Restoring Top of Atmosphere Reflectance Spectra during Partial and Annular Solar Eclipses: a Proof of Concept with the Absorbing Aerosol Index Measured by the TROPOMI/S5P Instrument
Abstract
Solar eclipses reduce the top of atmosphere (TOA) reflectances as measured by Earth observation satellites. Air quality products that are derived from absolute TOA UV-VIS reflectances, such as the Absorbing Aerosol Index (AAI), are severely distorted in the shadow of the Moon. These distortions result in anomalies or, if discarded, in gaps in derived air quality maps and time series. The availability of air quality satellite data during solar eclipses, however, may be of particular interest to users studying solar eclipses and their effect on the Earth's atmosphere. The reduction of the measured TOA UV-VIS reflectances during partial and annular eclipses can readily be explained by the reduction of the solar irradiance. Given the measurement time and spatial coordinates of a ground pixel on the Earth's surface, we compute the obscuration fraction of the solar disk by the lunar disk, taking into account solar limb darkening. We restore the TOA reflectance spectra in the penumbra of the annular solar eclipses on 26 December 2019 and 21 June 2020, measured by the TROPOMI/S5p instrument, as if the solar irradiance was not blocked by the Moon. With these restored reflectance spectra, we compute a solar eclipse corrected version of the AAI. The corrected AAI shows a significant reduction of the Moon's shadow signature and is of the same order of magnitude as the AAI of the previous orbits without an eclipse. The glint, which commonly increases the AAI and was hidden in the Moon's shadow, is visible in the corrected product. We test the validity of our approach by comparing the restored TOA spectra to measured TOA spectra of the previous orbit without an eclipse. Our results show that measured TOA reflectance spectra, and the corresponding AAI, may benefit from a solar eclipse correction. Anomalies or gaps in AAI maps or time series, caused by the Moon's shadow, may be solved. Potentially, other air quality products derived from TOA reflectance spectra may be restored as well. After a solar eclipse correction, satellite air quality data may be used to study solar eclipses and their impact on the Earth's atmosphere.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMA003.0020T
- 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