Analysis of Stray Light in a Brewer Spectrophotometer Brewers are not Perfect!

Transcription

1 Analysis of Stray Light in a Brewer Spectrophotometer Brewers are not Perfect! C. A. McLinden, D. I. Wardle, C. T. McElroy, &V. Savastiouk Environment Canada

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 3 June 2005 Brewer Workshop Beijing, China 2

3 Brewer Map 31 May 3 June 2005 Brewer Workshop Beijing, China 3

4 MSC Toronto 31 May 3 June 2005 Brewer Workshop Beijing, China 4

5 Mauna Loa Observatory 31 May 3 June 2005 Brewer Workshop Beijing, China 5

6 Eureka Weather Station 31 May 3 June 2005 Brewer Workshop Beijing, China 6

7 Global column ozone: rationale 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 achieved 1.0% RMS for daily average measurements with our best 3 instruments in Toronto. 31 May 3 June 2005 Brewer Workshop Beijing, China 7

8 Dispersion and spectral purity Given that we are to measure ozone from its UV spectrum, we need to know: (a) accurately the wavelengths of the measurement; in fact if 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 wavelengths is not interfering with the measurement. 31 May 3 June 2005 Brewer Workshop Beijing, China 8

9 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 3 June 2005 Brewer Workshop Beijing, China 9

10 The Brewer spectrophotometer optics sun Detector Order filter UV-vis Exit slits Slit selector Diffraction grating: 1800 lines/mm 1 st order: visible nm Neutral density filters 2 nd order: UV nm Also: 1200 and 3600 lines/mm Diffraction grating turned by stepper motor 31 May 3 June 2005 Brewer Workshop Beijing, China 10

11 Spectrometer layout mm The Brewer spectrometer is a modified Ebert type using one mirror. F P Exit slit #1 Entrance slit The instrument axis, defined as the normal to the front plate (FP) passing through the centre of the grating surface also passes through the vertical plate (VP) which locates the mirror. correction lens The entrance slit and exit slit#1 (not #3) are ~equidistant from the axis. The exit and exit directions of the central ray are parallel to the axis. The correction lens reduces the coma and astigmatism of the basic Ebert design. V P 31 May 3 June 2005 Brewer Workshop Beijing, China 11

12 Spectrometer Gratings and Marks Brewer gratings all have the same dispersion in the UV determined by n/d = 3600 lines per mm * order Usually as follows: ---- order in Grating pitch Blazed for UV BLUE RED Mark II S nm 2 Mark V S nm 2 1 Mark IV S nm 3 2 Mark III D nm 1 Mark VI S nm 1 measuring: ozone NO 2 ozone at low 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 3 June 2005 Brewer Workshop Beijing, China 12

13 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. m 2 ) 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 from spectral lamps, c. 200 from HeCd Lasers. 31 May 3 June 2005 Brewer Workshop Beijing, China 13

14 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 3 June 2005 Brewer Workshop Beijing, China 14

15 Slit #0 slit function Slit 0 W 297 a fwhi= 636 jw= May 3 June 2005 Brewer Workshop Beijing, China 15

16 How do Brewers Compare?? log10(normalized signal) all available Brewer laser scans 0 007MKIVf 009MKIV MKII 014MKII 015MKII 017MKII MKIII 029MKV 033MKII MKII 042MKV 053MKII MKII 069MKV 071MKIV MKIV 082MKIVe 084MKIV 085MKIII MKIII 109MKIVe 111MKIII MKII 128MKIII 507mkivf mkiii mkiii 3700 wavelength setting (nm*10) 645mkiii Note the log scale! 31 May 3 June 2005 Brewer Workshop Beijing, China 16

17 Class I Single Brewers log10(normalized signal) class I SB's 007MKIVf 012MKII 014MKII 015MKII 017MKII 029MKV 033MKII 037MKII 053MKII 055MKII 069MKV 082MKIVe 109MKIVe 113MKII May 3 June 2005 Brewer wavelength Workshop setting Beijing, (nm*10) China 17

18 Class II Single Brewers log10(normalized signal) log10(normalized signal) class II SB's 325 (signal on Brewers 007MKIVf 009MKIV 009MKIV 012MKII 042MKV 014MKII 071MKIV 015MKII 079MKIV 017MKII 084MKIV 029MKV 033MKII 037MKII 042MKV 053MKII 055MKII 069MKV 071MKIV 079MKIV 082MKIVe 084MKIV 109MKIVe 113MKII wavelength setting (nm*10) wavelength setting (nm*10) 31 May 3 June 2005 Brewer Workshop Beijing, China 18

19 Cass I SB - #017 log10(normalized signal) class I SB's minus atypical # MKIVf 012MKII 014MKII 015MKII 029MKV 033MKII 037MKII 053MKII 055MKII 069MKV 082MKIVe 109MKIVe 113MKII May 3 June 2005 Brewer wavelength Workshop setting Beijing, (nm*10) China 19

20 Class I - #s 007, 017, 109 log10(normalized signal) class I's minus #017, #007 and # MKII 014MKII 015MKII 029MKV 033MKII 037MKII 053MKII 055MKII 069MKV 082MKIVe 113MKII wavelength setting (nm*10) 31 May 3 June 2005 Brewer Workshop Beijing, China 20

21 Extended Scan Singles #s 007, two extended-scan class I SB's, (007 and 109). 007MKIVf 109MKIVe -2 log10(normalized signal) May 3 June 2005 Brewer wavelength Workshop setting Beijing, (nm*10) China 21

22 Three Single Brewers 012, 014, two of the triad SB's, (014,015), and another (012). 012MKII 014MKII 015MKII -2 log10(normalized signal) #014, #015 Are Toronto TRIAD Instruments May 3 June 2005 Brewer Workshop Beijing, China 22 wavelength setting (nm*10)

23 Double Brewers double on 325 nm 021MKIII 085MKIII 107MKIII 111MKIII 128MKIII log10(normalized signal) wavelength setting (nm*10) Note: #085 scan was done before the replacement of the ground quartz 31 May 3 June 2005 Brewer Workshop Beijing, China 23

24 0-1 Double Brewers using 353nm double Brewers on 353 nm 521mkiii 585mkiii 645mkiii -2 log10(normalized signal) wavelength setting (nm*10) Secondary peak at 325 nm is from impurity of the 353 nm laser 31 May 3 June 2005 Brewer Workshop Beijing, China 24

25 325 & 352 nm laser scans & 353nm lasers on same single Brewer 007MKIVf 507mkivf -2 log10(normalized signal) wavelength setting (nm*10) 31 May 3 June 2005 Brewer Workshop Beijing, China 25

26 325 & shifted 353 laser scans measured at 325nm. 007 measured at 353nm, shifted to 325nm. -2 log10(normalized signal) wavelength setting (nm*10) 31 May 3 June 2005 Brewer Workshop Beijing, China 26

27 325 & 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 energy wings 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 3 June 2005 Brewer Workshop Beijing, China 27

28 The data Multislit laser scan CZ , (5th segment) d 4d May 3 June 2005 Brewer Workshop Beijing, China 28

29 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 3 June 2005 Brewer Workshop Beijing, China 29

30 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 3 June 2005 Brewer Workshop Beijing, China 30

31 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 3 June 2005 Brewer Workshop Beijing, China 31

32 Ozone in Toronto - #s 085, May 3 June 2005 Brewer Workshop Beijing, China 32

33 Ozone at Sodankyla 31 May 3 June 2005 Brewer Workshop Beijing, China 33

34 Airmass Dependence in the presence of an ETC error 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) This is NOT what was happening in Sodankyla! 31 May 3 June 2005 Brewer Workshop Beijing, China 34

35 Stray Light Correction do3 = ( -6%-0.8% ) /( ) = 3.8E-3 %/DU slant column 31 May 3 June 2005 Brewer Workshop Beijing, China 35

36 Courtesy A. Cede 31 May 3 June 2005 Brewer Workshop Beijing, China 36

37 CPFM on WB-57F 31 May 3 June 2005 Brewer Workshop Beijing, China 37

38 6.00 -log(-log(normalized Intensity)) Stray Light Function HeCd 325 nm 1. I(i) = I(i) / Imax 2. f = -log(-log(i(i))) Y = * X Y = * X CPFM Stray Light Function Composition and Photodissociative Flux Measurement Y = * X Y = * X Y = * X Y = * X Pixel Number 31 May 3 June 2005 Brewer Workshop Beijing, China 38

39 Laserscans Single-Brewer stray light rejection is as measured by scanning 325 nm HeCd laserline Response Function (-) #009, Mk IV #014, Mk II #015, Mk II #071, Mk IV Stray light wing Stray light shoulder Instrument function core, FWHM=0.55 nm Wavelength (nm) 31 May 3 June 2005 Brewer Workshop Beijing, China 39

40 Nature of Stray Light Effect 6 F = Σ a l log[ I l ] l=3 X = ( F o 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 3 June 2005 Brewer Workshop Beijing, China 40

41 Stray Light Simplified Brewer ozone is based on slit positions l = 3 to l = 6 F = a 3 log[ I 3 + βi 6 ] + Σ a l log[ I l ] F = a 3 log[ I 3 ( 1+ βi 6 / I 3 ) ] + Σ a l log[ I l ] F = Σ a l log[ I l ] + a 3 log[ 1+ βi 6 / I 3 ] l = 3 to l = 6 l = 4 to l = 6 Sum over l = 4 to 6 β is stray light fraction = F + a 3 log[ 1+ βi 6 / I 3 ] ~ F + ζ I 6 / I 3 with ζ=a 3 β Now X = ( F o - F ) / ( Δα * μ ) Where F is the true ozone ratio 31 May 3 June 2005 Brewer Workshop Beijing, China 41

42 Rearranging X = ( F o - F ) / ( Δα * μ ) X = [ F o ( F + ζ I 5 / I 2 ) ] / (Δαμ ) = X - ζ I 5 / I 3 / ( Δαμ ) Now I 3 = I o3 exp( - μα 3 X ) X = X - ζ I 5 * exp( μ α 3 X ) / I o2 / ( Δαμ ) For μ α 3 X small and I 5 only lightly attenuated: X ~ X - ζ I o5 * (μ α 3 X + (μ α 3 X ) 2 /2 ) / I o3 / (Δαμ ) X ~ X - ζ I o5 /I o3 X - ζ I o5 /I o3 μα 3 X 2 /2 since Δα ~ α 3 X ~ X ( 1 - ζ I o5 /I o3 - ζ I o5 /I o3 μα 3 X /2 ) Let ξ = ζα 3 / 2 and recognize that I o5 ~ I o3 X ~ X ( 1 - ζ - ξμx ) X is the true ozone 31 May 3 June 2005 Brewer Workshop Beijing, China 42

43 Stray Light Correction do3 = ( -6%-0.8% ) /( ) = 3.8E-3 %/DU slant column 31 May 3 June 2005 Brewer Workshop Beijing, China 43

44 Stray light is modelled by 1. Calculating transmitted solar irradiances from 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, (x stray x core ) / x core 31 May 3 June 2005 Brewer Workshop Beijing, China 44

45 Modelled Fractional Stray Light Signal for Brewer #014 - Multiple latitudes and months (ozone profiles), SZAs considered (which is why there is some scatter) Fraction Stray Light Slit # May 3 June 2005 Slant Brewer Column, Workshop x (DU) Beijing, China 45

46 Comparing Modelled Signals with Observations from Brewer #007 (Fairbanks, May 5, 2001) - compare log(s i )-log(s 5 ), where S i 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 2 0 i=4 log(s i )-log(s 5 ) -2-4 i=1-6 i=0-8 Measurements Model with stray Model corrected Airmass, 31 May 3 June 2005 Brewer Workshop Beijing, China 46 i=3 i=2

47 Modelled Stray light Error in Ozone Columns Relative Error Error ( ) here caused primarily by stray light wing; thus (009)> (014) #009, Mk IV #014, Mk II Slant Column, x (DU) 31 May 3 June 2005 Brewer Workshop Beijing, China 47

48 Modelled Stray light Error in Ozone Columns Relative Error ** Model predicts non-zero error even for small slant columns Error ( ) here caused primarily by stray light shoulder; thus (014)> (009) #009, Mk IV #014, Mk II Slant Column, x (DU) 31 May 3 June 2005 Brewer Workshop Beijing, China 48

49 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 3 June 2005 Brewer Workshop Beijing, China 49

50 Parameterizing Laserscans Fitting a Lorentzian to the shoulder region and a constant to the wings seems reasonable (3 parameters) Laserscan Fit to shoulder+wing abs(difference) # May 3 June 2005 Brewer Wavelength Workshop Beijing, (A) China 50

51 Parameterizing Laserscans Question: is a laserscan measured using slit 1 representative of stray light for slits 2-5? Slit # Internal reflection at Slit 5 Lorentzian Fits #015 Yes, although subtle differences are evident when examining Lorentzian fitted parameters: Slit Amplitude Width (x10-3 ) (A) May 3 June 2005 Wavelength (A) Brewer Workshop Beijing, China 51

52 Correcting Brewer Measurements Brewer equation with stray light can be written as x = (F F 0 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 a i log(s i ) where I = 1 to 5 F 0 was calculated with stray light removed during the Langley analysis 31 May 3 June 2005 Brewer Workshop Beijing, China 52

53 Correcting Brewer Measurements After applying parameterization of F to model columns, remaining stray light error is <2 DU for SCD up to 4000 DU. 6 4 #015 Remaining Error (DU) Slant Column, x (DU) 31 May 3 June 2005 Brewer Workshop Beijing, China 53

54 The End Thank you! 31 May 3 June 2005 Brewer Workshop Beijing, China 54