Abstract
Dual-scan ground-based Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements of tropospheric nitrogen dioxide (NO2) and aerosols have been carried out in Uccle (50.8° N, 4.35° E; Brussels region, Belgium) for two years, from March 2018 to February 2020. The MAX-DOAS instrument has been operating in both UV and Visible wavelength ranges in a dual-scan configuration consisting of two sub-modes: (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 dSCDs at six different wavelength intervals along every azimuthal direction and by applying a new Optimal-Estimation-based inversion approach, the horizontal distribution of the NO2 near-surface concentrations and vertical column densities (VCDs) and the aerosols near-surface extinction coefficient are retrieved along ten 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 have been conducted with airborne, mobile, and satellite datasets, and overall a good agreement is found. The comparison with TROPOMI observations reveals that the characterization of the horizontal distribution of tropospheric NO2 VCDs by ground-based measurements, the appropriate sampling of TROPOMI pixels, and an adequate a priori NO2 profile shape in TROPOMI retrievals lead to a better consistency between satellite and ground-based datasets.
Highlights
Aerosols and nitrogen dioxide (NO2) play a crucial role in the tropospheric chemistry
The MAX-DOAS instrument was operating in both UV and Visible wavelength ranges in a dual-scan configuration consisting of two sub-modes: (1) an elevation scan in a fixed viewing azimuthal direction and (2) an azimuthal scan in a fixed low elevation angle (2o)
The 20 comparison with TROPOspheric Monitoring Instrument (TROPOMI) observations reveals that the characterization of the horizontal distribution of tropospheric NO2 vertical column densities (VCDs) by ground-based measurements, the appropriate sampling of TROPOMI pixels, and an adequate a priori NO2 profile shape in TROPOMI retrievals lead to a better consistency between satellite and ground-based datasets
Summary
Aerosols and nitrogen dioxide (NO2) play a crucial role in the tropospheric chemistry. Multi-azimuthal MAX-DOAS measurements offer many possibilities on air-quality monitoring, such as a better characterization of the effective NO2 field around the station These ground-based datasets can be valuable for validating satellite missions with fine spatial resolution in regions where the NO2 horizontal distribution is heterogeneous, such as urban and sub-urban areas. 55 In this study, a new aerosol and NO2 horizontal distribution inversion approach based on two years (March 2018-February 2020) of dual-scan multi-wavelength MAX-DOAS measurements in Uccle (Brussels-Capital region, Belgium) is presented. In every azimuthal viewing direction, parameterized NO2 near-surface concentrations, NO2 tropospheric columns and aerosol extinctions measured at six different wavelengths are used as input in a new horizontal distribution inversion approach.
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