Preprints
https://doi.org/10.5194/egusphere-2023-1461
https://doi.org/10.5194/egusphere-2023-1461
06 Jul 2023
 | 06 Jul 2023

A physically-based correction for stray light in Brewer spectrophotometer data analysis 

Vladimir Savastiouk, Henri Diémoz, and C. Thomas McElroy

Abstract. Brewer ozone spectrophotometers have become an integral part of the global ground-based ozone monitoring network collecting data since the early 1980s. The double monochromator Brewer version (MkIII) was introduced in 1992. With the Brewer hardware being so robust, both single and double monochromator instruments are still in use. The main difference between the single Brewers and the double Brewers is the much lower stray light in the double instrument. Laser scans estimate the rejection level of the single Brewers to be 10-4.5 while the doubles improve this to 10-8, virtually eliminating the effects of stray light. For a typical single monochromator Brewer, stray light leads to an underestimation of ozone of approximately 1 % at 1000 DU ozone SCD and can exceed 5 % at 2000 DU, while underestimation of sulphur dioxide reaches 30 DU when no sulphur dioxide is present. This is because even a small additional stray light at shorter wavelengths significantly reduces the slant ozone at large values. An algorithm for stray light correction based on the physics of the instrument response to stray light which adds light from longer wavelengths to shorter ones has been developed. The simple assumption is that count rates measured at any wavelength have a contribution from stray light from longer, and thus brighter, wavelengths because of the ozone cross-section gradient leading to a rapid change in intensity as a function of wavelength. Using the longest measured wavelength (320 nm) as a proxy for the overall brightness provides an estimate of this contribution. The sole parameter, in the order of 0.2 to 0.6 %, that describes the percentage of light at the longest wavelength to be subtracted from all channels is determined by matching ozone calculations from the single and the double monochromator Brewers. Removing this additional count rate from the signal mathematically before deriving ozone corrects for the extra photons scattering within the instrument that produces the stray light effect. Analysing historical data from co-located single and double monochromator Brewers can provide an estimate of how the stray light contribution changes over time in an instrument. The corrected count rates of the measured wavelengths can also be used to improve other calculations: the sulphur dioxide column, the aerosol optical depth, the effective temperature of the ozone layer or any other products. Also presented, is an initial analysis of signs consistent with the stray light effect in the double monochromator Brewers. A multi-platform code to correct the count rates for stray light and saving the corrected values in a new B-file for use with any existing Brewer data analysis software is available to the global Brewer user community at https://zenodo.org/record/8097039 (Savastiouk and Diémoz, 2023).

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.
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Journal article(s) based on this preprint

20 Oct 2023
A physically based correction for stray light in Brewer spectrophotometer data analysis
Vladimir Savastiouk, Henri Diémoz, and C. Thomas McElroy
Atmos. Meas. Tech., 16, 4785–4806, https://doi.org/10.5194/amt-16-4785-2023,https://doi.org/10.5194/amt-16-4785-2023, 2023
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

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This paper describes a way to significantly improve ozone measurements at low sun elevations and...
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