A new strategy for localized NO2 retrievals
Scientists at BIRA-IASB have developed a new strategy for the retrieval of NO2 near-surface concentrations and tropospheric vertical column densities (VCDs) in different azimuthal viewing directions and applied it to one year (March 2018 – March 2019) of 2-D MAX-DOAS observations performed at the station of Uccle (Brussels Capital region, Belgium).
This 2-D instrument operation consists of two sub-modes:
- an elevation scan in a fixed main viewing azimuthal direction pointing to the northeast of Uccle, and
- an azimuthal scan in a fixed low elevation angle (2° above the horizon).
By applying a vertical profile inversion algorithm in the main azimuthal direction and a parameterization technique in the other azimuthal directions, NO2 near-surface concentrations and VCDs have been retrieved in 10 different azimuthal directions around the Space Pole in Uccle (Brussels).
Validation of the resulting near-surface NO2 concentrations in Brussels
In a first step, the performance of the new retrieval strategy has been verified using ancillary data. A comparison of the 2-D MAX-DOAS near-surface NO2 concentrations with the in-situ air-quality network stations in Brussels revealed that both instruments are highly correlated but the in-situ stations measure systematically higher NO2 concentrations than the MAX-DOAS. The different vertical and horizontal representativeness of both datasets can explain these findings.
Then the seasonal variability of the NO2 near-surface concentrations in Brussels was investigated. It is found that higher NO2 concentrations are observed during autumn and winter due to larger emissions and the longer lifetime of NO2 under lower temperatures.
Validation of TROPOMI tropospheric NO2 columns above Brussels
In the second part of the study, one year of Sentinel-5P/TROPOMI tropospheric NO2 columns measured above Brussels were validated using the 2-D MAX-DOAS tropospheric NO2 VCDs. The use of MAX-DOAS tropospheric NO2 VCDs in more than one azimuthal direction improves the spatial colocation with S-5P/TROPOMI observations, and consequently the validation results.
However, despite this improvement, S-5P/TROPOMI is still found to systematically underestimate the MAX-DOAS tropospheric NO2 columns. The capability of the MAX-DOAS instrument to provide information about the vertical NO2 profiles in addition to the better horizontal distribution characterization revealed that this underestimation by the satellite is due to the use of an inadequate NO2 vertical profile shape in the satellite retrieval over urban areas like Brussels.
Dimitropoulou, E., Hendrick, F., Pinardi, G., Friedrich, M.M., Merlaud, A., Tack, F., De Longueville, H., Fayt, C., Hermans, C., Laffineur, Q., Fierens, F., and Van Roozendael, M. (2020). Validation of TROPOMI tropospheric NO₂ columns using dual-scan multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements in Uccle, Brussels. Atmospheric Measurement Techniques, 13(10), 5165-5191. https://doi.org/10.5194/amt-13-5165-2020