Articles | Volume 10, issue 9
Atmos. Meas. Tech., 10, 3345–3358, 2017
Atmos. Meas. Tech., 10, 3345–3358, 2017

Research article 13 Sep 2017

Research article | 13 Sep 2017

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

Stephen Conley1,6, Ian Faloona1, Shobhit Mehrotra1, Maxime Suard1, Donald H. Lenschow2, Colm Sweeney4, Scott Herndon3, Stefan Schwietzke4,5, Gabrielle Pétron4,5, Justin Pifer6, Eric A. Kort7, and Russell Schnell5 Stephen Conley et al.
  • 1Department of Land, Air, & Water Resources, University of California, Davis, CA 95616, USA
  • 2Mesoscale and Microscale Meteorology Laboratory, National Center for Atmospheric Research, Boulder, CO 80307, USA
  • 3Aerodyne Research, Inc, Billerica, MA 01821, USA
  • 4Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80305, USA
  • 5NOAA Earth System Research Laboratory, Boulder, CO, USA
  • 6Scientific Aviation, Inc., Boulder, CO, USA
  • 7Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI, USA

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 smaller-scale 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.

Short summary
This paper describes a new method of quantifying surface trace gas emissions (e.g. methane) from small aircraft (e.g. Mooney, Cessna) in about 30 min. This technique greatly enhances our ability to rapidly respond in the event of catastrophic failures such as Aliso Canyon and Deep Water Horizon.