Deriving tropospheric NO2 layer heights from TROPOMI observations
Abstract
Satellite observations of tropospheric NO2 columns are being used in many applications from air quality studies to emission estimates. While the NO2 absorption signal is relatively strong and easy to extract, quantification of the light path length and thus the sensitivity of the measurements is less accurate. As the vertical sensitivity of the measurements varies strongly in the lower atmosphere, a priori assumptions are needed on the NO2 vertical distribution to convert the observed slant path absorptions to vertical column amounts. These a priori profiles are usually taken from atmospheric model runs.
The TROPOMI instrument, launched in October 2017 on the European Sentinel 5 precursor satellite, provides measurements at spatial resolution of up to 3.5 x 5.5 km2. At these scales, it is challenging to provide appropriate a priori NO2 profiles as regional models having comparable resolution are difficult to run on a global scale. As a result, often low resolution a rpiori data is used which can become a limitation for the accuracy of tropospheric NO2 products from high-resolution satellite instruments. In this study, we evaluate the potential to use the wavelength dependence of Rayleigh scattering for extracting information on the vertical distribution of the observed NO2 directly from the satellite spectra. Sensitivity studies on synthetic spectra indicate that a simple semi-empirical approach can be used to determine an effective NO2 layer height parameter, which can be used to select an appropriate a priori profile for the air mass factor calculation without the need for model data. Application of the method to TROPOMI measurements demonstrates that qualitatively, situations with NO2 in the boundary layer can be identified and distinguished from measurements where NO2 is found in elevated layers. This proves that the method works in principle. A quantitative application is however limited to scenes with large NO2 signals as uncertainties in the determination of the layer height propagate to the derived air mass factors and therby to teh tropospheric columns.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMA027...04R
- Keywords:
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- 0305 Aerosols and particles;
- ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 0345 Pollution: urban and regional;
- ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 0365 Troposphere: composition and chemistry;
- ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 3360 Remote sensing;
- ATMOSPHERIC PROCESSES