Brewer Ozone Spectrophotometer Internal Stray Light Issues Vladimir Savastiouk Tom McElroy Some...
Transcript of Brewer Ozone Spectrophotometer Internal Stray Light Issues Vladimir Savastiouk Tom McElroy Some...
Brewer Ozone SpectrophotometerInternal Stray Light Issues
Vladimir SavastioukTom McElroy
Some points for discussion
September 12-16, 2011 13th Brewer Workshop Beijing, China 2
Topics for Today
• Follow on from the cross-section talk
• Differential cross-sections
• Instrumental stray light
• Conclusions
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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
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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
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Comparison of Brewers and Dobsons
Dobson AD
Brewer #007
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Stray Light
From SAUNA campaign data
Work byAlexander Cede
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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!
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Non-linear Retrievalx = (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 – extraterrestrialk – nonlinearity constant s – power lawmu – ozone airmass m – Rayleigh airmass
F – absorption function x – Single Brewer O3
A – differential absorption coefficient B – Rayleigh scattering coefficient
} Fromdispersion test
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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
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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