Horizontal distribution of tropospheric NO₂ and aerosols derived by dual-scan multi-wavelength multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements in Uccle, Belgium

Abstract. Dual-scan ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements of tropospheric nitrogen dioxide (NO2) and aerosols were carried out in Uccle (50.8∘ N, 4.35∘ E; Brussels region, Belgium) for 2 years from March 2018 to February 2020. The MAX-DOAS instrument operated in both UV and visible wavelength ranges in a dual-scan configuration consisting of two submodes: (1) an elevation scan in a fixed viewing azimuthal direction and (2) an azimuthal scan in a fixed low elevation angle (2∘). By analyzing the O4 and NO2 differential slant column density (dSCD) at six different wavelength intervals along every azimuthal direction and by applying a new optimal-estimation-based inversion approach (the so-called mapping MAX-DOAS technique), the horizontal distribution of the NO2 near-surface concentrations and vertical column densities (VCDs) as well as the aerosol near-surface extinction coefficients are retrieved along 10 azimuthal directions. The retrieved horizontal NO2 concentration profiles allow the identification of the main NO2 hotspots in the Brussels area. Correlative comparisons of the retrieved horizontal NO2 distribution were conducted with airborne, mobile, air quality model, and satellite datasets, and overall good agreement is found. The comparison with TROPOMI observations from operational and scientific data products reveals that the characterization of the horizontal distribution of tropospheric NO2 VCDs by ground-based measurements and an adequate a priori NO2 profile shape in TROPOMI retrievals lead to better consistency between satellite and ground-based datasets.