CAVIAR meeting May, 13, 2010 – UCL, London 1/11 Cambridge Centre for Atmospheric Science Broad Band Cavity Enhanced Absorption Spectroscopy of the Water.

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

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Further work

CAVIAR meetingMay, 13, 2010 – UCL, London