Validation of TROPOMI tropospheric NO<sub>2</sub> columns using dual-scan MAX-DOAS measurements in Uccle, Brussels

Abstract

Abstract. Ground-based Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements of aerosols and tropospheric nitrogen dioxide (NO2) were carried out in Uccle (50.8° N, 4.35° E) Brussels, during one year from March 2018 until March 2019. The instrument was operated in both UV and visible (Vis) wavelength ranges in a dual-scan configuration consisting of two sub-modes: (1) an elevation scan in a fixed viewing azimuthal direction (the so-called main azimuthal direction) pointing to the Northeast and (2) an azimuthal scan in a fixed low elevation angle (2°). By applying a vertical profile inversion algorithm in the main azimuthal direction and a parameterization technique in the other azimuthal directions, near-surface NO2 concentrations (VMRs) and vertical column densities (VCDs) were retrieved in ten different azimuthal directions. The dual-scan MAX-DOAS dataset allows partly resolving the horizontal distribution of NO2 around the measurement site and studying its seasonal variations. Furthermore, we show that measuring the tropospheric NO2 VCDs in different azimuthal directions improves the spatial colocation with measurements from the Sentinel-5 Precursor (S5P), leading to a reduction of the spread in validation results. By using NO2 vertical profile information derived from the MAX-DOAS measurements, we also resolve a systematic underestimation in S5P NO2 data due to the use of inadequate a-priori NO2 profile shape data in the satellite retrieval.

Assessment of the TROPOMI tropospheric NO<sub>2</sub> product based on airborne APEX observations

“…reference observations at higher resolution and this is consistent with the findings in other studies Ialongo et al, 2019;Zhao et al, 2019;Dimitropoulou et al, 2020;Verhoelst et al, 2020).…”

“…However, highly polluted areas typically exhibit strong NO2 vertical and horizontal gradients (see e.g. Dieudonné et al, 2013;Ialongo et al, 2019;Zhao et al, 2019;Dimitropoulou https://doi.org/10.5194/amt-2020-148 Preprint. Discussion started: 29 June 2020 c Author(s) 2020.…”

“…The sharp gradients between pollution plumes and background areas cannot be resolved properly at the horizontal scale of the model (~100 km x 100 km). In Dimitropoulou et al (2020), TROPOMI tropospheric NO2 VCDs were recalculated based on high-resolution MAX-DOAS profiles, while in Ialongo et al 2019a priori NO2 profiles were extracted from the Copernicus atmospheric monitoring service (CAMS) regional CTM (Marécal et al, 2015; https://www.regional.atmosphere.copernicus.eu). These transformations generally led to increased NO2 VCDs, resulting in a better agreement with reference ground-based measurements.…”

Validation of tropospheric NO<sub>2</sub> column measurements of GOME-2A and OMI using MAX-DOAS and direct sun network observations

“…The provision of this information for all ground-based measurements would thus be very valuable to further improve the comparison method. Note that in urban areas, the representativeness of MAX-DOAS observations for comparison with satellite data could be improved by making use of measurements in different azimuth directions (Ortega et al, 2015;Gratsea et al, 2016;Schreier et al, 2019;Dimitropoulou et al, 2020).…”

“…For example, possibilities exist to use estimates of the horizontal extent of MAX-DOAS measurements to improve the colocation with satellite data. MAX-DOAS instruments can also be operated in 20 multiple azimuthal scan mode, which could be used to further refine the colocation with satellite pixels (Brinksma et al, 2008;Gratsea et al, 2016;Ortega et al, 2015;Schreier et al, 2019;Dimitropoulou et al, 2020). Finally, imaging MAX-DOAS systems such as the IMPACT instrument (Peters et al, 2019) which provides fast sampling of the full (360°) azimuthal range, may lead to significant improvements in tropospheric NO2 validation close to source regions.…”