Using near-surface atmospheric measurements as a proxy for quantifying field-scale soil gas flux

We present a new method for deriving surface soil gas flux at the field scale, which is less fieldwork intensive than traditional chamber techniques and less expensive than those derived from airborne or space surveys. The “open-field” technique uses aspects of chamber and micrometeorological methods combined with a mobile platform and GPS to rapidly derive soil gas fluxes at the field scale. There are several assumptions in using this method, which will be most accurate under stable atmospheric conditions with little horizontal wind flow. Results show that soil gas fluxes, when averaged across a field site, are highly comparable between the open-field method and traditional chamber acquisition techniques. Atmospheric dilution is found to reduce the range of flux values under the open-field method, when compared to chamber-derived results at the field scale. Under ideal atmospheric conditions it may be possible to use the open-field method to derive soil gas flux at an individual point; however this requires further investigation. The open-field method for deriving soil–atmosphere gas exchange at the field scale could be useful for a number of applications including quantification of leakage from CO2 geological storage sites, diffuse degassing in volcanic and geothermal areas, and greenhouse gas emissions, particularly when combined with traditional techniques.

Mass spectrometric multiple soil-gas flux measurement system with a portable high-resolution mass spectrometer (MULTUM) coupled to an automatic chamber for continuous field observations

We developed a mass spectrometric soil-gas flux measurement system using a portable high-resolution multi-turn time-of-flight mass spectrometer, called MULTUM, and we combined it with an automated soil-gas flux chamber for the continuous field measurement of multiple gas concentrations with a high temporal resolution. The developed system continuously measures the concentrations of four different atmospheric gases (NO2, CH4, CO2, and field soil–atmosphere flux measurements of greenhouse gases (NO2, O2) ranging over 6 orders of magnitude at one time using a single gas sample. The measurements are performed every 2.5 min with an analytical precision (2 standard deviations) of ±34 ppbv for NO2; ±170 ppbv, CH4; ±16 ppmv, CO2; and ±0.60 vol %, O2 at their atmospheric concentrations. The developed system was used for the continuous field soil–atmosphere flux measurements of greenhouse gases (NO2, CH4, and CO2) and O2 with a 1 h resolution. The minimum quantitative fluxes (2 standard deviations) were estimated via a simulation as 70.2 µgNm-2h-1 for NO2; 139 µgCm-2h-1, CH4; 11.7 mg C m−2 h−1, CO2; and 9.8 g O2 m−2 h−1, O2. The estimated minimum detectable fluxes (2 standard deviations) were 17.2 µgNm-2h-1 for NO2; 35.4 µgCm-2h-1, CH4; 2.6 mg C m−2 h−1, CO2; and 2.9 g O2 m−2 h−1, O2. The developed system was deployed at the university farm of the Ehime University (Matsuyama, Ehime, Japan) for a field observation over 5 d. An abrupt increase in NO2 flux from 70 to 682 µgNm-2h-1 was observed a few hours after the first rainfall, whereas no obvious increase was observed in CO2 flux. No abrupt NO2 flux change was observed in succeeding rainfall events, and the observed temporal responses at the first rainfall were different from those observed in a laboratory experiment. The observed differences in temporal flux variation for each gas component show that gas production processes and their responses for each gas component in the soil are different. The results of this study indicate that continuous multiple gas concentration and flux measurements can be employed as a powerful tool for tracking and understanding underlying biological and physicochemical processes in the soil by measuring more tracer gases such as volatile organic carbon, reactive nitrogen, and noble gases, and by exploiting the broad versatility of mass spectrometry in detecting a broad range of gas species.

Using near-surface atmospheric measurements as a proxy for quantifying field-scale soil gas flux

We present a new method for deriving surface soil gas flux at the field scale, which is less field-work intensive than traditional chamber techniques and less expensive than those derived from airborne or space surveys. The technique uses aspects of chamber and micrometeorological methods combined with a mobile platform and GPS to rapidly derive soil gas fluxes at the field-scale. There are several assumptions in using this method, which will be most accurate under stable atmospheric conditions with little horizontal wind flow. Results show that soil gas fluxes, when averaged across a field site, are highly comparable between the method presented and traditional chamber acquisition techniques. Atmospheric dilution is found to reduce the range of flux values under the open field-scale method, when compared to chamber derived results. Under ideal atmospheric conditions it may be possible to use the presented method to derive soil gas flux at an individual point, however this requires further investigation. The new method for deriving soil-atmosphere gas exchange at the field-scale could be useful for a number of applications including quantification of CCS leakage, diffuse degassing in volcanic and geothermal areas and greenhouse-gas emissions.

A mass spectrometric multiple soil-gas flux measurement system with portable high-resolution mass spectrometer MULTUM coupled to automatic chamber for continuous field observation

We developed a mass spectrometric soil-gas flux measurement system using a portable high-resolution multi-turn time-of-flight mass spectrometer, called MULTUM, combined with an automated soil-gas flux chamber for continuous field measurement of multiple gas concentrations. The developed system continuously measures concentrations of four different atmospheric gases (i.e., N2O, CH4, CO2, and O2), of which the concentrations range over six orders of magnitude at a time within a single gas sample. The measurements were performed every 2.5 min with analytical precisions (two standard deviations) of ±34 ppbv for N2O, ±170 ppbv for CH4, ±16 ppmv for CO2, and ±0.60 vol% for O2 at their atmospheric concentrations. The developed system was used for continuous field soil–atmosphere flux measurements of greenhouse gases (GHGs: N2O, CH4, and CO2) and O2 with 1 h resolution. The minimum quantitative fluxes (two standard deviations) were estimated through simulation as 70.2 µg N m−2 h−1 for N2O, 139 µg C m−2 h−1 for CH4, 11.7 mg C m−2 h−1 for CO2, and 9.8 g O2 m−2 h−1 (negative) for O2, whereas the estimated minimum detectable fluxes (two standard deviations) were 17.2 μg N m−2 h −1 for N2O, 35.4 μg C m−2 h−1 for CH4, 2.6 mg C m−2 h−1 for CO2, and 2.9 g O2 m−2 h−1 for O2. The developed system was deployed in the University Farm of the Ehime University (Matsuyama, Ehime, Japan) for a field observation over five days. Interestingly, an abrupt increase in N2O flux from 70 to 682 µg N m−2 h−1 was observed a few hours after the first rainfall, whereas no obvious increase in the CO2 flux was observed, although the temporal responses were different from those observed in a laboratory experiment. No abrupt N2O flux change was observed in succeeding rainfalls. Continuous multiple-gas flux and concentration measurements can be a powerful tool for tracking and understanding of underlying biological and physicochemical processes in the soil through measuring more tracer gases, such as volatile organic carbon gases, reactive-nitrogen gases, and noble gases by taking advantage of the broad versatility of mass spectrometry in detecting broad range of gas species.