First MAX-DOAS Observations of Tropospheric NO2 and H2CO in Central Africa: Impact on Air Quality and Validation of the Tropomi Satellite Instrument

Abstract

The research presented in this thesis focuses on the issue of air pollution in Central Africa. The deterioration of air quality in this region has been prominently observed, as evidenced by various recent studies. Based on satellite observations, these recent studies have identified significant peaks in nitrogen dioxide, formaldehyde, and aerosol pollution. Initial interpretations point to biogenic emissions from the extensive forests in the region, subject to various challenges related to biomass burning, forest fires for agricultural land preparation, as well as charcoal production widely used as an energy source and for cooking by a large part of the local population. Validating such hypotheses requires ground-level observations to understand the situation on the ground and contribute to the validation of satellite data and model simulations. Unfortunately, Africa remains significantly undersampled compared to other parts of the world, with a near absence of ground measurement stations in Central Africa and a lack of regulation.

In this context, this thesis focuses on the operationalization and implementation of a remote sensing instrument to measure specific pollutants. We installed an instrument based on the the Differential Optical Absorption Spectroscopy (DOAS) technique in Kinshasa. After processing the recorded spectra from May 2017 to July 2021, a database of NO₂, H₂CO, and Aerosol optical depth (AOD), strong indicators of air pollution in the region, was established. Observations reveal significant pollution from these molecules in Kinshasa and its surroundings, with high tropospheric column values observed during the dry season. Daily cycles have also been identified, showing elevated values around noon. The database also facilitated the initial validation of the TROPOMI instrument aboard the S5P satellite. This validation demonstrated good agreement between TROPOMI and ground observations, using a realistic a priori profile for local conditions, with a median bias of around 10% for both compounds (NO₂ and H₂CO).

Another aspect of this thesis involved evaluating the performance of the GEOS-Chem model, a chemistry-transport model simulating pollution in the study area. This evaluation is crucial to determine the extent to which the model can accurately replicate real atmospheric conditions in the Kinshasa region and its surroundings. The preliminary results indicate that simulations including biomass burning emissions inventory align remarkably well with TROPOMI observations, while those without accounting for biomass burning exhibit a non-cyclical seasonal behavior. The impact of biomass burning is particularly pronounced, especially during the dry season for NO₂ pollution and throughout the year for H₂CO.

The overall results of this thesis confirm significant pollution from NO₂ and H₂CO in Kinshasa and its surroundings. TROPOMI satellite products align well with ground observations, taking into account the a priori measured by the ground instrument. The impact of biomass burning is strongly significant in the observed pollution. Recommen dations are made to deepen studies by installing more ground instruments in different cities in the sub-region and to use these results to raise awareness and encourage local authorities to implement control and regulation measures.