Continuous methane isotope ratio δ13C(CH4) δ2H(CH4) at UK tall tower sites

Methane (CH₄) is an attractive target for emissions reduction due to its short atmospheric lifetime and emission from distinct source sectors. The primary UK emissions are from the fossil fuel, agricultural and waste landfill sectors that have different δ¹³C(CH₄) and δ²H(CH₄) isotopic signatures. Top-down estimates of total emissions are already made using continuous measurements of CH₄ amount fraction using a tall tower network and inversion modelling but there are limited observations for isotope ratio. Continuous measurements of the isotope ratio in atmospheric CH₄ provide an additional observable to disaggregate the relative emissions by source sector.

NPL have developed Boreas, an automated cryogenic preconcentrator coupled to an optical isotope ratio spectrometer (OIRS). Boreas purifies CH₄ from a ~5 L ambient air, removing air matrix gases and delivers a sample of CH₄ in N₂ at ~550 ppm to the spectrometer, improving measurement precision. The OIRS is calibrated using mixtures prepared gravimetrically from a single high-purity CH₄ parent that has been characterised for δ¹³C and δ²H by mass spectrometry, and the measurements are referenced to a whole air working standard that is sampled in sequence with the air.

Boreas has been deployed to an atmospheric monitoring station and makes simultaneous measurements of δ¹³C(CH₄) and δ²H(CH₄) at hourly intervals, with a repeatability of 0.07‰ for δ¹³C(CH₄) and 0.9‰ for δ²H(CH₄). We will show results from four years of continuous measurements of δ¹³C(CH₄) and δ²H(CH₄) at a GAW regional station in the Southeast of England, and a new deployment in Scotland.