Ground-based Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) observations of NO2 and H2CO at Kinshasa and comparisons with TROPOMI observations
[en] We present a database of MAX-DOAS (Multi-
AXis Differential Optical Absorption Spectroscopy) ground-
based observations of NO2 and H2CO tropospheric verti-
cal column densities (VCDtropo) performed for the first time
in the city of Kinshasa. These measurements were con-
ducted between November 2019 and July 2021 and pro-
cessed 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 col-
umn observations from the TROPOspheric Monitoring In-
strument (TROPOMI) over Kinshasa. In the validation, we
experiment with three different comparison cases of increas-
ing complexity. In the first case, a direct comparison between
MAX-DOAS observations (hourly average of MAX-DOAS
VCDtropo at overpass) and TROPOMI shows an underesti-
mation of TROPOMI with a median bias of −38 % for NO2
and −39 % for H2CO based on monthly comparison. The
second case takes into account the different vertical sensi-
tivities of the two instruments and the a priori profile. We
note significant changes in the median bias for both com-
pounds: −12 % for NO2 and +11 % for H2CO. The third
case builds on the second case by considering also the direc-
tion of sight of the MAX-DOAS. For this third case, we find
a median bias of +44 % for NO2 and a median bias of +4 %
for H2CO. However this case is impacted by low sampling
and is considered less reliable. The findings from this study
underscore the significance of employing a realistic a priori
profile in TROPOMI column extraction, particularly within
heavily polluted urban zones like Kinshasa. The investiga-
tion also highlights the necessity for prudence when inte-
grating the MAX-DOAS line of sight due to the noise gener-
ated during subsampling and the limited horizontal sensitiv-
ity of MAX-DOAS observations. Importantly, the study fur-
ther reveals the pronounced pollution levels of NO2, H2CO
and aerosols in both the city of Kinshasa and its adjacent re-
gions, underscoring the imperative for consistent monitoring
and effective regulatory measures by local authorities.
Mahieu, Emmanuel ; Université de Liège - ULiège > Département d'astrophysique, géophysique et océanographie (AGO) > Groupe infra-rouge de physique atmosphérique et solaire (GIRPAS)
Ground-based Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) observations of NO2 and H2CO at Kinshasa and comparisons with TROPOMI observations
BELSPO - Belgian Federal Science Policy Office ARES - Académie de Recherche et d'Enseignement Supérieur
Funding text :
This research was partly funded by UR
SPHERES of the Université de Liège and by the Belgian Federal
Science Policy Office (BELSPO) through the EQUATOR (Emis-
sion QUantification of Atmospheric tracers in the Tropics using Ob-
seRvations from satellites, 2021–2025) project.
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