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

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.