Natural emissions of VOC and NOx over Africa constrained by TROPOMI HCHO and NO₂ data using the MAGRITTEv1.1 model

Natural emissions (vegetation, soil, lightning) are the dominant sources of non-methane biogenic volatile organic compounds (BVOCs) and nitrogen oxides (NOx ≡ NO + NO2) released into the atmosphere over Africa. BVOCs and NOx interact with each other and strongly impact their own chemical lifetimes and degradation pathways, in particular through their influence on hydroxyl radical levels. To account for this intricate interplay between NOx and VOCs, we design and apply a novel inversion setup aiming at the simultaneous optimisation of monthly VOC and NOx emissions in 2019 in a regional chemistry-transport model, based on TROPOMI HCHO and NO2 satellite observations. The TROPOMI-based inversions suggest substantial underestimations of natural NOx and VOC emissions used as a priori in the model. The annual flux over Africa is increased from 125 to 165 Tg yr−1 for isoprene, and from 1.9 to 2.4 TgN yr−1 and from 0.5 to 2.0 TgN yr−1 for the soil and lightning NO emissions, respectively. Despite the NOx emission increase, evaluation against in situ NO2 measurements at seven rural sites in Western Africa displays significant model underestimations after optimisation. The large increases in lightning emissions are supported by comparisons with TROPOMI cloud-sliced upper-tropospheric NO2 volume mixing ratios, which even remain underestimated by the model after optimisation. Our study strongly supports the application of a bias correction to the TROPOMI HCHO data and the use of a double-species constraint (vs single-species inversion), based on comparisons with isoprene columns retrieved from the Cross-track Infrared Sensor (CrIS).