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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/amt-2019-420
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2019-420
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 05 Feb 2020

Submitted as: research article | 05 Feb 2020

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This preprint is currently under review for the journal AMT.

Recommendations for spectral fitting of SO2 from MAX-DOAS measurements

Zoë Y. W. Davis1 and Robert McLaren2 Zoë Y. W. Davis and Robert McLaren
  • 1Department of Earth and Space Science, York University, Toronto, M3J 1P3, Canada
  • 2Department of Chemistry, York University, Toronto, M3J 1P3, Canada

Abstract. Fitting SO2 dSCDs from MAX-DOAS measurements of scattered sunlight is challenging because actinic light intensity is low in wavelength regions where the SO2 absorption features are strongest. SO2 dSCDs were fit with different wavelength windows (λlow to λhigh) from ambient measurements with calibration cells of 2.2 × 1017 and 2.2 × 1016 molec cm−2 inserted in the light path at different viewing elevation angles. SO2 dSCDs were the least accurate and fit errors were the largest for fitting windows with λlow < 307 nm or λlow > 312 nm. The SO2 dSCDs also exhibited an inverse relationship with the SO2 absorption cross-section for fitting windows with λlow < 307 nm. Spectra measured at low viewing elevation angles (i.e., α = 2o) exhibited less accurate SO2 dSCDs for the same fitting windows compared to higher angles. The use of a 400 nm short-pass filter or a polynomial to account for stray light (the offset function), increased the accuracy of the SO2 dSCDs for many different fitting windows, decreased fit errors, and decreased the dSCDs’ dependence on the SO2 absorption features. The inaccuracies at lower fitting wavelengths were increased by stray light originating from light with λ > 400 nm. Deviation of the SO2 dSCD from the true value depended on the SO2 concentration for some fitting windows rather than exhibiting a consistent bias. Uncertainties of the SO2 dSCD reported by the fit algorithm were significantly less than the true error for many windows, particularly for the measurements without the filter or offset function. For retrievals with the filter or offset function, increasing λhigh > 320 nm tended to decrease the reported fit uncertainty but did not increase the accuracy. Based on the results of this study, a short-pass filter and a fitting window of 307.5 < λ < 319 nm are recommended. If a filter is not available or conflicts with other species to be determined (NO2, HCHO, etc.), the offset function should be enabled, and a fit window 307.5 < λ < 319 nm is still recommended.

Zoë Y. W. Davis and Robert McLaren

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Zoë Y. W. Davis and Robert McLaren

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Latest update: 17 Feb 2020
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Short summary
MAX-DOAS is a technique that can be used for measuring pollutant concentrations and vertical profiles in the atmosphere using remote sensing of sky-scattered light via telescope. Measuring SO2 is particularly challenging because of low light intensities in regions where SO2 absorbs solar radiation. Here we have performed experiments that document inaccuracies in these measurements as a function of spectral fitting windows. We provide recommendations for measuring SO2 with greater accuracy.
MAX-DOAS is a technique that can be used for measuring pollutant concentrations and vertical...
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