Atmospheric Measurement Techniques
www.atmos-meas-tech.net
1867-1381
1867-8548
2
2
2009
10.5194/amt-2-713-2009
http://www.atmos-meas-tech.net/2/713/2009/
http://www.atmos-meas-tech.net/2/713/2009/amt-2-713-2009.html
http://www.atmos-meas-tech.net/2/713/2009/amt-2-713-2009.pdf
713
723
2009-11-16
Broadband Cavity Enhanced Differential Optical Absorption Spectroscopy (CE-DOAS) – applicability and corrections
U. Platt
ulrich.platt@iup.uni-heidelberg.de
J. Meinen
D. Pöhler
T. Leisner
Inst. for Environmental Physics (IUP), Atmosphere and Remote Sensing, Ruprecht-Karls-Universität Heidelberg, Germany
Institute for Meteorology and Climate Research, Aerosols and Heterogeneous Chemistry in the Atmosphere (IMK-AAF), Forschungszentrum Karlsruhe GmbH, Germany
Atmospheric trace gas measurements by cavity assisted long-path absorption
spectroscopy are an emerging technology. An interesting approach is the
combination of CEAS with broadband light sources, the broadband CEAS
(BB-CEAS). BB-CEAS lends itself to the application of the DOAS technique to
analyse the derived absorption spectra. While the DOAS approach has enormous
advantages in terms of sensitivity and specificity of the measurement, an
important implication is the reduction of the light path by the trace gas
absorption, since cavity losses due to absorption by gases reduce the
quality (Q) of the cavity. In fact, at wavelength, where the quality of the
BB-CEAS cavity is dominated by the trace gas absorption (especially at very
high mirror reflectivity), the average light path will vary nearly inversely
with the trace gas concentration and the strength of the band will become
only weakly dependent on the trace gas concentration c in the cavity, (the
differential optical density being proportional to the logarithm of the
trace gas concentration). Only in the limiting case where the mirror
reflectivity determines Q at all wavelength, the strength of the band as
seen by the CE-DOAS instrument becomes directly proportional to the
concentration <I>c</I>. We investigate these relationships in detail and present
methods to correct for the cases between the two above extremes, which are
of course the important ones in practice.
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