Abstract
Abstract. The TROPOspheric Monitoring Instrument (TROPOMI) onboard the Copernicus Sentinel-5 Precursor (S-5P) platform will measure ultraviolet earthshine radiances at high spectral and improved spatial resolution (pixel size of 7 km × 3.5 km at nadir) compared to its predecessors OMI and GOME-2. This paper presents the sulfur dioxide (SO2) vertical column retrieval algorithm implemented in the S-5P operational processor UPAS (Universal Processor for UV/VIS Atmospheric Spectrometers) and comprehensively describes its various retrieval steps. The spectral fitting is performed using the differential optical absorption spectroscopy (DOAS) method including multiple fitting windows to cope with the large range of atmospheric SO2 columns encountered. It is followed by a slant column background correction scheme to reduce possible biases or across-track-dependent artifacts in the data. The SO2 vertical columns are obtained by applying air mass factors (AMFs) calculated for a set of representative a priori profiles and accounting for various parameters influencing the retrieval sensitivity to SO2. Finally, the algorithm includes an error analysis module which is fully described here. We also discuss verification results (as part of the algorithm development) and future validation needs of the TROPOMI SO2 algorithm.
Highlights
Sulfur dioxide enters the Earth’s atmosphere via both natural and anthropogenic processes
This paper presents the sulfur dioxide (SO2) vertical column retrieval algorithm implemented in the Sentinel-5 Precursor (S-5P) operational processor UPAS (Universal Processor for UV/VIS Atmospheric Spectrometers) and comprehensively describes its various retrieval steps
When applying the algorithm to Ozone Monitoring Instrument (OMI) and GOME-2 data, across-track/viewing-angle-dependent residuals of SO2 were found over clean areas and negative SO2 SCDs are found at high solar zenith angles (SZAs) which need to be corrected
Summary
The total uncertainty (accuracy and precision) on the SO2 columns produced by the algorithm presented in Sect. 2 is composed of many sources of error (see e.g., Lee et al, 2009). The total uncertainty (accuracy and precision) on the SO2 columns produced by the algorithm presented in Sect. Several of them are related to the instrument, such as uncertainties due to noise or knowledge of the slit function. These instrumental errors propagate into the uncertainty on the slant column. Model errors can affect the slant column results or the air mass factors. The total retrieval uncertainty on the SO2 vertical columns can be derived by error propagation, starting from Eq (1) and if one assumes uncorrelated retrieval steps (Boersma et al, 2004; De Smedt et al, 2008): σN2v =. Where σNs and σNbasck are the errors on the slant column NS and on the background correction NSback, respectively. (m is the weighting function, Eq 6), which is often used to characterize the sensitivity of the retrieved column to a change in the true profile
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