubert et al., 2019 ). Ongoing GHG emissions and their consequences make it increasingly clear that negative emissions, e.g.…”

“…However, the above-mentioned methods have been developed to investigate mainly diffuse degassing and so none of them is capable of directly quantifying advective gas fluxes of intense gas exhalations such as fumaroles or mofettes as the flow rates are either too high or the exhalations too focused. For strong advective degassing from vents, a robust method is to channel the exhaled gas and measure its velocity and concentration to determine the mass flow (Rogie et al., 2000 ; Lübben & Leven, 2022 ). However, to our knowledge, no such design has been published that focuses on continuous, unattended operation, high temporal resolution, low-cost components and adaptability to different magnitudes of degassing.…”