Brewer Network Status

C.T. McElroy

Manchester COST Meeting 2

From the COST Agenda:State of the art:– Ozone, UV, AOD, Umkehr, NO2, SO2

Too much for one talkToo much for Me!

26 September 2013

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?)

26 September 2013

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??

26 September 2013

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

26 September 2013

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] Dobsons through 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?]

26 September 2013

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

26 September 2013

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

26 September 2013

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

26 September 2013

Manchester COST Meeting 10

Nitrogen Dioxide Measurements

26 September 2013

Photo: Canadian Aviation Museum

Moving forward:…

Manchester COST Meeting 11

Nitrogen Dioxide Measurements

26 September 2013

• 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 & Diemoz et 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

A New Calibration Procedure which Accounts for Non-linearity in Single-monochromator Brewer Ozone Spectrophotometers

Zahra Vaziri 1, Omid Moeini 1, Vladimir Savastiouk 2, David Barton 1, Tom McElroy 1

1 York University2 Full Spectrum Science Inc.

CMOS, CGU and CWRA 2013 CongressMay 30th

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

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Single and Double Ozone Measurement Comparison

• Single #009 • Double #119

Brewer Instrument Physical Model• Brewer 009 and 119 Slit Function:

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(Omid Moeini, 2013)

Single Brewer Physical Model#009

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Double Brewer Physical Model#119

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Methodology continued• 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.

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Linear model for the single Brewer

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 19

Results

Single Brewer• Data from the Single Brewer

number 009 at sunset• Filter Change• Nonlinear factor• Total column

ozone amount• F0

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ResultsLinear model for the double Brewer

• Data from the Double Brewer number 119 at sunset

ResultsNon-Linear model for the double Brewer

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• Finite slit width may be the contributing factor to the non-linearity factor.

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

23Still some work to do but we hope to publish preliminary conclusions in the near future.

Manchester COST Meeting 24

Funding for this work is provided by:

26 September 2013

The CSA/ABB/NSERC Industrial Research Chair inAtmospheric Remote sounding at York University

CSAThe Canadian Space Agency

ASCAgence spatiale canadienne

ABB Incorporated, Quebec City, Canada

Natural Sciences and EngineeringResearch Council of Canada

And travel to attend this meeting funded under the COST action

TimminsFirst test flight near midnightSeptember 12, 2013

Photos courtesy Paul Loewen, USAK and CSA. CSA

Methodology continued• Least square method is

applied for N observations:

• Langley process:– The solution is moved

a step Δvk away

– Δvk is estimated

– vk is replaced by vk+ Δvk

– Process is done in a loop until Δvk

approaches zero 26

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Model Comparison

Physical Model Non-Linear Model

Linear Model

Single Brewer 009

ɣ 4.61x10-7 1.14x10-7 --------

X 218.58 299.08 284.9

F0 2795.52 2431.43 2740.32

Double Brewer 119

ɣ 7.89x10-8 1.12x10-8 -------

X 273.95 274.29 275.68

F0 1673.22 1623.51 1624.39