Atmospheric Measurement Techniques Discussions
www.atmos-meas-tech-discuss.net
1867-8610
3
4
2010
10.5194/amtd-3-3675-2010
http://www.atmos-meas-tech-discuss.net/3/3675/2010/
http://www.atmos-meas-tech-discuss.net/3/3675/2010/amtd-3-3675-2010.html
http://www.atmos-meas-tech-discuss.net/3/3675/2010/amtd-3-3675-2010.pdf
3675
3723
2010-08-23
Absolute accuracy and sensitivity analysis of OP-FTIR retrievals of CO<sub>2</sub>, CH<sub>4</sub> and CO over concentrations representative of ''clean air'' and ''polluted plumes''
T. E. L. Smith
thomas.smith@kcl.ac.uk
M. J. Wooster
M. Tattaris
D. W. T. Griffith
King's College London, Environmental Monitoring and Modelling Research Group, Department of Geography, Strand, London, WC2R 2LS, UK
NERC National Centre for Earth Observation, UK
University of Wollongong, Centre for Atmospheric Chemistry, Wollongong NSW 2522, Australia
When compared to established point-sampling methods, Open-Path Fourier
Transform Infrared (OP-FTIR) spectroscopy can provide path-integrated
concentrations of multiple gases simultaneously, in situ and
near-continuously. Concentrations can be retrieved from the measured
IR spectra using a forward model coupled to a non-linear least squares
fitting procedure, without requiring ''background'' spectral
measurements unaffected by the gases of interest. However, few studies
have investigated the accuracy of such retrievals for CO<sub>2</sub>,
CH<sub>4</sub> and CO, particularly across a broad concentration range
covering ambient to highly polluted air (e.g. from biomass burning or
industrial plumes). Here we perform such an assessment using data
collected by a field-portable FTIR spectrometer. The FTIR was
positioned to view a fixed IR source placed at the other end of an
IR-transparent cell filled with the gases of interest, whose target
concentrations were varied by up to two orders of
magnitude. Retrievals made using the forward model are complicated by
absorption line pressure broadening, the effects of temperature on
absorption band shape and by convolution of the gas absorption lines
and the instrument line shape (ILS). Despite this, with optimal
forward model parameterisation (i.e. the wavenumber range used in the
retrieval, gas temperature, pressure and ILS), concentration
retrievals for all gases were able to be made to within 5% of the
true value. Sensitivity to the aforementioned model inputs was also
investigated. CO retrievals were shown to be most sensitive to the ILS
(a function of the assumed instrument FOV), which is due to the
narrow nature of CO absorption lines and their consequent sensitivity
to convolution with the ILS. Conversely, CO<sub>2</sub> retrievals were
most sensitive to assumed atmospheric parameters, particularly
temperature. The analysis suggests that trace gas concentration
retrieval errors can remain well below 10%, even with the
uncertainties in atmospheric pressure and temperature that might arise
when studying plumes in field situations (e.g. at uncertain altitudes
or temperatures). Our findings provide confidence that FTIR-derived
trace gas retrievals of CO<sub>2</sub>, CH<sub>4</sub> and CO based on
forward modeling can yield accurate results, even over very large
concentration ranges that can prove difficult to retrieve via standard
classical least squares (CLS) techniques.
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