Application of Gauss's theorem to quantify localized surface emissions from airborne measurements of wind and trace gases

Abstract

Abstract. Airborne estimates of greenhouse gas emissions are becoming more prevalent with the advent of rapid commercial development of trace gas instrumentation featuring increased measurement accuracy, precision, and frequency, and the swelling interest in the verification of current emission inventories. Multiple airborne studies have indicated that emission inventories may underestimate some hydrocarbon emission sources in US oil-and gas-producing basins. Consequently, a proper assessment of the accuracy of these airborne methods is crucial to interpreting the meaning of such discrepancies. We present a new method of sampling surface sources of any trace gas for which fast and precise measurements can be made and apply it to methane, ethane, and carbon dioxide on spatial scales of ∼ 1000 m, where consecutive loops are flown around a targeted source region at multiple altitudes. Using Reynolds decomposition for the scalar concentrations, along with Gauss's theorem, we show that the method accurately accounts for the smallerscale turbulent dispersion of the local plume, which is often ignored in other average "mass balance" methods. With the help of large eddy simulations (LES) we further show how the circling radius can be optimized for the micrometeorological conditions encountered during any flight. Furthermore, by sampling controlled releases of methane and ethane on the ground we can ascertain that the accuracy of the method, in appropriate meteorological conditions, is often better than 10 %, with limits of detection below 5 kg h −1 for both methane and ethane. Because of the FAA-mandated minimum flight safe altitude of 150 m, placement of the aircraft is critical to preventing a large portion of the emission plume from flowing underneath the lowest aircraft sampling altitude, which is generally the leading source of uncertainty in these measurements. Finally, we show how the accuracy of the method is strongly dependent on the number of sampling loops and/or time spent sampling the source plume.

Methane emissions from underground gas storage in California

“…This mass balance approach involves executing a cylindrical flight pattern (i.e. stacked, approximately constant altitude circles at altitudes from the minimum safe flight altitude to the top of the emissions plume) while measuring methane concentrations and wind speeds and the application of Gauss's Theorem to estimate the flux divergence through the cylinder (Conley et al 2014, Conley et al 2017. The airborne system is flown on a fixed wing single engine mooney aircraft, extensively modified for research as described in (Conley et al 2014).…”

Advancing Scientific Understanding of the Global Methane Budget in Support of the Paris Agreement

“…First, mud volcanoes can be considered point sources with spatial dimensions comparable to oil and gas production/processing facilities. Their emission estimates could be empirically verified using the types of offsite downwind measurement methodologies employed during oil and gas field campaigns over the last decade in the United States and internationally (e.g., Conley et al, ). Focusing on some of the largest emitters worldwide, for example, in onshore areas around the Caspian Sea could be used to validate the estimation methods used in global models (Etiope et al, ).…”

“…Global Biogeochemical Cycles offsite downwind measurement methodologies employed during oil and gas field campaigns over the last decade in the United States and internationally (e.g., Conley et al, 2017). Focusing on some of the largest emitters worldwide, for example, in onshore areas around the Caspian Sea could be used to validate the estimation methods used in global models (Etiope et al, 2019).…”

High-resolution spatiotemporal patterns of China’s FFCO2 emissions under the impact of LUCC from 2000 to 2015

“…In contrast, the measurements underlying topdown approaches, such as aircraft and satellites, have been used to determine emissions based on the amounts and rates of observed trace gases (Cohen and Wang 2014, Dlugokencky and Tans 2020, Conley et al 2017. This application of top-down measurements has led to an accuracy of global CO 2 concentrations estimated from space-borne observations better than 1% (less than 4 ppmv) and could decrease the uncertainty in regional estimates of CO 2 sources and sinks (Rayner and O'Brien 2001).…”

“…Liu et al (2017) combined surface measurements with satellite data into a regional highresolution atmospheric transport model (Kort et al 2012, Zumkehr et al 2017 for central and southern Europe, revealing the spatiotemporal patterns of the FFCO 2 signal and additional changes in the variability in atmospheric CO 2 associated with FFCO 2 emissions. A new airborne method (Wecht et al 2014, Mitchell et al 2015 has been applied to quantify localized surface emissions at spatial scales of~1000 m, with an error less than 10% accounting for smaller-scale turbulent dispersion (Conley et al 2017).…”