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, France
2 Institut Pierre Simon Laplace (IPSL), Laboratoire de Météorologie Dynamique (LMD),
UMR CNRS 8539, Ecole Polytechnique, 91128 Palaiseau Cedex, France
3 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
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1. CONTEXT
Precision required on CO2 IPDA (Integrated Path Differential
Absorption) measurements from space to infer CO2 surface fluxes with
a constant pressure weighting function :
- 1 ppm in total random bias
- 0.1 ppm differential (interregional) bias
IPDA measurement principle
ρ=
τ
p surf
∫ WF ( p )dp
Measurement biases caused by :
- Error on differential optical depth τ measurement
- Error on pressure weighting function
determination WF(p)
0
1  Poff
τ = ln
2  Pon



σ~on ( p) − σ~off ( p)
WF ( p ) =
ma g (1 + mw / ma ρ w ( p ))
Uncertainties on spectroscopic parameters induce errors in WF(p) calculation
15th CLRC - Toulouse, FRANCE
22-26 June 2009
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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-line
position
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
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2. ABSORPTION LINE SELECTION (2/2)
Requirement on high sensitivity of the WF to the lowest atmosphere
Selection of the R30 CO2
absorption line at
4875.75 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 in
lowest part of atmosphere
15th CLRC - Toulouse, FRANCE
22-26 June 2009
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3. SPECTROSCOPY OF CO2 AT 2μm : DIODE
LASER ABSORPTION SPECTROMETER
Tunability of 2051 nm Nanoplus DFB
laser diode : ~ 4873  4879 cm-1
5 lines of (2001)III (0000)I band of
C12O16 in this range : R26  R34
Multipass
absorption
cell
15th CLRC - Toulouse, FRANCE
22-26 June 2009
DFB laser diode
5
3. SPECTROSCOPY OF CO2 AT 2μm :
METHODOLOGY (1/2)
Cell filled with gas :
N : number of molecules/cm3
DFB laser
diode
Detector
p : pressure
T : temperature
I0(σ)
I(σ)
Beer-Lambert law
Tgas (σ ) =
I (σ )
= exp[− α (σ , T , p ).N (T , p ).l ]
I 0 (σ )
Absorption coefficient
Length of the cell
R30 line
T = 293 K, p = 371 mbar, l = 203.8 cm
Profiles fitted using a Voigt profile
15th CLRC - Toulouse, FRANCE
22-26 June 2009
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3. SPECTROSCOPY OF CO2 AT 2μm :
METHODOLOGY (2/2)
Determination of the baseline
Line centre from HITRAN database
FSR = 0.01cm-1
Normalisation and calibration
of the absorption spectrum
15th CLRC - Toulouse, FRANCE
22-26 June 2009
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3. SPECTROSCOPY OF CO2 AT 2μm : RESULTS
(1/4)
Intensities S0 (line strengths)
Measurement at
5 temperatures
Normalisation at
reference temperature
 E" hc  1 1  
T 
 −  
S = S 0  0  exp −
k  T T0  
T 

S0 (10-22 cm-1/(molecule.cm-2))
Line
σ (cm-1)
R26
This work
(uncertainty)
HITRAN
2004
Diff.
(%)
Regalia et al
2006
(uncertainty)
Diff.
(%)
Toth et al
2006
(uncertainty)
Diff.
(%)
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
15th CLRC - Toulouse, FRANCE
22-26 June 2009
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3. SPECTROSCOPY OF CO2 AT 2μm : RESULTS
(2/4)
Air-broadening coefficient γa and temperature dependence coefficient η
Measurements at 5 different T: results
standardized at reference temperature :
P
γ =γ0
P0
 T0 
 
T 
η
Determination of η from :
ln γ (T ) = −η ln(T) + ln (γ 0 ( 296 K ) ) + η ln(296)
15th CLRC - Toulouse, FRANCE
22-26 June 2009
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3. SPECTROSCOPY OF CO2 AT 2μm : RESULTS
(3/4)
Air-broadening coefficient γa and temperature dependence coefficient η
γ0 (cm-1/atm) at 296 K
Line
η
This work
(uncertainty)
HITRAN
2004
Diff.
(%)
Toth et al.
2007
(uncertainty)
Diff.
(%)
This work
(uncertainty)
HITRAN
2004
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
15th CLRC - Toulouse, FRANCE
22-26 June 2009
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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 γa
and 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
15th CLRC - Toulouse, FRANCE
22-26 June 2009
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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 :
δρ ( psurf , T ( p ), δX ) =
τ
p surf
∫ WF ( p, T ( p ), δX )dp
0
−
τ
p surf
∫ WF ( p, T ( p ), δX
= 0 )dp
0
15th CLRC - Toulouse, FRANCE
22-26 June 2009
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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% on
pressure-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 profile
SAW : Sub Arctic Winter Profile
TRO : Tropical profile
Extreme differential interregional biases
are the difference between bias between
TRO and SAW profiles or between two
different reflecting surface pressure
(at 1000 hPa and 900 hPa)
15th CLRC - Toulouse, FRANCE
22-26 June 2009
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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
15th CLRC - Toulouse, FRANCE
22-26 June 2009
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5. RESULTS and CONCLUSION
• New spectroscopic parameters measurements have been
conducted at Reims University with previously unreached
precisions on CO2 R30 absorption line
• Potential biases due to spectroscopic uncertainties on this
line have been investigated
• Random global bias due to these uncertainties is 1.6 ppm
and just fulfilled the requirements on CO2 sources and sinks
determination
• Interregional differential biases in extreme cases due to
temperature or reflecting surface pressure is 0.084 ppm and
fulfilled the requirements
• Spectroscopic parameters determination should
nevertheless be improved especially concerning the pressure
shift coefficient
15th CLRC - Toulouse, FRANCE
22-26 June 2009
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