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

September 12-16, 2011 13th Brewer Workshop Beijing, China 6

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

September 12-16, 2011 13th Brewer Workshop Beijing, 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

The End

Thank you….