Brewer Network Status

1. C.T. McElroy Brewer Network Status

2. From the COST Agenda: State of the art: – Ozone, UV, AOD, Umkehr, NO2, SO2 Too much for one talk Too much for Me! Manchester COST Meeting

3. Goal for Ozone Measurements • Accuracy and precision 1DU • Where are we now? ~3 DU (~1%) • What are the outstanding issues? • Ozone coefficients • Wavelength assignment accuracy • Stray light effects and corrections • Transfer of calibrations and tracking of accuracy • Frequency of calibrations (2-year program?) Manchester COST Meeting

4. Goal for UV Measurements • Maintain a 1% accuracy in absolute irradiance • Improve wavelength stability (as for Ozone) • Adequate local standards for maintaining performance between major calibration events • ‘Universal’ standards?? Manchester COST Meeting

5. Wavelength Calibrations • Wavelength jitter from Hg lamp • Variations in dispersion if internal lamp is used • Small differences between UV and Ozone port dispersion – (FOV effects; moving discharge) • Comparison spectra – ATLAS, Kelly Chance, satellites – have their own calibration problems • Errors in calibration = absolute errors in ozone coefficients • Still have not done a direct cross-section comparison in a laboratory setting Manchester COST Meeting

6. Stray Light • Comparisons during TOMS 3F and [at least] 3 Sodankyla campaigns illustrated the problem in both single Brewers and [double] Dobsonsthrough comparisons with double Brewers at large ozone slant paths • Tomi and Esko produced a QOS paper putting forward a correction scheme • Not the most desirable approach • Will outline a possible alternative formulation • Why is this important? Trends at high-latitude stations may be being distorted due to changes in the record due to singles being replaced by doubles [Next assessment?; Dobsons?] Manchester COST Meeting

7. Aerosol Measurements • The community has largely accepted the need for special calibration locations • IOS has been transferring calibrations • Not clear how rigorous system analysis there is • Combining observations from different model Brewers has not been exploited Manchester COST Meeting

8. Umkehr • Carl Mateer led the charge to go to the optimal assimilation method for Umkehr analysis • This is based on the Rodgers method • Until now, this has not made use of the ‘multiple observations strategy’ put forward in Rodgers’ papers • NOAA has now done this and is retrieving mean profiles directly for a period of time (e.g.: a month) with a significant reduction in the a priori content of the solution profile Manchester COST Meeting

9. Sulphur Dioxide • Brewer measurements – particularly the single Brewer measurements – fall off at large zenith angles • Originally this was thought to be because of discrepancies in the ozone cross-sections at different wavelengths • The significantly better results from the double instrument demonstrated that stray light is the real cause • SO2 measurements use the shortest slit. The effect of stray light is greater here because of the gradient in the ozone cross-section (possible contamination of ozone) • Little progress has been made on corrections for the SO2 measurements Manchester COST Meeting

10. Nitrogen Dioxide Measurements Photo: Canadian Aviation Museum Moving forward:… Manchester COST Meeting

11. Nitrogen Dioxide Measurements • Brewer-type multi-wavelength ratio measurements of NO2 have been made since the early 1970s (3 & 5 wavelengths) • The MkIV Brewer was designed to measure both O3 and NO2 • Filterwheel 3 added for grating order switching (1200 l/mm grating) • Comparisons with array spectrometers showed discrepancies • Barton & Diemozet al. investigated systematic issues affecting data • Exhaustive study by Diemoz, Savastiouk & Siani investigated theoretically the use of 3 different approaches to using Brewers • It is likely that the new step-scanning algorithm investigated will provide the best results and make MkIV data a useful contribution • Recommendation: • (as in the study) validate the conclusions of the study • Propagate the measurement algorithm • Validate with NDACC comparisons • Move filter wheel 3 into the foreoptics filter area Manchester COST Meeting

12. CMOS, CGU and CWRA 2013 Congress May 30th A New Calibration Procedure which Accounts for Non-linearity in Single-monochromatorBrewer Ozone Spectrophotometers Zahra Vaziri1, OmidMoeini1, Vladimir Savastiouk2, David Barton 1, Tom McElroy 1 1 York University2 Full Spectrum Science Inc.

13. Instrumental Stray Light (ISL) Double Brewer has an advantage over the single Brewer • Depends on characteristics of source and detector • Causes errors in photometry and measurement of absorptivity • Leads to underestimation of the Ozone column amount for larger airmass • In the Brewer instruments the main source of ISL is the grating surface

14. Single and Double Ozone Measurement Comparison • Single #009 • Double #119

15. Brewer Instrument Physical Model • Brewer 009 and 119 Slit Function: (OmidMoeini, 2013)

16. Single Brewer Physical Model #009

17. Double Brewer Physical Model #119

18. Methodology continued Linear model for the single Brewer • Model components to be determined: vk=(x, ɣ, bj, F0)k • Least Square Method is used: • Initial values for vk are determined. • Values for b, ɣ and x are guesstimated. • Initial value for F0 is estimated from the linear Langley plot.

19. Stray Light Correction Methodology Linear Model: F=log(I1/I2) F0=log(I10/I20) Non-Linear model: Where: Fm : Modeled Absorption Function α : Absorption Coefficient x : Ozone Column Amount µ : Ozone Airmass F0 : Absorption at Zero Airmass ɣ : Non-Linearity Factor bj : Filter Change Factor NDj: Filter Number j

20. Results Single Brewer • Data from the Single Brewer number 009 at sunset • Filter Change • Nonlinear factor • Total column ozone amount • F0

21. Results Linear model for the double Brewer • Data from the Double Brewer number 119 at sunset

22. Results • Finite slit width may be the • contributing factor to the • non-linearity • factor. Non-Linear model for the double Brewer

23. Conclusion & Future Work • The model proposes corrections to the observations. • Using corrections in reverse eliminate the effects of stray light • The Double Brewer suffers less from non-linearity than the Single Brewer. • The non-linearity factor for the Double Brewer is an order of magnitude less than the Single Brewer. • The Instrument model agrees very well with the observations made with both the double Brewer and the single Brewer  the algorithm is reliable and is reacting to the amount of non-linearity present. • Next step  Transfer calibration including the stray light correction from one instrument to another Still some work to do but we hope to publish preliminary conclusions in the near future.

24. Funding for this work is provided by: The CSA/ABB/NSERC Industrial Research Chair in Atmospheric Remote sounding at York University CSA The Canadian Space Agency ASC Agencespatialecanadienne ABB Incorporated, Quebec City, Canada Natural Sciences and Engineering Research Council of Canada And travel to attend this meeting funded under the COST action Manchester COST Meeting

25. Timmins First test flight near midnight September 12, 2013 Photos courtesy Paul Loewen, USAK and CSA. CSA

26. Methodology continued • Least square method is applied for N observations: • Langley process: • The solution is moved a step Δvkaway • Δvkis estimated • vk is replaced by vk+ Δvk • Process is done in a loop until Δvkapproaches zero

27. Model Comparison