brewer ozone spectrophotometer internal stray light issues vladimir savastiouk tom mcelroy some...
TRANSCRIPT
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
September 12-16, 2011 13th Brewer Workshop Beijing, 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
September 12-16, 2011 13th Brewer Workshop Beijing, 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
September 12-16, 2011 13th Brewer Workshop Beijing, China 5
Comparison of Brewers and Dobsons
Dobson AD
Brewer #007
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Stray Light
From SAUNA campaign data
Work byAlexander Cede
September 12-16, 2011 13th Brewer Workshop Beijing, 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!
September 12-16, 2011 13th Brewer Workshop Beijing, China 8
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
September 12-16, 2011 13th Brewer Workshop Beijing, 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
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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