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

2. September 12-16, 201113th Brewer WorkshopBeijing, China 2 Topics for Today Follow on from the cross-section talk Differential cross-sections Instrumental stray light Conclusions

3. September 12-16, 201113th Brewer WorkshopBeijing, China 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

4. September 12-16, 201113th Brewer WorkshopBeijing, China 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

5. September 12-16, 201113th Brewer WorkshopBeijing, China 5 Comparison of Brewers and Dobsons Dobson AD Brewer #007

6. September 12-16, 201113th Brewer WorkshopBeijing, China 6 Stray Light From SAUNA campaign data Work by Alexander Cede

7. September 12-16, 201113th Brewer WorkshopBeijing, China 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!

8. September 12-16, 201113th Brewer WorkshopBeijing, China 8 Non-linear Retrieval x = (F o - B m - F) / (A mu) x = X - k ( mu * X ) s equating… (A mu) [ X - k ( mu * X ) s ] - (F o - B m - F) = 0.0 X – ‘true’ ozoneF o – extraterrestrial k – nonlinearity constants – power law mu – ozone airmassm – Rayleigh airmass F – absorption functionx – Single Brewer O 3 A – differential absorption coefficient B – Rayleigh scattering coefficient } From dispersion test

9. September 12-16, 201113th Brewer WorkshopBeijing, China 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

10. September 12-16, 201113th Brewer WorkshopBeijing, China 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

11. The End Thank you….