Brewer Ozone Spectrophotometer

1. Brewer Ozone Spectrophotometer Internal Stray Light Issues Vladimir Savastiouk Tom McElroy Some points for discussion

2. Topics for Today • Follow on from the cross-section talk • Differential cross-sections • Instrumental stray light • Conclusions 13th Brewer Workshop

3. 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’ • This is at odds with doing a linear 2-parameter fit for the calibration of single Brewers 13th Brewer Workshop

4. Instrument Stray Light • Both single Brewers and Dobsons have a stray light problem at large ozone paths • Some quantification of the problem has been done – both analytically and by observation • A correction for the effect is needed • The use of a 2-point calibration is NOT an appropriate response to the issue 13th Brewer Workshop

5. Comparison of Brewers and Dobsons Brewer #007 Dobson AD 13th Brewer Workshop

6. Stray Light Work by Alexander Cede From SAUNA campaign data 13th Brewer Workshop

7. Problems with a 2-parameter Fit • The two parameter fit has no physical basis • 2-parameter linear fit is a poor representation of the observed performance of the single Brewers (SAUNA) • Poor performance at low ozone values • Poor performance at large slant amounts • Inability to independently calibrate single instruments Not recommended! 13th Brewer Workshop

8. Non-linear Retrieval x = (Fo - B m - F) / (A mu) x = X - k ( mu * X )s equating… (A mu) [ X - k ( mu * X )s ] - (Fo - B m - F) = 0.0 X – ‘true’ ozone Fo – extraterrestrial k – nonlinearity constant s – power law mu – ozone airmass m – Rayleigh airmass F – absorption function x – Single Brewer O3 A – differential absorption coefficient B – Rayleigh scattering coefficient } From dispersion test 13th Brewer Workshop

9. Solutions • SAUNA suggested that: • s = 2.16 • k = 0.016 (with ozone in cm) • But… • can fix s and / or k or retrieve all 4 parameters • probably best to retrieve s from a comparison between singles and doubles (changing ozone) • A non-linear, least squares solution works 13th Brewer Workshop

10. Conclusion • At this point it is suggested that the s and k determined in the SAUNA campaign be used with single Brewer MkII and MkV • Then do a retrieval for the Fo using calculated cross-sections and a comparison to a calibrated instrument or direct Langley plot 13th Brewer Workshop

11. The End Thank you….