CAVIAR meeting April, 30, 2010 – UCL, London 1/16 Cambridge Centre for Atmospheric Science Broad...
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Transcript of CAVIAR meeting April, 30, 2010 – UCL, London 1/16 Cambridge Centre for Atmospheric Science Broad...
CAVIAR meetingApril, 30, 2010 – UCL, London 1/16
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,
CAVIAR meetingApril, 30, 2010 – UCL, London
Summary
An optimized Broadband Cavity Enhanced Absorption Spectroscopy (BBCEAS) experimental set-up has been developed and utilized to measure water vapour
optical depth profiles at room temperature and pressure over a representative range of monomer concentrations between 1.31 × 1017 and 5.42 × 1017 molecules cm-3. These measurements were made between 1120 and 1380 nm a region of weak
water monomer absorption, where a large uncertainty is currently associated with the semi-empirical MT_CKD continuum model, on account of 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 were systematically
inconsistent with those derived theoretically – about three time larger.
The feature work is aimed to break down this inconsistency and to expand experiment towards higher temperatures.
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CAVIAR meetingApril, 30, 2010 – UCL, London
Experimental set-up
3/16
CAVIAR meetingApril, 30, 2010 – UCL, London
Advancement of the experimental set-up
selecting proper mirror reflectivity
wavelength and FWHM calibration – neon spectral lamp
minimization of the stray light – adjustment of the f-matcher
light tightening of the whole system – now we can estimate the real dark current
accounting for time variations of the dark current
accounting for time variations of the light source
accounting for detuned cavity signal
measurement of mirror reflectivity
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CAVIAR meetingApril, 30, 2010 – UCL, London
Advancement of the experimental set-up
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CAVIAR meetingApril, 30, 2010 – UCL, London
Advancement of the experimental set-up
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CAVIAR meetingApril, 30, 2010 – UCL, London
Advancement of the experimental set-up
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CAVIAR meetingApril, 30, 2010 – UCL, London
Experimental procedure
A sequence of consecutive measurements were used aiming to collect data
Neon 1 – Neon Dark 1 - Detuned F1 - D1 – F1 – I1 – I2 – D2 – I3 – I4 – D3 –I5 – I6 – D4 – I7 – I8 – D5…(n-1) – F2 – Dn – Detuned F2 - Neon 2 – Neon Dark 2
where the water signals I1,8, I2,7, I3,6 and I4,5 were recorded with a wet air flow rate of 1.50 slm and dry air flow rates of 4.50, 1.50, 0.50, and 0.00 slm respectively.
These four dry air flow rates were used to generate a range of RHs within the absorption cell, and hence a representative range of H2O concentrations. A number of dark spectra were recorded between D5 and D(n-1) in order to allow sufficient time for the absorption cell to re-stabilize at RH = 0.0% before recording F2 and Det.F2.
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CAVIAR meetingApril, 30, 2010 – UCL, London
Experimental procedure
Exp. Run
RH/ %
T/ °CI1 I2 I3 I4 I5 I6 I7 I8
1 21.6 45.7 70.2 90.2 92.6 73.1 45.9 22.2 20.5
2 21.7 44.7 69.9 89.2 92.3 74.9 46.4 22.4 19.4
3 21.4 43.9 69.0 88.3 91.1 70.7 44.5 21.6 22.7
4 24.9 47.5 73.5 90.4 92.0 73.1 46.7 23.0 22.0
5 22.2 46.2 71.4 89.5 91.8 72.7 45.0 21.5 20.2
6 21.6 45.3 71.8 89.1 91.9 76.0 45.8 22.4 20.1
Mean 22.2 45.6 71.0 89.5 92.0 73.4 45.7 22.2 20.8
Relative humidity, mean H2O concentration and temperature measured inside the absorption cell during each experimental run.
Mean Water Content I1 I2 I3 I4 I5 I6 I7 I8
[H2O] × 10-17/ molecules cm-3 1.31 2.69 4.18 5.27 5.42 4.33 2.69 1.31
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CAVIAR meetingApril, 30, 2010 – UCL, London
Analysis results
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CAVIAR meetingApril, 30, 2010 – UCL, London 11/16
CAVIAR meetingApril, 30, 2010 – UCL, London
Analysis results
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CAVIAR meetingApril, 30, 2010 – UCL, London
Analysis results
I2
I1
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CAVIAR meetingApril, 30, 2010 – UCL, London
Analysis results
I4
I5
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CAVIAR meetingApril, 30, 2010 – UCL, London
Analysis results
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
15/16
CAVIAR meetingApril, 30, 2010 – UCL, London
Further work
16/16
Experiments at higher temperatures
Experiment within spectral range 930-1200 nm
Measurement of mirror relativities by means PS CRDS
Accuracy of the retrieved monomer concentrations
Understanding the nature of detuned signal
CAVIAR meetingApril, 30, 2010 – UCL, London
Further work
CAVIAR meetingApril, 30, 2010 – UCL, London