Examining ground-based in situ monitoring capability to improve understanding of the atmospheric methane budget
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Date
23/01/2023Author
Drinkwater, Alice
Metadata
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.