Analysis of Stray Light in a Brewer · PDF fileAnalysis of Stray Light in a BrewerAnalysis of...

Analysis of Stray Light in a BrewerAnalysis of Stray Light in a Brewer SpectrophotometerSpectrophotometer

C. A. McLinden, D. I. Wardle,C. T. McElroy, &V. Savastiouk

Environment Canada

Brewers are not Perfect!

31 May –

3 June 2005 Brewer Workshop Beijing, China 2

Stolen Stuff:•

David Wardle –

many slides

•

Tom Grajnar

and Mike Brohart

–

data•

Volodya

Savastiouk

–

many calculations…

•

Jim Kerr –

much code•

Chris McLinden

-

models

31 May –


Brewer Map

31 May –


MSC Toronto

31 May –


Mauna Loa Observatory

31 May –


Eureka Weather Station

31 May –


Global column ozone:

Develop confidence in prediction of the future (ozone);models are tuned to reproduce currently measured amounts,also to reproduce measured values during past 20 years.

Target accuracy 1.0% 2-sigma. (or maybe 2 or 3 DU)

Reasonable to have 20-50? instruments deployed over the world using the measurements with those from space instruments.

Over the last 20 years we have almost achieved1.0% RMS for daily average measurementswith our best 3 instruments in Toronto.

rationale

31 May –


Dispersion and spectral purity…

Given that we are to measure ozone fromits UV spectrum, we need to know:

(a)

accurately the wavelengths of the measurement; in factif the wavelength uncertainty is less than 0.01nm it is ok. if >0.02nm, it is a problem. Thus we need δλ/λ

~

1/30,000.

(b)

that the much stronger radiation at longer wavelengthsis not interfering with the measurement.

31 May –


Spectrometer

•

The idea is to have monochromatic light at the exit slit

•

Diffraction grating is used to separate different wavelengths and send them at different angles

31 May –


Neutral density filters

Diffraction grating

Slit selector

Exit slits

Order filter

Detectorsun

UV-vis

turned by stepper motor

Diffraction grating: 1800 lines/mm1st

order: visible 570-650 nm

2nd

order: UV 285-325 nmAlso: 1200 and 3600 lines/mm

The Brewer spectrophotometer optics

31 May –


correctionlens

Exitslit #1

Entranceslit

100.6 mm

The Brewer spectrometer is amodified Ebert type using onemirror.

The instrument axis, defined asthe normal to the front plate(FP) passing through the centreof the grating surface alsopasses through the vertical plate(VP) which locates the mirror. The entrance slit and exit slit#1(not #3) are ~equidistant fromthe axis. The exit and exit directions ofthe central ray are parallel to theaxis.

The correction lens reduces thecoma and astigmatism of thebasic Ebert design.

F P

V P

Spectrometer layout

31 May –


Spectrometer Gratings and “Marks”Brewer gratings all have the same dispersion in the UVdetermined by n/d = 3600 lines per mm * order

Usually as follows:

----

order in -----Grating pitch Blazed for UV BLUE RED

Mark II S

1800

620 nm

2 Mark V S

1800

620 nm

2

1

Mark IV S

1200 1000 nm 3

2

Mark III D

3600

330 nm

1Mark VI S

3600

330 nm

1

measuring:

ozone NO2

ozone atlow sun

Mark III is a double spectrometer (D); roughly the same transmission characteristics as the singles (S), except for “stray light”.Another variation is that earlier Brewers have a much smaller Slit#0 .

31 May –


Some dogma………………For a single wavelength input to a linear spectrometer we can write

Signal = intensity of input * f( ,

s

)

where is the wavelength of the input radiation& s is the wavelength setting.f(

, s

) is a response function (count rate. W-1. m2)note: the dispersion function is s

= G( steps )We often do a line scan in which we

use a constant input

, and vary the setting s

.What is more relevant is changing the input given a constant setting.We’ve looked principally at two types of line scan file, c. 9000 from spectral lamps,c. 200 from HeCd Lasers.

31 May –


Dogma continued………………For a spectrum of input radiation:

Signal(s

) = P() * f(

,

s

) * d

Where P() is the spectral irradiance (watts m-2

nm-1)

Most spectrometer users assume the above can be simplified to:Signal(s

) = P()*R(s

)*q(

-

s

)*d

R may be called the responsivity and q the “slit function,”and q()

normalized to 1. i.e.: q(

) d

== 1.0…..Not entirely correct.

31 May –


Slit #0 “slit function”

-0.6 -0.4 -0.2 0 0.2 0.4 0.6

Slit 0W 297a 34.55

47763666

fwhi=636jw=557

31 May –


2800 2900 3000 3100 3200 3300 3400 3500 3600 3700-8

-7

-6

-5

-4

-3

-2

-1

0

wavelength setting (nm*10)

log1

0(no

rmal

ized

sig

nal)

all available Brewer laser scans

007MKIVf009MKIV012MKII014MKII015MKII017MKII021MKIII029MKV033MKII037MKII042MKV053MKII055MKII069MKV071MKIV079MKIV082MKIVe084MKIV085MKIII107MKIII109MKIVe111MKIII113MKII128MKIII507mkivf521mkiii585mkiii645mkiii

Note the log scale!

How do Brewers Compare??

31 May –

3 June 2005 Brewer Workshop Beijing, China 172800 2900 3000 3100 3200 3300 3400 3500 3600 3700-8

-7

-6

-5

-4

-3

-2

-1

0


log1

0(no

rmal

ized

sig

nal)

class I SB's (signal @300nm<1e-4.2)

007MKIVf012MKII014MKII015MKII017MKII029MKV033MKII037MKII053MKII055MKII069MKV082MKIVe109MKIVe113MKII

Class I Single Brewers

31 May –


2800 2900 3000 3100 3200 3300 3400 3500 3600 3700-8

-7

-6

-5

-4

-3

-2

-1

0


log1

0(no

rmal

ized

sig

nal)

325 on single Brewers

007MKIVf009MKIV012MKII014MKII015MKII017MKII029MKV033MKII037MKII042MKV053MKII055MKII069MKV071MKIV079MKIV082MKIVe084MKIV109MKIVe113MKII

2800 2900 3000 3100 3200 3300 3400 3500 3600 3700-8

-7

-6

-5

-4

-3

-2

-1

0


log1

0(no

rmal

ized

sig

nal)

class II SB's (signal @300nm>1e-4.2)

009MKIV042MKV071MKIV079MKIV084MKIV

Class II Single Brewers

31 May –


-7

-6

-5

-4

-3

-2

-1

0


log1

0(no

rmal

ized

sig

nal)

class I SB's minus atypical #017

007MKIVf012MKII014MKII015MKII029MKV033MKII037MKII053MKII055MKII069MKV082MKIVe109MKIVe113MKII

Cass I SB -

#017

31 May –


2800 2900 3000 3100 3200 3300 3400 3500 3600 3700-8

-7

-6

-5

-4

-3

-2

-1

0


log1

0(no

rmal

ized

sig

nal)

class I's minus #017, #007 and #109

012MKII014MKII015MKII029MKV033MKII037MKII053MKII055MKII069MKV082MKIVe113MKII

Class I -

#s 007, 017, 109

31 May –


-7

-6

-5

-4

-3

-2

-1

0


log1

0(no

rmal

ized

sig

nal)

two extended-scan class I SB's, (007 and 109).

007MKIVf109MKIVe

Extended Scan Singles #s 007, 109

31 May –


-7

-6

-5

-4

-3

-2

-1

0


log1

0(no

rmal

ized

sig

nal)

two of the triad SB's, (014,015), and another (012).

012MKII014MKII015MKII

Three Single Brewers –

012, 014, 015

#014, #015Are Toronto TRIAD Instruments

31 May –


2800 2900 3000 3100 3200 3300 3400 3500 3600 3700-8

-7

-6

-5

-4

-3

-2

-1

0


log1

0(no

rmal

ized

sig

nal)

double Brewers on 325 nm

021MKIII085MKIII107MKIII111MKIII128MKIII

Note: #085 scan was done before the replacement of the ground quartz

Double Brewers

31 May –


2800 2900 3000 3100 3200 3300 3400 3500 3600 3700-8

-7

-6

-5

-4

-3

-2

-1

0


log1

0(no

rmal

ized

sig

nal)

double Brewers on 353 nm

521mkiii585mkiii645mkiii

Secondary peak at 325 nm is from impurity of the 353 nm laser

Double Brewers using 353nm

31 May –


325 & 352 nm laser scans ….

2800 2900 3000 3100 3200 3300 3400 3500 3600 3700-8

-7

-6

-5

-4

-3

-2

-1

0


log1

0(no

rmal

ized

sig

nal)

325 & 353nm lasers on same single Brewer

007MKIVf507mkivf

31 May –


325 & shifted 353 laser scans…

2800 2900 3000 3100 3200 3300 3400 3500 3600 3700-8

-7

-6

-5

-4

-3

-2

-1

0


log1

0(no

rmal

ized

sig

nal)

007 measured at 325nm.007 measured at 353nm,shifted to 325nm.

31 May –


325 & 353 nm comments………So yes the shape is the same, or is it?

Actually the centre is ~10% narrower as the optics dictate

has ~10% less energywings are ~20% lower

have ~20% less energy

The reasoning is that the ratio of good to bad should be constant regardless of slit width,assuming the aberration-and-diffraction-

determined width is

smaller than the geometrical (slit-size-

determined) width,which appears to be the case.

31 May –


The data

290 295 300 305 310 315 320 325 330 335 340-2

-1

0

1

2

3

4

5

6

7Multislit laser scan CZ09405.145, (5th segment)

0123452d4d

31 May –


How to do laser scans

•

Need to do at least at two neutral density filter wheel positions to capture both the peak and the “stray light”

•

It looks like we can use laser scans to find neutral density factors for the single instruments, but not for the doubles.

•

Need to characterize the ND filters separately for the doubles

31 May –


How to do laser scans

•

Important: there must be no changes in the optical configuration or laser position when doing multiple ND filters

31 May –


Conclusion

•

We are learning a great deal about the Brewers with laser scans

•

We need to help Tom and Mike with establishing rules on how to do laser scans in the field

•

We also need to propagate this information to the rest of the Brewer community

31 May –


Ozone in Toronto -

#s 085, 039

31 May –


Ozone at Sodankyla

31 May –


Airmass Dependence

x = ( Fo -

F ) / ( alpha * mu ) …with dFo the error in ETC…

x’

= ( Fo + dFo -

F ) / ( alpha * mu ) = x -

dFo / ( alpha * mu )

%x = x -

[ x + dFo / ( alpha * mu ) ] / x * 100

= -

dFo / ( alpha * mu ) / x * 100

let sx = mu * x slant column ozone

%x = %x( mu = 1 ) / mu

So at 2400 DU more we expect a 1%(mu = 0) to be 1% / 6 = 0.15% (mu=6)

…in the presence of an ETC error

This is NOT what was happening in Sodankyla!

31 May –


dO3 = ( -6%-0.8% ) /( 2400 - 600 ) = 3.8E-3 %/DU slant column

Stray Light Correction

31 May –


Courtesy A. Cede

31 May –


CPFM on WB-57F

31 May –


CPFM Stray Light

Function

660 680 700 720Pixel Number

-2.00

0.00

2.00

4.00

6.00-lo

g(-lo

g(N

orm

aliz

ed In

tens

ity))

Y = -0.00589632 * X + 2.37681Y = 0.00456359 * X + -4.94138

Y = -0.0558675 * X + 37.8466Y = 0.0675285 * X + -47.6797

Y = 1.43717 * X + -993.059 Y = -1.55749 * X + 1086.16

1. I(i) = I(i) / Imax2. f = -log(-log(I(i)))

HeCd 325 nmStray Light Function

Composition andPhotodissociativeFlux Measurement

31 May –


290 300 310 320 33010-5

10-4

10-3

10-2

10-1

100

Wavelength (nm)

Res

pons

e Fu

nctio

n (-)

#009, Mk IV#014, Mk II#015, Mk II#071, Mk IV

Single-Brewer stray light rejection is 10-5-10-4

as measured by scanning 325 nm HeCd laserline

Instrument function core, FWHM=0.55 nm

Stray light wing

Stray light shoulder

Laserscans

31 May –


Nature of Stray Light Effect6

F = Σ

al

log[ Il ]l=3

X = ( Fo

– F ) / ( Δα

μ

)

Stray light adds signal at each wavelength. largely from longer wavelengths (spectrum gradient)

Assume that the stray light is from the longest wavelength, and affects the shortest the most…

31 May –


Stray Light Simplified

F = a3

log[ I3

+ βI6 ] + Σ

al

log[ Il ]

F = a3

log[ I3

( 1+ βI6

/ I3 ) ] + Σ

al

log[ Il ]

F = Σ

al

log[ Il ] + a3

log[ 1+ βI6

/ I3 ]

= F’

+ a3

log[ 1+ βI6

/ I3 ] ~ F’

+ ζ

I6

/ I3

with ζ=a3

βNow X = ( Fo

-

F ) / ( Δα

* μ

)

Brewer ozone is based on slit positions l = 3 to l = 6Sum over l = 4 to 6

β

is stray light fraction

Where F’

is thetrue ozone ratio

l = 4 to l = 6

l = 3 to l = 6

31 May –


RearrangingX = ( Fo

-

F ) / ( Δα

* μ

) X = [ Fo

– ( F’ + ζ

I5 / I2

) ] / (Δαμ

)= X’

-

ζ

I5 / I3

/ ( Δαμ

)Now I3

= Io3

exp( -

μ α3

X’

)X = X’

-

ζ

I5

* exp( μ α3

X’

) / Io2

/ ( Δαμ

)For μ α3

X’

small and I5

only lightly attenuated:X ~ X’

-

ζ

Io5 * (μ α3

X’

+ (μ α3

X’)2/2 ) / Io3

/ (Δαμ

)X ~ X’

-

ζ

Io5

/Io3 X’

-

ζ

Io5

/Io3 μ α3

X’2/2 since Δα

~ α3X ~ X’

( 1 -

ζ

Io5

/Io3

-

ζ

Io5

/Io3 μ α3

X’/2 )Let ξ

= ζ α3 / 2 and recognize that Io5 ~ Io3

X ~ X’

( 1 -

ζ

-

ξ μ X’

)

X’

is the true ozone

31 May –


dO3 = ( -6%-0.8% ) /( 2400 - 600 ) = 3.8E-3 %/DU slant column

Stray Light Correction

31 May –


Stray light is modelled by

1.

Calculating transmitted solar irradiances from 290-350 nm at 0.05 nm resolution, multiplying by measured responsitivity, and convolving with laser scan to get synthetic Brewer measurements for each slit

2.

Deriving ETC by performing Langley on synthetic data

3.

Applying Brewer algorithm to synthetic data and ETC

Stray light errors are determined by

1.

Modeling Brewer column including stray light

2.

Modeling

Brewer column including only instrument function core

3.

Calculating fractional difference, (xstray

– xcore

) / xcore

31 May –


Modelled Fractional Stray Light Signal for Brewer #014-

Multiple latitudes and months (ozone profiles), SZAs considered

(which is why there is some scatter)

0 1000 2000 3000 4000

10-2

10-1

100

Slant Column, x (DU)

Frac

tion

Stra

y Li

ght

012345

Slit #

31 May –

3 June 2005 Brewer Workshop Beijing, China 461 2 3 4 5-10

-8

-6

-4

-2

0

2

Airmass,

log(

S i)-lo

g(S 5)

MeasurementsModel with strayModel corrected

Comparing Modelled Signals with Observations from Brewer #007 (Fairbanks, May 5, 2001)

-

compare log(Si

)-log(S5

), where Si

is signal at slit i

-

Small differences may remain due to assumed Rayleigh, solar flux, slit widths; slit 0 and 1 suggest slightly too much stray light in model

i=4

i=3

i=2

i=1i=0

31 May –


Modelled Stray light Error in Ozone Columns

500 1000 1500 2000 2500 3000 3500-1

-0.8

-0.6

-0.4

-0.2

0


Rel

ativ

e Er

ror

#009, Mk IV#014, Mk II

Error ()

here caused primarily by stray light wing; thus(009)>(014)

31 May –


Modelled Stray light Error in Ozone Columns

200 400 600 800 1000 1200 1400 1600-0.08

-0.07

-0.06

-0.05

-0.04

-0.03

-0.02

-0.01

0


Rel

ativ

e Er

ror

#009, Mk IV#014, Mk II

** Model predicts non-zero error even for small slant

columns

Error ()

here caused primarily by stray light shoulder; thus(014)>(009)

31 May –


Modelled Stray light Error

-

Non-zero error at small x is at 1-1.5% level

-

Jim Kerr (personal communication) estimated

this to be about at ~1% error by

analyzing

Brewer measurements

-

A small model-measurement inconsistency may remain

31 May –


Parameterizing Laserscans

Fitting a Lorentzian to the shoulder region and a constant to the wings seems reasonable (3 parameters)

-300 -200 -100 0 100

10-6

10-4

10-2

100

Wavelength (A)

LaserscanFit to shoulder+wingabs(difference)

#015

31 May –

3 June 2005 Brewer Workshop Beijing, China 51-400 -300 -200 -100 0 100 200

10-5

10-4

10-3

10-2

10-1

100

Wavelength (A)

12345

Parameterizing Laserscans

Question: is a laserscan measured using slit 1 representative of stray light for slits 2-5?

Internal reflection at Slit 5

#015

Yes, although subtle differences are evident whenexamining Lorentzianfitted parameters:

Slit Amplitude

Width

(x10-3)

(A) 1

5.05

13.4

2

5.26

12.93

5.84

12.8

4

5.73

12.6

Slit #

Lorentzian Fits

31 May –


Correcting Brewer Measurements

Brewer equation with stray light can be written as

x = (F –

F0

–

F) /

where F is the stray light contribution to F (that is, subtracting F from F removes the stray light), and is a combination of the fraction of stray light at slits 2, 3, 4, and 5

F (calculated in the model) for a particular Brewer can be expressed as a combination of the signals (S) at each slit

F = i ai

log(Si

) where I = 1 to 5

F0

was calculated with stray light removed during the Langley analysis

31 May –


Correcting Brewer Measurements

After applying parameterization of F to model columns, remaining stray light error is <2 DU for SCD up to 4000 DU.

#015

0 1000 2000 3000 4000-4

-2

0

2

4

6


Rem

aini

ng E

rror

(DU

)

31 May –

3 June 2005 Brewer Workshop Beijing, China 54The End –

Thank you!

Analysis of Stray Light in a Brewer · PDF fileAnalysis of Stray Light in a BrewerAnalysis of...

Documents

Transcript of Analysis of Stray Light in a Brewer · PDF fileAnalysis of Stray Light in a BrewerAnalysis of...