Quantification of greenhouse gas fluxes by a new airborne laser absorption spectrometer

Kurzfassung

A better understanding of the sources and sinks of anthropogenic greenhouse gases is necessary for improving on long term climate projections. This requires sufficient high quality observations of these climate-relevant gases that are currently limited. Aircraft provide a flexible platform for sensing trace gases in the lower troposphere, where most emissions from anthropogenic sources enter the atmosphere. Airborne spectroscopic instruments allow for observation of trace gas amounts at high temporal resolution through fast instrument response times coupled with low uncertainties. The availability of on-board meteorological data acquisition systems further allows for reducing uncertainty on emission estimates through simultaneous observations of important atmospheric state variables like the local wind field. This study describes the development, setup and performance of a Quantum/Interband cascade laser based system developed for simultaneous airborne measurements of methane (CH4), ethane (C2H6), carbon dioxide (CO2), carbon monoxide (CO), nitrous oxide (N2O) and water vapor (H2O). It highlights the required refinements over the commercial system for use on two research aircraft, including a custom developed retrieval software (JFIT) and a frequent two-point calibration to reduce measurement uncertainties. The instrument has been thoroughly characterized in the laboratory and aboard a C-130 aircraft during the National Aeronautics and Space Administration’s (NASA) Atmospheric Carbon and Transport (ACT)-America field campaign in fall 2017, including an inter-instrumental comparison with a calibrated cavity ring-down instrument and flask samples. The validated instrument was deployed in summer 2018 aboard the Cessna 208B owned by the German Aerospace Center (DLR) to obtain a top-down estimate of CH4 emissions in the Upper Silesian Coal Basin (USCB), located in southern Poland. Fugitive CH4 emissions emanating predominantly from the hard coal mines around Katowice are estimated for two research flights using a traditional mass-balance approach and a model based approach exploiting model-generated local meteorology with assimilated Wind-Lidar soundings and the FLEXible PARTicle dispersion model (FLEXPART). The mass balance approach digesting data from several constant-altitude transects, downwind of the USCB area, revealed a net flux of Φ = 503 ± 104 kt CH4 yr^−1 for a morning flight on June 6th. An afternoon flight with deeper boundary layer yields a similar flux of Φ = 507 ± 105 kt CH4 yr^−1. Albeit mass balance derived emission estimates are higher (12%/13%) than reported in the European Pollutant Release and Transfer Register (EPRTR 2017) they are distinctly lower (30%/30%) than values reported in the Emission Database for Global Atmospheric Research (EDGAR v4.3.2) inventory. The model approach yields similar results of Φ = 412 ± 58 kt yr^−1 and Φ = 442 ± 62 kt yr^−1 that are closer to the E-PRTR 2017 inventory (8%/2%) and allows, in principle, for estimating emissions on a facility level from large area survey flights. The uncertainties associated with this inverse estimation however depend significantly on the quality of the meteorological input data.