15th CLRC - Toulouse, FRANCE 22-26 June 2009 1

LASER DIODE ABSORPTION SPECTROSCOPY FOR ACCURATE CO2 LINE PARAMETERS AT 2 μm.

CONSEQUENCES FOR SPACE-BASED DIAL MEASUREMENTS AND POTENTIAL BIASES.

Lilian JOLY 1, Fabien MARNAS 2, Fabien GIBERT 2, Bruno GROUIEZ 1,Pierre H. FLAMANT 2, Didier BRUNEAU 3, Georges DURRY 1,3, Bertrand PARVITTE 1 and Virginie ZENINARI 1

1 Groupe de Spectrométrie Moléculaire et Atmosphérique (GSMA), UMR CNRS 6089, UFR Sciences Exactes et Naturelles, BP 1039, 51687 Reims Cedex, France2 Institut Pierre Simon Laplace (IPSL), Laboratoire de Météorologie Dynamique (LMD), UMR CNRS 8539, Ecole Polytechnique, 91128 Palaiseau Cedex, France3 Institut Pierre Simon Laplace (IPSL), LATMOS, UMR CNRS 8190, CNRS-Réduit de Verrières, B.P. 3, 91371 Verrières-le-Buisson Cedex, France

15th CLRC - Toulouse, FRANCE 22-26 June 2009 2

1. CONTEXTPrecision required on CO2 IPDA (Integrated Path DifferentialAbsorption) measurements from space to infer CO2 surface fluxes witha constant pressure weighting function :- 1 ppm in total random bias- 0.1 ppm differential (interregional) bias

( )∫=

surfp

dppWF0

τρ

IPDA measurement principleMeasurement biases caused by :- Error on differential optical depth τ measurement- Error on pressure weighting function determination WF(p)

=

on

off

PP

ln21τ ( )( )pmmgm

pppWF

wawa

offon

ρσσ/1

)(~)(~)(

+

−=

Uncertainties on spectroscopic parameters induce errors in WF(p) calculation

15th CLRC - Toulouse, FRANCE 22-26 June 2009 3

2. ABSORPTION LINE SELECTION (1/2)

- Availability of appropriate laser sources (power, spectral properties…)- Absence of interference with other species (especially water vapor)- Minimum temperature sensitivity (E’’ around 200 to 400 cm-1)- Optimal differential optical thickness (τ ~ 1)

Domain Wavenumber(cm-1) Wavelength (nm) Line strength

(cm2.cm-1.mol-1) E'' (cm-1) line ON-lineposition

1.6 μm 6367.223459 1570.54 1.205. 10-23 316.77 R28 Center

1.6 μm 6361.250904 1572.0 1.823 10-23 133.4 R18 Edge

2 μm 4875.748957 2050.967 1.741 10-22 362.79 R30 Wing

15th CLRC - Toulouse, FRANCE 22-26 June 2009 4

2. ABSORPTION LINE SELECTION (2/2)Requirement on high sensitivity of the WF to the lowest atmosphere

Selection of the R30 CO2absorption line at4875.75 cm-1 in the(2001)III (0000)I band of CO2 as one of the most suitable line for DIAL measurement from space.

Weighting function peaking inlowest part of atmosphere

15th CLRC - Toulouse, FRANCE 22-26 June 2009 5

3. SPECTROSCOPY OF CO2 AT 2μm : DIODE LASER ABSORPTION SPECTROMETER

Multipass absorption cell DFB laser diode

Tunability of 2051 nm Nanoplus DFBlaser diode : ~ 4873 4879 cm-1

5 lines of (2001)III (0000)I band of C12O16 in this range : R26 R34

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3. SPECTROSCOPY OF CO2 AT 2μm : METHODOLOGY (1/2)

DFB laser diode

Cell filled with gas :

N : number of molecules/cm3

p : pressure

T : temperature

Detector

I0(σ) I(σ)

( ) ( )( ) ( ) ( )[ ]lpTNpT

IITgas .,.,,exp0

σασσσ −==

Beer-Lambert law

Absorption coefficient Length of the cell

R30 lineT = 293 K, p = 371 mbar, l = 203.8 cmProfiles fitted using a Voigt profile

15th CLRC - Toulouse, FRANCE 22-26 June 2009 7

3. SPECTROSCOPY OF CO2 AT 2μm : METHODOLOGY (2/2)

Determination of the baseline

FSR = 0.01cm-1

Line centre from HITRAN database

Normalisation and calibrationof the absorption spectrum

15th CLRC - Toulouse, FRANCE 22-26 June 2009 8

3. SPECTROSCOPY OF CO2 AT 2μm : RESULTS (1/4)

Intensities S0 (line strengths)

Line σ (cm-1)

S0 (10-22 cm-1/(molecule.cm-2))

This work(uncertainty)

HITRAN2004

Diff.(%)

Regalia et al2006

(uncertainty)

Diff.(%)

Toth et al2006

(uncertainty)

Diff.(%)

R26 4873.1290 1.984(0.35 %) 2.295 -16 1.996

(2 %) -0.6 1.985(0.75 %) -0.1

R28 4874.4481 1.747(0.51 %) 2.019 -16 1.760

(1.9 %) -0.7 1.745(0.85 %) 0.1

R30 4875.7487 1.507(0.39 %) 1.741 -16 1.509

(1.9 %) -0.1 1.504(0.73) 0.2

R32 4877.0305 1.278(0.54 %) 1.474 -15 1.284

(2 %) -0.5 1.273 (0.86 %) 0.4

R34 4878.2932 1.060(1.1%) 1.225 -16 1.065

(1.9 %) -0.5 1.057(0.75%) 0.3

Measurement at5 temperatures

−−

=

0

00

11"expTTk

hcETT

SSNormalisation atreference temperature

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3. SPECTROSCOPY OF CO2 AT 2μm : RESULTS (2/4)

Measurements at 5 different T: results standardized at reference temperature :

η

γγ

=

TT

PP 0

00

Determination of η from :

( ) )296ln( )296(ln ln(T) )( ln 0 ηγηγ ++−= KT

Air-broadening coefficient γa and temperature dependence coefficient η

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3. SPECTROSCOPY OF CO2 AT 2μm : RESULTS (3/4)

Air-broadening coefficient γa and temperature dependence coefficient η

Line

γ0 (cm-1/atm) at 296 K η

This work(uncertainty)

HITRAN 2004

Diff.(%)

Toth et al.2007

(uncertainty)

Diff.(%)

This work(uncertainty)

HITRAN2004

Diff.(%)

R26 0.0727(0.41 %) 0.0692 4.8 0.0704

(1.9 %) 3.2 0.656(3 %) 0.78 -19

R28 0.0713(0.28 %) 0.0687 3.7 0.0699

(2 %) 2.0 0.662(2.5 %) 0.78 -18

R30 0.0702(0.15 %) 0.0684 2.6 0.0693

(2 %) 1.3 0.657(0.45 %) 0.78 -19

R32 0.0691(1.4 %) 0.0681 1.4 0.0688

(2 %) 0.4

R34 0.0675(1.4 %) 0.0678 -0.4 0.0684

(1.9 %) -1.3

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3. SPECTROSCOPY OF CO2 AT 2μm : RESULTS (4/4)

S0, γa, and η revisited for five CO2 lines R26 to R34 in (2001)III (0000)I band with significant precision improvement

For R30 line (2050.967 nm) of particular interest for DIAL from space : uncertainties are 0.4 % on S0, 0.15 % on γaand 0.45 % on η

First measurements reported on temperature dependance coefficient η whose value is fixed (0.78) in HITRAN for the entire absorption band.

A value of 0.66 is derived in this work

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4. INFLUENCE ON IPDA POTENTIAL BIASES : METHODOLOGY

The peaking weighting function of R30 transition relaxed requirements on global bias to 1.64 ppm and on the interregional bias to 0.164 ppm

WF(p) random error resulting from error on surface pressure, humidity and temperature vertical profiles have been computed to correspond to 0.5 ppm, 0.3 ppm and 0.1 ppm, respectively, for dry-air CO2 mixing ratios measurements

Uncertainty on line strength parameter yield to a constant bias (as Voigt profile is proportionnal with S0)

Assuming a perfect knowledge on meteorological parameters, an error δX on a spectroscopic parameter will yield a bias :

( )( )( ) ( )( )∫∫ =

−=surfsurf ppsurf

dpXpTpWFdpXpTpWFXpTp

00

0,,,,

),(,

δ

τ

δ

τδδρ

15th CLRC - Toulouse, FRANCE 22-26 June 2009 13

4. INFLUENCE ON IPDA POTENTIAL BIASES : RESULTS (1/2)

Bias estimated for uncertainties of 0.15% on γa, 0.45% on η and 2.6% onpressure-shift coefficient [Toth et.al 2007]using a Voigt profile for absorption line

Bias estimated for 3 cases of ESA’s reference meteorological profiles:STA : Standard profileSAW : Sub Arctic Winter ProfileTRO : Tropical profile

Extreme differential interregional biasesare the difference between bias betweenTRO and SAW profiles or between two different reflecting surface pressure(at 1000 hPa and 900 hPa)

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4. INFLUENCE ON IPDA POTENTIAL BIASES : RESULTS (2/2)

STA profil : sum of absolute bias 1.6 ppm

Differential biases :

Quadratic sum of pressure induced differential bias : 0.039 ppm

Quadratic sum of temperature induced differential bias : 0.074 ppm

Total quadratic sum : 0.084 ppm

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5. RESULTS and CONCLUSION• New spectroscopic parameters measurements have beenconducted at Reims University with previously unreachedprecisions on CO2 R30 absorption line• Potential biases due to spectroscopic uncertainties on thisline have been investigated • Random global bias due to these uncertainties is 1.6 ppm and just fulfilled the requirements on CO2 sources and sinksdetermination• Interregional differential biases in extreme cases due to temperature or reflecting surface pressure is 0.084 ppm andfulfilled the requirements • Spectroscopic parameters determination should nevertheless be improved especially concerning the pressureshift coefficient