Linking airborne in-situ and column greenhouse gas measurements using an atmospheric transport model

An intensive flight campaign, CoMet (Carbon dioxide and Methane Mission), was conducted during May and June 2018, with the aim to better understand important sources of carbon dioxide (CO2) and methane (CH4) in Europe as well as to test and validate the newly developed Integrated Path Differential Absorption (IPDA) Lidar CHARM-F operated by DLR. On board the German research aircraft HALO, both in-situ and remote sensing measurements were taken simultaneously by the Jena Instrument for Greenhouse gas measurements (JIG) and CHARM-F. The in-situ measurement is highly precise and traceable to WMO standards, but challenge arises here, as these two data streams are not directly comparable. This study aims to link these two different measurement techniques by using the regional Lagrangian transport model (STILT), and to establish the traceability of the measurements from the CHARM-F lidar to WMO standards. The STILT model uses meteorological fields from the ECMWF IFS model to derive footprints from a measurement receptor point, defining the area whose fluxes influence the atmospheric mixing ratio signal. The model then calculates the mixing ratio enhancement of the trace gases CO2 and CH4 by multiplying the footprint with the anthropogenic emissions inventory from EDGAR v4.3 and biospheric CO2 fluxes from the diagnostic Vegetation Photosynthesis and Respiration Model (VPRM). The trace gas concentrations at the receptor then can be calculated by adding this enhancement to the concentration from background. In order to reduce the model uncertainties when bridging the two data streams, an optimized anthropogenic emission inventory was used and a bias correction is applied using the in-situ measurements. This corrected and characterized model output is then used to reproduce the CHARM-F observations, i.e. vertical partial column (VPC) CO2 and CH4 mixing ratios at the aircraft position along the flight track. By comparing the modeled VPC mixing ratios to the CHARM-F observations, the consistency of the CHARM-F measurement with the highly precise and accurate in situ measurements can be established, while taking into account the uncertainty of the model bridging the two data streams.