Quantifying methane emissions by combining multi-instrument airborne observations with regional atmospheric transport modelling: application to Madrid waste sites

<jats:p>Abstract. Regulating the emission of methane (CH4) from anthropogenic sources plays an important role in global climate mitigation strategies. The waste sector is responsible for some of the strongest localized sources and therefore its control has the potential for high impact reduction measures. As shown in a related preceding publication, airborne lidar and imaging spectrometer data provide reliable measurements of CH4 columns and can be used to estimate CH4 fluxes from sources such as landfills. However, incomplete knowledge of the transport of greenhouse gases from the source to the measurement point is one of the main sources of uncertainty in flux determination. In this paper, we improve the accuracy over previous emission estimates by applying a new analysis method, based on high-resolution regional-scale weather simulations to describe the atmospheric transport. We use this model to perform a combined fit to the CH4 measurements from airborne lidar and passive remote sensing, as well as in situ measurements. The key improvement over previous estimates comes from explicitly accounting for the complex, time-varying wind field on the measurement day, which caused significant CH4 accumulation that biased earlier cross-sectional flux estimates. This investigation focuses on two waste facilities close to Madrid, Spain, which were overflown by the German research aircraft HALO during a flight of the CoMet 2.0 Arctic mission. The estimated emission rates for the two sites were determined to be 4.0 ± 1.2 t h-1 (Pinto landfill) and 4.7 ± 0.7 t h-1 (Valdemingómez waste site). </jats:p>