| Aircraft and ground-based sampling of atmospheric greenhouse gas composition over the
British Isles was conducted between 2014 and 2016 as part of the Greenhouse gAs UK and
Global Emissions (GAUGE) project. We report a case study focussing on two research
aircraft flights conducted on 12 May 2015 to sample inflow and outflow across
the British Isles. We have employed the NAME Lagrangian dispersion model to
simulate CH4 mole fraction enhancements corresponding to aircraft and ground-based
sample times and locations, using CH4 surface fluxes derived from a composite
flux inventory, which included both anthropogenic and natural sources. For each
sampling location, variations in the baseline CH4 mole fraction were derived using the
MOZART global chemical transport model, and added to the NAME enhancements to
produce a dataset of modelled CH4 mole fractions which can be compared to the
measurements.
Using a multiple variable regression technique, we derive CH4 fluxes for the British Isles
region from both aircraft and ground-based datasets. We discuss the applicability of our
approach for both datasets, and conclude that in this case the assumptions inherent in our
method are much better satisfied for the aircraft data than for the ground-based data. Using
the aircraft data we derive a possible range of scale factors for the prior inventory flux of
0.53 – 0.97, with a central estimate of 0.82 based on our assessment of the most
likely apportionment of model uncertainty. This leads to a posterior estimate of the
British Isles CH4 flux of 67 kg s−1 – 121 kg s−1, with a central value of 103 kg s−1. |