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

Stephen Conley,Gabrielle Pétron,Scott Herndon,Stefan Schwietzke,Eric A Kort,Russell Schnell,Donald H Lenschow,Justin Pifer,Shobhit Mehrotra,Ian Faloona,Colm Sweeney,Maxime Suard

doi:10.5194/amt-10-3345-2017

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 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.

Highlights

Accurate national inventories of greenhouse gas emissions is of paramount importance in developing strategies to understand global emissions
We flew 2 days measuring an controlled ethane release provided by Aerodyne Research, Inc., 4 days measuring a controlled natural gas release provided by the Pacific Gas & Electric Company (PG & E), and six power plant flights where our estimates are compared with reported hourly power plant CO2 emissions
The main uncertainty arises from the effluent below the lowest flight altitude, but this is minimized by targeting a downwind distance determined by large eddy simulations (LES) studies to provide very little change in the plume flux divergence from the lowest loop to the ground

Summary

Introduction

Accurate national inventories of greenhouse gas emissions (primarily carbon dioxide – CO2; methane – CH4; and nitrous oxide – N2O) is of paramount importance in developing strategies to understand global emissions. Atmospheric scientists have long striven to use measurements from global surface networks, aircraft campaigns, and satellites to try to determine emissions based on the amounts and build-up rates of observed trace gases. Top-down methods suffer from difficulties attributing sources and generalizing measurements made over a relatively short time period. Attempts to reconcile these two distinct methods on global (Muhle et al, 2010) and continental scales (Gerbig et al, 2003; Miller et al, 2013) have often indicated an apparent underestimation by the bottom-up methods of a factor 1.5 or more

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Conclusion