Figure 1: Example of a footprint computed from an aircraft flight. The plane trajectory is shown as a blue line on the bottom right of the figure. Background grey values represent methane wetland emissions as estimated by a bottom-up model called JSBACH-HIMMELI and the coloured dots represent the footprint value computed using the Lagrangian particle dispersion model FLEXPART. Higher footprint values indicate a stronger influence of the surface at that location.

The MAGIC2021 campaign took place in August 2021 and aimed at constraining methane emissions estimates provided by bottom-up models to improve our understanding of the methane cycle in the region. Methane emissions at high latitudes are poorly quantified and the region is particularly sensitive to climate change which could lead to feedback loops that are still not well understood.

During the campaign, airborne measurements of methane mole fraction were performed in the atmospheric boundary layer, the part of the atmosphere closest to the Earth’s surface. Flights over wetland areas allowed to isolate contributions from local wetland to the methane mole fraction values measured.

Félix langot, doctoral student at LMD, used an atmospheric modelling technique called Lagrangian particle dispersion modelling to reconstruct the paths that our sampled air parcels had taken to reach measurement locations. Knowing this allowed in turn to compute where and for how long these air parcels were close enough to the ground to be influenced by local emissions. This is known as the footprint of our measurements.

Combining footprints with measured mole fraction values allowed to estimate methane fluxes over a local area. These results could then be compared to bottom-up estimates of methane emissions from wetlands in the region and assess their performance.