Validation of Boreas: an instrument for simultaneous measurement of amount fraction and stable isotope ratios in methane

Methane (CH₄) is the second most important anthropogenic greenhouse gas in terms of its impact on climate to date and its current rates of emissions. It has a wide variety of emission sources including agriculture, landfills and fossil fuel combustion alongside a short atmospheric lifetime, making it a justifiable short-term target of emissions mitigation strategies. To aid the global drive on reducing emissions, it is integral to quantify emissions estimates for the validation of CH₄ inventories. Each source of CH₄ is identified by distinct isotopic ratios which proves a useful appliance in the sectoral division of amount fraction measurements.

We have developed Boreas, an automated preconcentrator sample preparation system coupled to an infrared laser spectrometer, capable of making hourly measurements of δ¹³C(CH₄) and δ²H(CH₄) of ambient air samples. Deployed at National Physical Laboratory’s atmospheric monitoring site in May 2021, Boreas makes continuous, high-frequency hourly measurements capable of capturing pollution events.

As concentration calibration and linearity has been verified using synthetic mixtures, we present a method of validation of Boreas measurements, comparing them to simultaneous one-minute-average measurements made by cavity-ring down spectroscopy. We use Keeling and Miller-Tans analysis to examine the source signature δ¹³C(CH₄) and δ²H(CH₄) of local pollution events, distinguishing agricultural sources from northern hemisphere background. We will show that the calibrated concentration reported by Boreas agrees with that of a Picarro G2401 making simultaneous measurements over a wide range of ambient background and polluted air. Moreover, we will show that the preconcentration step has no concentration dependence e.g., breakthrough or carry over.