CAVIAR meeting May, 13, 2010 – UCL, London 1/11 ambridge Centre for Atmospheric Science Broad Band Cavity Enhanced Absorption Spectroscopy of the Water Vapour Continuum in the Near-IR Prof. R. Jones, Dr. A. Shillings, A. Wilson, Dr. V. Tsanev,
Mar 28, 2015
CAVIAR meetingMay, 13, 2010 – UCL, London 1/11
Cambridge Centre for Atmospheric Science
Broad Band Cavity Enhanced Absorption Spectroscopy
of the Water Vapour Continuum in the Near-IR
Prof. R. Jones, Dr. A. Shillings, A. Wilson, Dr. V. Tsanev,
Experimental set-up
The Broadband Cavity Enhanced Absorption Spectroscopy (BBCEAS) experimental
set-up for the 1230-1380 nm region has been advanced by:
measuring directly the mirror reflectivity
wavelength and FWHM calibration using neon spectral lamp
minimization of the stray light by selection and adjustment of f-matching of optical
fibres
minimization of light leaks in the system to improve measurement of “pure” dark
current
accounting for time variations of the dark current
accounting for time variations of the light source intensity
accounting for a ‘detuned’ cavity signal (origin unclear)
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Experimental set-up
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Experimental results
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Experimental results
Analysis
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1
1
1
R
ODe ODcav
cav
R
lX
I
Io
1
)(1)(
)( lX
I
Io )(1)(
)(
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Analysis
I2
I1
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Analysis
1)(
)(
I
Io
Analysis
Plot comparing predicted RH with measured RH
0
50
100
150
200
250
300
0 25 50 75 100
Measured RH/ %
Fit
ted
RH
/ %
0
2
4
6
8
10
12
14
16
18
Fit
ted
[H
2O
] x
10-1
7/ m
ole
cule
scm
-3
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The system has been utilized to measure with resolution 1.6 nm the water vapour
optical depth profiles at room temperature and pressure and monomer concentra-
tions between 1.31×1017 and 5.42×1017 molecules cm-3 (RH 20-95%).
Measurements were made between 1120 and 1380 nm a region of weak water
monomer absorption, where the semi-empirical MT_CKD model uncertainty is
associated with the limited experimental data available.
The comparison of the experimental optical depth profiles with corresponding
theoretical models, calculated using spectral data from HITRAN and the
MT_CKD continuum model, revealed the presence of a weak, smoothly-varying
continuum absorption profile superposed upon the dominant monomer absorption
features.
The measured optical depth profiles are reproducible but they are
systematically inconsistent with those derived theoretically.
Summary of results
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CAVIAR meetingMay, 13, 2010 – UCL, London
The feature work is aimed to
Rationalise the inconsistency between experiment and model
Expand experiment towards higher temperatures.
Extend measurements to 1000 to 1100 nm region
Future work
11/11
Further work
CAVIAR meetingMay, 13, 2010 – UCL, London