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Brewer Ozone Spectrophotometer

Brewer Ozone Spectrophotometer. Ozone Cross-section Issues. Vladimir Savastiouk Vitali Fioletov Tom McElroy. - Some points for discussion. Topics for Today. Ozone measurement methodology Differential cross-sections Instrumental stray light Progress on a Brewer relational database

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Brewer Ozone Spectrophotometer

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  1. Brewer Ozone Spectrophotometer Ozone Cross-section Issues Vladimir Savastiouk Vitali Fioletov Tom McElroy - Some points for discussion

  2. Topics for Today • Ozone measurement methodology • Differential cross-sections • Instrumental stray light • Progress on a Brewer relational database • Implications for the WOUDC • Conclusions 13th Brewer Workshop

  3. Brewer Mathematics • Brewer ozone measurements are based on a differential measurement method using 4 wavelengths Absorption Function (of intensities on slits 3 to 6): F = -log(I3) + 0.5log(I4) + 2.2log(I5) -1.7log(I6) (Suppresses absorptions linear with wavelength, makes the result independent of absolute intensity is insensitive to SO2 and minimizes changes due to small wavelength shifts) • High-resolution laboratory cross-sections are used and effective cross-sections determined at each wavelength using accurate dispersion information and measured slit functions (ILS) for each instrument 13th Brewer Workshop

  4. Different Cross-sections • Differential cross-sections: dF/dozone = differential cross-section were computed using different laboratory data at various temperatures…. 13th Brewer Workshop

  5. Sciamachy Ozone These cross-sections agree with the composite ones very well on this plot. but the composite file seems to fit the data a little better. John Burrows’ group is looking at this comparison. But no news…. 13th Brewer Workshop

  6. EffectiveCross-sections 13th Brewer Workshop

  7. Differential Cross-sections for 32 Brewers GOME 10% Brion Sciamachy Bass & Paur 13th Brewer Workshop Temperature [C]

  8. Impact • Satellites determine the ozone profile including the height-dependent effects of temperature on apparent absorption • Columns are summed up from these data • Ground-based UV instruments directly infer a total column ozone • If the differential cross-section has a zero temperature coefficient the results will be temperature independent 13th Brewer Workshop

  9. The Brewers USE B&P at -45C! GOME 10% Brion Sciamachy Bass & Paur The Brewer is believed to produce ozone values with a small temperature dependence between -55 and -35 C based on the Bass & Paur data 13th Brewer Workshop Temperature [C]

  10. The Brewer • Because the Brewer characterizes its own ILS, each Brewer will have its own, accurate differential cross-section • This will be a primary reference for the satellite system 13th Brewer Workshop

  11. Calibrating Single Brewers • It has been demonstrated that Brewers can be independently characterized and absolutely calibrated and be in excellent agreement • This makes the absolute reference robust since it does not rely on one instrument remaining ‘stable’ 13th Brewer Workshop

  12. Agreement? • Some satellite data agree well with the Brewer data over a range of latitudes • But the cross-sections are different – the Brewer uses Bass & Paur (-45 C) currently • Satellite data may have been inadvertently ‘tuned’ to agree in the quest for better data • Also, there are systematic errors in both single Brewer data and Dobson data (AD) at large slant column amounts (e.g.: high latitude, winter) because of instrumental stray light 13th Brewer Workshop

  13. Conclusion • At this time it is not clear what cross-sections should be chosen • Very high spectral resolution is an advantage to bring together satellites, ground based and Lidar measurements • It is difficult, experimentally, to collect absolute cross-section data that can be ratioed with the accuracy needed to define consistent differential cross-sections with the required accuracy (for the Brewer, the absolute cross-section at the shortest wavelength is about 5 times larger than the differential cross-section) 13th Brewer Workshop

  14. Data Handling • Changes to both Brewer and Dobson data are likely to be required • Brewer data should also be recalculated with instrument-specific weighting coefficients • The Dobson and Brewer data will likely be dependent to some degree on the local ozone profiles and temperature profiles 13th Brewer Workshop

  15. WOUDC • Database capable of multiple versions of the data required • Transparency with respect to correction algorithms • Track method used for the re-analysis of each station’s data • In order of priority: • Resubmit corrected data • Reprocess from raw data at the WOUDC • Correct data in the WOUDC 13th Brewer Workshop

  16. The End Thank you…. 13th Brewer Workshop

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