Ground-based MAX-DOAS observations of NO2 and H2CO at Kinshasa and comparisons with TROPOMI observations

Abstract. We present a database of MAX-DOAS (Multi-AXis Differential Optical Absorption Spectroscopy) ground-based observations of NO₂ and H₂CO performed for the first time in the city of Kinshasa. These measurements were conducted between November 2019 and July 2021 and processed using the standardized inversion tools developed in the ESA FRM4DOAS (Fiducial Reference Measurements for Ground-Based DOAS Air-Quality Observations) project. The retrieved geophysical quantities are used to validate column observations from the TROPOspheric Monitoring Instrument (TROPOMI) in Kinshasa. In the validation, we experiment three different comparison cases of increasing complexity. In the first case, a direct comparison between MAX-DOAS observations (average +/- 60 minutes around overpass) and TROPOMI shows an underestimation of TROPOMI with a median bias of -40 % (s=0.26 and R=0.41) for NO₂ and -26 % (s=0.24 and R=0.28) for H₂CO. The second case takes into account the different vertical sensitivities of the two instruments and the apriori profile. We note a slight decrease of the biases and a strong improvement of the linear regression parameter, about -35 % (s=0.72 and R=0.74) for NO₂ and 1 % (s=1.01 and R=0.66) for H₂CO. The third case, which is considered more realistic than the first two, builds on the second case by considering also the direction of sight of the MAX-DOAS. For this third case, we find a bias of -2 % (s= 1.09; R= 0.59) for NO₂ and 13 % (s= 1.51; R= 0.60) for H₂CO. Those results indicate a large impact of the vertical sensitivity and horizontal heterogeneity in this validation process at this site. In order to evaluate the capability of the GEOS-Chem model in this region, we performed the comparisons between TROPOMI and the simulations made for 2020. We found a bias of 16 % (s= 0.42 and R = 0.80) for NO₂ and bais of 61 % (s= 0.05 and R = 0.24) for H₂CO.