A new approach to eliminate the A new approach to eliminate the broadband absorption inbroadband absorption in

DOAS spectraDOAS spectra

M. Ródenas1, E. Soria2, J.D. Martín2

1Fundación CEAM, EUPHORE Labs, 46980 Paterna, Valencia, Spain 2Dpto. Ingeniería Electrónica. Univ. de Valencia, Burjassot-Valencia, Spain

Speaker: Mila Ródenas

Fundación CEAM: EUPHOREFundación CEAM: EUPHOREFundación CEAM: EUPHOREFundación CEAM: EUPHORE

EUPHORE is one of the major research platforms in Europe with outstanding analytical infrastructure to investigate chemical

processes related to tropospheric chemistry.

Measurement of contaminants Provide information for models Air quality policies

improvement

Special focus on the Mediterranean Basin

Doas scheme

Doas scheme

DOAS Setup White-multireflection cell: 8m base-path

(EUPHORE) Optical Pathlengths: 128-386-640-896-

1154m Resolution: 0.04, 0.08, 0.16 nm/pixel

(FWHM=0.18, 0.35, 0.76 nm, resp.) Several sets of mirrors for detection of

different compounds Range: UV and Visible (200 – 1000 nm) Automatic adjustment of mirrors in the

chamber Automatic positioning of optical fibers Spectrometer (Acton-500 Pro) and

detector (PDA-3904 Hamamatsu) are

thermostated

DOAS setup at EUPHOREDOAS setup at EUPHOREDOAS setup at EUPHOREDOAS setup at EUPHORE

Lambert-Beer Law:Lambert-Beer Law:Lambert-Beer Law:Lambert-Beer Law:

iii LcII ·exp0

Broadband structures in DOAS spectraBroadband structures in DOAS spectraBroadband structures in DOAS spectraBroadband structures in DOAS spectra

Longitud de onda (nm)

Inte

nsid

ad (u

.a.)

Scattering de Mie

Scattering de Rayleigh

I0 ()

OD'

'0

Longitud de onda (nm)

Inte

nsid

ad (u

.a.)

Scattering de Mie

Scattering de Rayleigh

I0 ()

OD'

'0

BroadbandNarrow bands

Origin of broadbandOrigin of broadbandOrigin of broadbandOrigin of broadband

• Mie scattering (aerosols, particles, λ-

1.3)

• Raileigh scattering (molecules, λ-4)

• Low frequency contribution of the compound of interest

• Change of beam position on dielectrical mirrors, etc

• Change of lamp spectrum

• Compounds with broad structure, etc

''ln0

NLccI

IOD MR

jjj

a

jjj

ChoicesChoices

Binomial smoothing

Polynomials

Regression

Moving average

Savitzky-Golay

Fourier, etc

FILTERING of broadband structuresFILTERING of broadband structuresFILTERING of broadband structuresFILTERING of broadband structures

When to apply it?When to apply it?

During the fitting routine

Prior to the fitting routine

Applied methodApplied method

High-pass filtering

Subtraction of low-pass filter

DrawbacksDrawbacksDrawbacksDrawbacks

ANALYST MUST CHOOSE:

• polynomial degree, r

• region of analysis

• use or not pre-filters?

Classic DOAS approachClassic DOAS approachClassic DOAS approachClassic DOAS approach

Lambert-Beer Law j

jjjjr iddSaiPiF ),...)(,('·)()( 1,0,

Linear fitting (Pr, aj)

Non-linear fitting (dj,k)

ANALYSIS ALGORITHM: Classic MethodANALYSIS ALGORITHM: Classic MethodANALYSIS ALGORITHM: Classic MethodANALYSIS ALGORITHM: Classic Method

Reduction of diff. cross section if a pre-filter is used, i.e. INCREASE OF DETECTION LIMIT

Degrees of freedom

No exact rules (Need to check up residuals)

Does the broadband match exactly a polynomial? (structures interfering or higher residuals)

Dependence with the compounds absorption features

ANALYSIS ALGORITHM: Proposed methodANALYSIS ALGORITHM: Proposed methodANALYSIS ALGORITHM: Proposed methodANALYSIS ALGORITHM: Proposed method

Proposed approachProposed approachProposed approachProposed approach

Linear fitting (Pri, aji=aj)

Non-linear fitting (dj,k)

j

jjjjr iddSaiPiF ),...)(,('·)()( 1,0,

Fitting of moving windows of n channels centered in i (Pri, aji) using low r

AdvantagesAdvantagesAdvantagesAdvantages

• Less intervention of analyst

• Low polynomial orders used - width of analysis spectral range - shape of

compounds/broadband

• Better match between modelled and real broadband

• No need of pre-filter, i.e. NO DECREASE OF DETECTION LIMIT

Analysis conditionsAnalysis conditionsAnalysis conditionsAnalysis conditions

Combination of concentrations [0-100ppb] Spectral dispersion not allowed (i.e., linear fitting) Proposed Method: width of windows, n=80 channels

polynomial degree, r=3 Classic implementation with Polynomial degree: [1-9] & binomial

pre-filters Proposed implementation with MATLAB

RESULTS: Test on NORESULTS: Test on NO22 and Glyoxal and GlyoxalRESULTS: Test on NORESULTS: Test on NO22 and Glyoxal and Glyoxal

+ noise

0.0E+00

2.0E-04

4.0E-04

6.0E-04

8.0E-04

1.0E-03

1.2E-03

1.4E-03

1.6E-03

400 420 440 460 480

Wavelength (nm)

O D

(C

o m

po

u n

d s

)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

O D

(B

r o

a d

b a

n d

)

NO2GlyoxalBroadband

c

OD’ of 1 ppb of compound at L=1154m

ANALYSIS ALGORITHM: ResultsANALYSIS ALGORITHM: ResultsANALYSIS ALGORITHM: ResultsANALYSIS ALGORITHM: Results

NONO22: Concentration error (%): Concentration error (%)NONO22: Concentration error (%): Concentration error (%)

Concentration error %

-40

-20

0

20

40

60

80

100

10 20 30 40 50 60 70 80 90 100 110

Theoretical C [ppb]

G1

G2

G3

G4

G5

G6

G7

G8

G9

G1_500i

G2_500i

G3_500i

G4_500i

G5_500i

G6_500i

G7_500i

G8_500i

G9_500i

No Pre-Filter

Pre-Filter

Concentration error %

0

2

4

6

8

10

12

14

16

18

20

10 20 30 40 50 60 70 80 90 100 110

Theoretical C [ppb]

G1_500i

G2_500i

G3_500i

G4_500i

G5_500i

G6_500i

G7_500i

G8_500i

G9_500i

c

Concentration error %

0

2

4

6

8

10

12

14

16

18

20

10 20 30 40 50 60 70 80 90 100 110

Theoretical C [ppb]

G1_500i

G2_500i

G3_500i

G4_500i

G5_500i

G6_500i

G7_500i

G8_500i

G9_500i

This work

Pre-Filter Proposed Method

Cerror=(CTheo-Ccalc)/CTheo*100

ANALYSIS ALGORITHM: ResultsANALYSIS ALGORITHM: ResultsANALYSIS ALGORITHM: ResultsANALYSIS ALGORITHM: Results

GLYOXAL: Concentration error (%)GLYOXAL: Concentration error (%)GLYOXAL: Concentration error (%)GLYOXAL: Concentration error (%)

Glyoxal Concentration error (%)

0.0

0.1

1.0

10.0

100.0

1000.0

10000.0

10 20 30 40 50 60 70 80 90 100 110

Theoretical C [ppb]

Glyoxal Concentration error (%)

0.0

0.1

1.0

10.0

100.0

1000.0

10000.0

10 20 30 40 50 60 70 80 90 100 110

Theoretical C [ppb]

Pre-Filter

Proposed Method

No Pre-Filter

Residual Root Mean Square (RMS)

7.58

E-0

4

8.03

E-0

4

0.0E+00

2.0E-03

4.0E-03

6.0E-03

8.0E-03

1.0E-02

1.2E-02

1.4E-02

G1 G2 G3 G4 G5 G6 G7 G8 G9 ProposedMethod

ANALYSIS METHOD

No pre-filterPre-filter (500 iter)Pre-filter (2000 iter)

Results: Test on NOResults: Test on NO2 2 (355 – 408 nm)(355 – 408 nm)Results: Test on NOResults: Test on NO2 2 (355 – 408 nm)(355 – 408 nm)

NO2

355-408 nm

disp=0.16 nm/ch

Binomial filter:Classic

n=80 channels, r=3:Proposed

NO2

1.1

0

1.0

6

0

20

40

60

80

100

120

G1 G2 G3 G4 G5 G6 G7 G8 G9 ProposedMethod

ANALYSIS METHOD

No pre-filter

Pre-filter (500 iter)

Pre-filter (2000 iter)

(ppb)

N

CCE calcTheor

2

2/1 )(

NO2

1.10

1.06

0

5

10

15

20

25

G1 G2 G3 G4 G5 G6 G7 G8 G9 ProposedMethod

ANALYSIS METHOD

No pre-filter

Pre-filter (500 iter)

Pre-filter (2000 iter)

(ppb)

N

CCE calcTheor

2

2/1 )(

NO2

8.64

1.492.

965.

00

0

5

10

15

20

25

G1 G2 G3 G4 G5 G6 G7 G8 G9 ProposedMethod

ANALYSIS METHOD

No pre-filter

Pre-filter (500 iter)

Pre-filter (2000 iter)

(ppb)

306 – 408 nm

Results: Test on NOResults: Test on NO2 2 (320 – 400 nm)(320 – 400 nm)Results: Test on NOResults: Test on NO2 2 (320 – 400 nm)(320 – 400 nm)

NO2

320-400 nm

disp=0.16 nm/ch

Binomial filter:Classic

n=100 channels, r=3:Proposed

Similar results using the best fitting parameters of the classical method and using the proposed method

Strong dependence on the analysis range used in classic method

DEGREES OF FREEDOM REDUCED IN THE PROPOSED

METHOD

NO2

1.10

1.06

0

5

10

15

20

25

G1 G2 G3 G4 G5 G6 G7 G8 G9 ProposedMethod

ANALYSIS METHOD

No pre-filter

Pre-filter (500 iter)

Pre-filter (2000 iter)

(ppb)

N

CCE calcTheor

2

2/1 )(

Residual Root Mean Square (RMS)

7.58

E-04

8.03

E-04

0.0E+00

2.0E-03

4.0E-03

6.0E-03

8.0E-03

1.0E-02

1.2E-02

1.4E-02

G1 G2 G3 G4 G5 G6 G7 G8 G9 ProposedMethod

ANALYSIS METHOD

No pre-filterPre-filter (500 iter)Pre-filter (2000 iter)

ANALYSIS ALGORITHM: ConclusionsANALYSIS ALGORITHM: ConclusionsANALYSIS ALGORITHM: ConclusionsANALYSIS ALGORITHM: Conclusions

COMPARATIVE CLASSIC vs COMPARATIVE CLASSIC vs PROPOSEDPROPOSED ALGORITHM ALGORITHMCOMPARATIVE CLASSIC vs COMPARATIVE CLASSIC vs PROPOSEDPROPOSED ALGORITHM ALGORITHM

Features Classic Proposed

Correct results

Highly dependent on: Polynomial degree Pre-fi lter used? Spectral structure of

compounds

Yes

Concentration off set I nitial parameters

dependent Negligible (good

results)

Pre-fi lter Depends on spectra No

Sensitivity with initial parameters

Strong Weak

Wider ranges possible?

? Yes

User intervention High Low

*

MUCH HIGHER USER INTERVENTION NEEDED IN CLASSIC THAN IN PROPOSED METHOD

ANALYSIS ALGORITHM: Future worksANALYSIS ALGORITHM: Future worksANALYSIS ALGORITHM: Future worksANALYSIS ALGORITHM: Future works

Test of the algorithm to analyse other compounds

Use of different polynomials or curves

Pre – processing of spectra to enhance features(e.g.. derivatives)

Thanks for your

attention

ACKNOWLEDGEMENTS: GENERALITAT VALENCIANA, BANCAJA,

EUROCHAMP, AND ACCENT FOR SUPPORT.