Measurement of atmospheric methane and ethane at a suburban site using mid-IR absorption spectroscopy

Abstract

Methane (CH4) is the second most significant greenhouse gas after carbon dioxide. The concentration of methane in the atmosphere has been continuously increasing for several years and is currently at almost 2000 ppb. Methane emissions arise from a variable mix of thermogenic sources, such as oil, natural gas, and coal mining, or biogenic sources, such as wetlands and agriculture. Co-emitted gases such as ethane (C2H6) and other hydrocarbons can be utilised as a tracer to discern thermogenic (co-emission of ethane) and biogenic (no ethane is emitted) methane sources (Commane et al., 2023, https://doi.org/10.5194/amt-16-1431-2023). Ethane concentrations in the Northern hemisphere were decreasing between 1970 and 2010, which was attributed to better emission controls from oil and gas production, storage, and distribution, as well as exhaust emissions from cars and trucks. However, emissions are currently on the rise once again, linked to additional emissions from oil and gas production for example from the Eastern USA, (D Helmig et al., 2016, https://doi.org/10.1038/ngeo2721). In November 2023 we started CH4 / C2H6 measurements in ambient air from a rooftop air inlet (approx. 15 m above ground) at the Empa research campus in Dübendorf, Switzerland using a MIRA Ultra analyser (AERIS Technologies, USA). To correct for drift effects and to calibrate the analyser, two cylinders of compressed air were analysed in regular time intervals. To minimize interferences of water vapour, sample and calibration gases were dehumidified. Concentration measurements were compared to results of a CRDS analyser (model 2401, Picarro, USA) for CH4 and to a GC-MS system (model 5975C, Agilent Technologies, USA) for C2H6. Compressed air measurements demonstrate that the MIRA Ultra gas analyser meets the manufacturer's specifications of sensitivity for CH4 and C2H6 of < 1 ppb/s and < 500 ppt/s, respectively. We will present a comparison of C2H6/CH4 data from the MIRA ULTRA analyser and GC-MS / CRDS. We foresee to complement CH4/C2H6 measurements with CH4 isotope analysis (δ13C, δD-CH4) by TREX-QCLAS and relate temporal variations to differences in CH4 source contributions by means of FLEXPART simulations.