Examining ground-based in situ monitoring capability to improve understanding of the atmospheric methane budget

Abstract

Methane (CH₄) is the second most abundant anthropogenic greenhouse gas. Amount fractions have been rising in recent years, but we still do not have a complete understanding of the driving forces behind these changes. It is of great importance to understand what has been driving CH₄ increases in recent years so we can focus efforts on emissions reduction and therefore mitigate the harmful impacts of climate change. Stable isotope ratios of CH₄ (specifically δ¹³C and δD) may be the key to providing this understanding, as atmospheric isotope ratios are sensitive to changes in CH₄ sources and sinks. To address the gaps in our understanding, this thesis aims to assess ground-based in situ monitoring capability, to understand how CH₄ fluxes have varied over a recent 17 year period; develop the protocols for examining measurements from a new type of in situ instrument to study methane isotopes; and assess where new in situ instruments could be deployed in the future to aid in budget quantification. First, variations in large-scale regional CH₄ fluxes and δ¹³C emissions signatures are examined, 2004-2020. The GEOS-Chem 3D Chemical Transport Model is used to simulate CH₄ and δ1¹³C. Ground-based in situ data from the NOAA global greenhouse gas monitoring database provides a constraint in an inversion to solve for regional fluxes and δ¹³C source signatures. Results show that there has been a latitudinal shift from midlatitudinal Northern Hemisphere emissions to tropical emissions. Coevally, δ¹³C emissions source signatures have become lighter, with the greatest lightening occurring around the tropics. As such, increasing emissions are attributed to tropical wetland emissions. Second, data analysis protocol for a new CH₄ measuring instrument ‘Boreas’ is outlined. Boreas is a preconcentration system, attached to a laser spectrometer, which has the capability to measure CH₄ isotopologues (¹²CH₄, ¹³CH₄ and CH₃D) continuously to a high precision. The data analysis consists of a calibration on each isotopologue amount fraction; a drift correction to account for noise; and an internal calibration adjustment to ensure Boreas measurements are comparable with measurements of external laboratories. Real-world data from when Boreas was located at the National Physical Laboratory (Teddington, UK) is shown, demonstrating the capability of Boreas to differentiate between source types. Third, the thesis considers where to deploy continuous monitoring instruments around the world, to best capture variations in CH₄ isotope ratios. To examine this, δ¹³C and δD are simulated using GEOS-Chem over the period 2016-2020, and compared with typical precision on continuous monitoring instruments. The analysis covers current network site locations and a potential new location for instrument deployment. The results show such instruments would effectively capture isotope ratio variations on both a daily and monthly scale, especially in areas with active sources present. This potential future deployment would be useful to constrain the drivers behind global CH₄ variations, to prepare future emission reduction scenarios.