Vanessa Sherlock,
Alain Hauchecorne,
and Jacqueline Lenoble
When this research was performed, V. Sherlock and A. Hauchecorne were with Service d’Aéronomie, Centre National de la Recherche Scientifique, BP 3, 91371 Verrières-le-Buisson, France and
J. Lenoble was with the Laboratoire d’Optique Atmosphérique, Université Lille 1, Villeneuve d’Ascq, France.
V. Sherlock is now with the Meteorological Office, London Road, Bracknell, Berkshire RG12 2SZ, UK.
J. Lenoble is now at IRSA (Interactions entre Rayonnement Solaire et Atmosphere), Université Joseph Fourier, 17 Quai Claude Bernard, 38000 Grenoble, France.
We present a method for the independent calibration of Raman
backscatter water-vapor lidar systems. Particular attention is
given to the resolution of instrumental changes in the short and the
long terms. The method reposes on the decomposition of the
instrument function, which allows the lidar calibration coefficient to
be re-expressed as the product of two terms, one describing the
instrumental transmission and detection efficiency and the other
describing the wavelength-dependent convolution of the Raman
backscatter cross sections with the instrument function. The
origins of changes in instrument response necessitate the experimental
determination of the system detection efficiency. Two external
light sources for calibration are assessed: zenith observation of
diffuse sunlight and a xenon arc lamp. The results favor use of the
diffuse-sunlight measurement but highlight the need for simultaneous
sunphotometer measurements to constrain modeled aerosol optical
properties. Quantum mechanical models of the Raman cross sections
are described, and errors in determining the cross sections and their
convolution with the instrument function are discussed in
detail. The calibration coefficients deduced by using the
independent method are compared with coefficients deduced from Vaisala
H-Humicap radiosonde measurements. These results agree to within
current calibration errors (15%, unconstrained aerosol
parameters), and a change in calibration coefficient following
instrument modification is reproduced satisfactorily. Results from
modeling and intercomparison studies are extended to estimate the
calibration accuracy and the precision of the diffuse-sunlight method
with constrained modeled aerosol parameters. Changes in the
calibration coefficient in the short and the long terms should be
resolved to 4(6)% and 6(9)%, respectively, which is
comparable or better than the precision of existing dependent methods
of calibration. The reduction of the absolute calibration error
remains an outstanding issue for all calibration methods.
Demetrius D. Venable, David N. Whiteman, Monique N. Calhoun, Afusat O. Dirisu, Rasheen M. Connell, and Eduardo Landulfo Appl. Opt. 50(23) 4622-4632 (2011)
Ina Mattis, Albert Ansmann, Dietrich Althausen, Volker Jaenisch, Ulla Wandinger, Detlef Müller, Yuri F. Arshinov, Sergej M. Bobrovnikov, and Ilya B. Serikov Appl. Opt. 41(30) 6451-6462 (2002)
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The laser wavelength used in each
determination is given in the second column. For comparison purposes,
the cross section (10-30 cm-1/sr) is
reported for an exciting wavelength of 514 nm. Correction has been
applied for the λR-4 dependence
of the cross section where necessary. For the CARS cross sections, γ
is the collisional air-broadened linewidth at half-maximum in
cm-1/atm.
Ref. 16.
Ref. 31.
Ref. 34.
Ref. 35.
Ref. 36.
Table 2
Summary of Evaluation of Effective Cross Sections and
Associated Errorsa,b
Error Source
Breakdown of Error
Error (%)
H2O effective cross section
Anisotropy
8
Mixed state
3
ν3 Contribution
<2
Normalization
±5
N2 effective cross section
Anisotropy
5
Normalization
±5
Atmospheric and instrumental effects
Temperature dependence
-0.02/K
Line broadening
Monomode
2.0
Multimode
0.5
Instrument function
Random error
<1
Translation ±0.05 nm
-5 + 3
Convolution
-5 + 3
Total error
12
Line-broadening effects are expressed
relative to the dirac delta representation. The total error
calculation is described in the text.
Ratio of effective cross
sections: 0.49 ± 0.05 at 280 K.
Optical depths are given for 550 nm.
Boundary layer aerosol.
Table 4
Characteristic Measures for the Solar Zenith Angle
Dependence on the Ratio of Downwelling Zenith Radiances r(χ)
Modeled Atmosphere Observation Period
[r(25) - r(60)]/r(25) (%)
[r(60) - r(80)]/r(60) (%)
Atmosphere type
Molecular scatter and absorption
≥-3
7–12
Continental
≤3
8–10
Maritime
10
6
Maritime mineral
6
6
Observation period
May 1997
3–7, ≤1
6–7
August–September 1997
≤1
5
October 1997–March 1998
–
5–10
Note: Model results are compared with
observations, highlighting the likelihood of the presence of large
particles (the maritime-mineral class) for three of the five
observations in May 1997.
Table 5
Summary of Errors in the Diffuse Sunlight Determination of
TN2/TH2O
Error Source
Breakdown of Error
Error (%)
Illumination
–
Modeled spectral dependence
Atmospheric density changes
<2
H2O absorption
≤3
O3 absorption
±2
Aerosol properties
10–15
Model dispersion
<2
Solar spectrum
<2
Instrumental uncertainties
Photon counting
<3
Correction for neutral-density filter
<2
Convolution with instrument function
<2
The errors associated with H2O and O3
absorption, atmospheric density changes, and aerosol properties
correspond to the case of the current OHP calibration where these
modeled parameters are not constrained by simultaneous independent
measurements.
The relative detection efficiency deduced
from diffuse sunlight measurements with the bounds for the most
probable values in bold face.
The ratio of molecular nitrogen signals on the
two Raman channels (T/
R)N2 may be
regarded as a proxy for detection efficiency. Measured values of
(T/
R)N2 are given.
Calibration coefficient deduced from diffuse
sunlight measurements when the explicit method is used.
Calibration coefficient deduced from
simultaneous radiosonde measurements over the same period. The number
of soundings are in parentheses.
Table 7
Results for Determination of
TN2
/TH2O
when a Xenon Arc Lampa
is Used
Observation Series
TN2/TH2O (%)
Test Series
TN2/TH2O
(%)
1
1.83 ± 0.09
Unobscured
1.79 ± 0.05
2
1.86 ± 0.07
Partially obscured (left)
1.68 ± 0.03
3
1.90 ± 0.04
Partially obscured (right)
1.78 ± 0.04
4
1.99 ± 0.04
Note: Each observation series corresponds to different
illumination conditions. Values have been ordered from lowest to
highest to aid in comparison. The error in the observed ratios are due
to short time scale intensity fluctuations (see text for details).
In the test series the influence of changes in photocathode
illumination is investigated.
Xenon arc lamp, 75 W.
Table 8
Summary of Errors in the Xenon Lamp Calibration
Error Source
Error Breakdown
Error (%)
Illumination
5–10
Spectral dependence
Blackbody temperatures
<3
Manufacturers’ curves
±5
Short-term instability
2–5
Mirror and lens
<2
Instrumental uncertainties
Photon counting
<3
Correction for optical density
<3
Convolution with instrument function
<2
Xenon spectrum contribution
<2
Total
≥7, ≤14
Tables (8)
Table 1
Summary of Recent Experimental Determinations of the Raman
ν1Q-Branch Cross Section for Water
Vapora
The laser wavelength used in each
determination is given in the second column. For comparison purposes,
the cross section (10-30 cm-1/sr) is
reported for an exciting wavelength of 514 nm. Correction has been
applied for the λR-4 dependence
of the cross section where necessary. For the CARS cross sections, γ
is the collisional air-broadened linewidth at half-maximum in
cm-1/atm.
Ref. 16.
Ref. 31.
Ref. 34.
Ref. 35.
Ref. 36.
Table 2
Summary of Evaluation of Effective Cross Sections and
Associated Errorsa,b
Error Source
Breakdown of Error
Error (%)
H2O effective cross section
Anisotropy
8
Mixed state
3
ν3 Contribution
<2
Normalization
±5
N2 effective cross section
Anisotropy
5
Normalization
±5
Atmospheric and instrumental effects
Temperature dependence
-0.02/K
Line broadening
Monomode
2.0
Multimode
0.5
Instrument function
Random error
<1
Translation ±0.05 nm
-5 + 3
Convolution
-5 + 3
Total error
12
Line-broadening effects are expressed
relative to the dirac delta representation. The total error
calculation is described in the text.
Ratio of effective cross
sections: 0.49 ± 0.05 at 280 K.
Optical depths are given for 550 nm.
Boundary layer aerosol.
Table 4
Characteristic Measures for the Solar Zenith Angle
Dependence on the Ratio of Downwelling Zenith Radiances r(χ)
Modeled Atmosphere Observation Period
[r(25) - r(60)]/r(25) (%)
[r(60) - r(80)]/r(60) (%)
Atmosphere type
Molecular scatter and absorption
≥-3
7–12
Continental
≤3
8–10
Maritime
10
6
Maritime mineral
6
6
Observation period
May 1997
3–7, ≤1
6–7
August–September 1997
≤1
5
October 1997–March 1998
–
5–10
Note: Model results are compared with
observations, highlighting the likelihood of the presence of large
particles (the maritime-mineral class) for three of the five
observations in May 1997.
Table 5
Summary of Errors in the Diffuse Sunlight Determination of
TN2/TH2O
Error Source
Breakdown of Error
Error (%)
Illumination
–
Modeled spectral dependence
Atmospheric density changes
<2
H2O absorption
≤3
O3 absorption
±2
Aerosol properties
10–15
Model dispersion
<2
Solar spectrum
<2
Instrumental uncertainties
Photon counting
<3
Correction for neutral-density filter
<2
Convolution with instrument function
<2
The errors associated with H2O and O3
absorption, atmospheric density changes, and aerosol properties
correspond to the case of the current OHP calibration where these
modeled parameters are not constrained by simultaneous independent
measurements.
The relative detection efficiency deduced
from diffuse sunlight measurements with the bounds for the most
probable values in bold face.
The ratio of molecular nitrogen signals on the
two Raman channels (T/
R)N2 may be
regarded as a proxy for detection efficiency. Measured values of
(T/
R)N2 are given.
Calibration coefficient deduced from diffuse
sunlight measurements when the explicit method is used.
Calibration coefficient deduced from
simultaneous radiosonde measurements over the same period. The number
of soundings are in parentheses.
Table 7
Results for Determination of
TN2
/TH2O
when a Xenon Arc Lampa
is Used
Observation Series
TN2/TH2O (%)
Test Series
TN2/TH2O
(%)
1
1.83 ± 0.09
Unobscured
1.79 ± 0.05
2
1.86 ± 0.07
Partially obscured (left)
1.68 ± 0.03
3
1.90 ± 0.04
Partially obscured (right)
1.78 ± 0.04
4
1.99 ± 0.04
Note: Each observation series corresponds to different
illumination conditions. Values have been ordered from lowest to
highest to aid in comparison. The error in the observed ratios are due
to short time scale intensity fluctuations (see text for details).
In the test series the influence of changes in photocathode
illumination is investigated.
Xenon arc lamp, 75 W.