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

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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

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CAVIAR meetingApril, 30, 2010 – UCL, London

Further work

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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