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 7 field-work intensive than traditional chamber techniques and less expensive than those derived from 8 airborne or space surveys. The technique uses aspects of chamber and micrometeorological methods 9 combined with a mobile platform and GPS to rapidly derive soil gas fluxes at the field-scale. There are 10 several assumptions in using this method, which will be most accurate under stable atmospheric 11 conditions with little horizontal wind flow. Results show that soil gas fluxes, when averaged across a field 12 site, are highly comparable between the method presented and traditional chamber acquisition 13 techniques. Atmospheric dilution is found to reduce the range of flux values under the open field-scale 14 method, when compared to chamber derived results. Under ideal atmospheric conditions it may be 15 possible to use the presented method to derive soil gas flux at an individual point, however this requires 16 further investigation. The new method for deriving soil-atmosphere gas exchange at the field-scale could 17 be useful for a number of applications including quantification of CCS leakage, diffuse degassing in 18 volcanic and geothermal areas and greenhouse-gas emissions.


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The study of soil-atmosphere gas exchange has become more prominent over the past couple of 22 decades. Objectives for these studies are wide-ranging, for example: the study of volcanic degassing [1,2] ; 23 quantification of carbon budgets [3,4] ; greenhouse gas (GHG) emission studies [5] ; and identifying potential 24 leakage from Enhanced Oil Recovery (EOR) and Carbon Capture and Storage (CCS) sites [6][7][8] . Soil gas 25 emissions are directly measured at points or spots using chamber techniques [9] or over restricted areas 26 through micrometeorological methods [10] . At regional and national scales, airborne and space measure-27 ments are used to derive soil gas emissions using empirical and process-oriented models for post-pro-28 cessing. These regional scale methods lack detail required for field-scale studies (10 1 to 10 3 m 2 ) and 29 may be prohibitively expensive 5 . There is currently a lack of practical methods for quantifying soil gas 30 flux rates at field-scales, which are highly relevant to leakage and degassing studies. Feitz et al. [11] pro-31 vide a comparison of many of these techniques under a controlled gas release.

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Closed loop flux chamber-based analyses utilize an open-bottomed chamber with a known footprint 34 and volume placed on the soil surface, allowing gases emitted by the soil to accumulate within the cham-35 ber headspace. From analysis of the gas mixing ratios within the chamber over time, the flux of gas from 36 the soil can be derived for that small spot of the land surface. In contrast, an open loop technique passes 37 air through the sample chamber just once at a known flow rate, until a steady-state concentration is 38 observed, from which a flux rate is derived. Both techniques require measurements at a large number of 39 points to estimate field-scale fluxes via interpolation, with the caveats that sample density is sufficient to 40 represent site spatial variability and that flux is static with respect to time during the measurement period.

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Micrometeorological methods use eddy covariance (EC) techniques to derive soil gas flux. A three-di-2 mensional sonic anemometer is coupled to a gas analyser attached to a tower or mast, allowing meas-43 urements that incorporate areas of up to several square kilometers under the right atmospheric and 44 terrain conditions [12] . Continuous EC measurements over a period of time allow soil gas fluxes for a par-45 ticular parcel of land to be derived from absolute gas concentrations, temperature, and vertical and hor-46 izontal wind flows. To ensure that the path from soil surface up to the instrument sensors is correct, the technique is most suited to sites with low turbulence properties associated with short vegetation and level 48 ground.

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The use of chambers to represent soil-gas fluxes at field-scale is often highly time-intensive and prone 51 to interpolation-related uncertainty [13] , particularly for sites with heterogeneous soils or geology. Micro-52 meteorological methods can also be difficult to implement at the field-scale as they are reliant on wind 53 direction and non-turbulent atmospheric conditions between the ground-surface and the sensors.

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The objective of this paper is present a new method that uses aspects of chamber and micrometeor-56 ological methods combined with a mobile platform and GPS to rapidly derive soil gas fluxes at the field-57 scale. We assess this method against traditional chamber techniques for field locations within the UK 58 and Italy, and discuss the explicit and implicit assumptions inherent in the presented techniques.

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Development of a new field-scale soil CO2 flux quantification method was focused on creating a 62 mobile tool that could easily and quickly make measurements around a field site without the need for 63 stopping at individual locations. Here we describe the theoretical aspects, assumptions made and com-64 ponents used to undertake the measurements.

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Experimental theory. As we approach the ground surface, frictional drag reduces horizontal wind 67 speed to near-zero. The depth of this frictional influence depends on the roughness of the surface [14] . By

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The physical basis for this calculation is best described using a thought experiment. Suppose we 88 have a box of air at the ground surface with a known, uniform gas concentration. As we know the volume 89 of the box and the gas concentration, we know the weight of that gas within that box from the ideal gas

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the vertical wind field is directly coupled to the temperature field through buoyant forces [16] . It is also 105 assumed that a measurement point represents the entire box of air and that the air is fully mixed. As the 106 experimental theory is scalable, the size of the box can be reduced to near-zero and, therefore, the 107 assumption holds true. How well that measurement represents surrounding areas when interpolation is 108 applied in post-processing is unknown. The same issue is faced by traditional chamber methods, how-

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The results show that the average (mean and median) CO2 flux obtained using the chamber tech-

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The method presented for deriving surface soil gas flux at the field scale is less field-work intensive 178 than traditional chamber techniques and cheaper than those derived from airborne or space surveys.

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The use of chamber and micrometeorological methods together with a mobile platform and GPS allow

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The presented method of deriving soil-atmosphere gas exchange at the field-scale could be useful for a 187 number of applications including leakage, degassing and greenhouse-gas emission studies.