Abstract
Abstract. The Tropospheric Monitoring Instrument (TROPOMI), aboard the Sentinel-5 Precursor (S5P) satellite, launched on 13 October 2017, provides measurements of atmospheric trace gases and of cloud and aerosol properties at an unprecedented spatial resolution of approximately 7×3.5 km2 (approx. 5.5×3.5 km2 as of 6 August 2019), achieving near-global coverage in 1 d. The retrieval of nitrogen dioxide (NO2) concentrations is a three-step procedure: slant column density (SCD) retrieval, separation of the SCD in its stratospheric and tropospheric components, and conversion of these into vertical column densities. This study focusses on the TROPOMI NO2 SCD retrieval: the retrieval method used, the stability of the SCDs and the SCD uncertainties, and a comparison with the Ozone Monitoring Instrument (OMI) NO2 SCDs. The statistical uncertainty, based on the spatial variability of the SCDs over a remote Pacific Ocean sector, is 8.63 µmol m−2 for all pixels (9.45 µmol m−2 for clear-sky pixels), which is very stable over time and some 30 % less than the long-term average over OMI–QA4ECV data (since the pixel size reduction TROPOMI uncertainties are ∼8 % larger). The SCD uncertainty reported by the differential optical absorption spectroscopy (DOAS) fit is about 10 % larger than the statistical uncertainty, while for OMI–QA4ECV the DOAS uncertainty is some 20 % larger than its statistical uncertainty. Comparison of the SCDs themselves over the Pacific Ocean, averaged over 1 month, shows that TROPOMI is about 5 % higher than OMI–QA4ECV, which seems to be due mainly to the use of the so-called intensity offset correction in OMI–QA4ECV but not in TROPOMI: turning that correction off means about 5 % higher SCDs. The row-to-row variation in the SCDs of TROPOMI, the “stripe amplitude”, is 2.15 µmol m−2, while for OMI–QA4ECV it is a factor of ∼2 (∼5) larger in 2005 (2018); still, a so-called stripe correction of this non-physical across-track variation is useful for TROPOMI data. In short, TROPOMI shows a superior performance compared with OMI–QA4ECV and operates as anticipated from instrument specifications. The TROPOMI data used in this study cover 30 April 2018 up to 31 January 2020.
Highlights
Nitrogen dioxide (NO2) and nitrogen oxide (NO) – together usually referred to as nitrogen oxides (NOx) – enter the atmosphere due to anthropogenic and natural processes.Over remote regions NO2 is primarily located in the stratosphere, with concentrations in the range of 33–116 μmol m−2 (2 − 7 × 1015 molec. cm−2) between the tropics and high latitudes
Processing the Tropospheric Monitoring Instrument (TROPOMI) orbit for which the wavelength shifts are shown in Fig. 2a with QDOAS leads to almost identical wavelength shifts: the irradiance and TL average radiance shifts differ by 0.25 ± 0.10 × 10−3 nm and 0.65 ± 0.08 × 10−3 nm, respectively
The TROPOMI NO2 slant column density (SCD) retrieval describes the modelled reflectance in terms of a non-linear function of the relevant reference spectra and uses optimal estimation to minimise the difference between the measured and modelled reflectance. The results of this retrieval method compare very well with SCD retrievals performed with the QDOAS software (Danckaert et al, 2017) when using settings as close as possible to those of the TROPOMI processor
Summary
Nitrogen dioxide (NO2) and nitrogen oxide (NO) – together usually referred to as nitrogen oxides (NOx) – enter the atmosphere due to anthropogenic and natural processes. The TROPOMI NO2 retrieval (van Geffen et al, 2019; Eskes et al, 2020) uses the three-step approach introduced for the Ozone Monitoring Instrument (OMI) NO2 retrieval (the DOMINO approach; Boersma et al, 2007, 2011). NO2 vertical profile information from a chemistry transport model and data assimilation (CTM/DA) system that assimilates the satellite observations is used to separate the stratospheric and tropospheric components of the total SCD. These SCD components are converted to NO2 vertical stratospheric and tropospheric column densities using appropriate air-mass factors (AMFs). For convenience of the reader this paper uses the SI units and in most instances provides numbers in the more commonly used unit of molec. cm−2; the conversion factor between the two is 6.02214 × 1019 mol−1
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