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Absolute accuracy and sensitivity analysis of OP-FTIR retrievals of CO2, CH4 and CO over concentrations representative of "clean air" and "polluted plumes"

Journal Article


Abstract


  • 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.

    The trace gas pathlength amounts 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 CO2, CH4 and CO, particularly

    across broad concentration ranges covering those characteristic

    of 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 more than two orders of magnitude. Retrievals made

    using the 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 careful model

    parameterisation (i.e. the optimum wavenumber range, ILS,

    and assumed gas temperature and pressure for the retrieval),

    concentrations for all target gases were able to be retrieved

    to within 5%. 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

    field-of-view), which is due to the narrow nature of CO

    absorption lines and their consequent sensitivity to convolution

    with the ILS. Conversely, CO2 retrievals were most sensitive

    to assumed atmospheric parameters, particularly gas

    temperature. Our findings provide confidence that FTIRderived

    trace gas retrievals of CO2, CH4 and CO based on

    modeling can yield results with high accuracies, even over

    very large (many order of magnitude) concentration ranges

    that can prove difficult to retrieve via standard classical least

    squares (CLS) techniques. With the methods employed here,

    we suggest that errors in the retrieved trace gas concentrations

    should remain well below 10%, even with the uncertainties

    in atmospheric pressure and temperature that might

    arise when studying plumes in more difficult field situations

    (e.g. at uncertain altitudes or temperatures).

UOW Authors


  •   Smith, T E L. (external author)
  •   Wooster, Martin (external author)
  •   Tattaris, M (external author)
  •   Griffith, David

Publication Date


  • 2011

Citation


  • Smith, T. E.L., Wooster, M. J., Tattaris, M. & Griffith, D. W.T. (2011). Absolute accuracy and sensitivity analysis of OP-FTIR retrievals of CO2, CH4 and CO over concentrations representative of "clean air" and "polluted plumes". Atmospheric Measurement Techniques, 4 (1), 97-116.

Scopus Eid


  • 2-s2.0-79551653299

Ro Metadata Url


  • http://ro.uow.edu.au/scipapers/3061

Has Global Citation Frequency


Number Of Pages


  • 19

Start Page


  • 97

End Page


  • 116

Volume


  • 4

Issue


  • 1

Abstract


  • 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.

    The trace gas pathlength amounts 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 CO2, CH4 and CO, particularly

    across broad concentration ranges covering those characteristic

    of 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 more than two orders of magnitude. Retrievals made

    using the 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 careful model

    parameterisation (i.e. the optimum wavenumber range, ILS,

    and assumed gas temperature and pressure for the retrieval),

    concentrations for all target gases were able to be retrieved

    to within 5%. 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

    field-of-view), which is due to the narrow nature of CO

    absorption lines and their consequent sensitivity to convolution

    with the ILS. Conversely, CO2 retrievals were most sensitive

    to assumed atmospheric parameters, particularly gas

    temperature. Our findings provide confidence that FTIRderived

    trace gas retrievals of CO2, CH4 and CO based on

    modeling can yield results with high accuracies, even over

    very large (many order of magnitude) concentration ranges

    that can prove difficult to retrieve via standard classical least

    squares (CLS) techniques. With the methods employed here,

    we suggest that errors in the retrieved trace gas concentrations

    should remain well below 10%, even with the uncertainties

    in atmospheric pressure and temperature that might

    arise when studying plumes in more difficult field situations

    (e.g. at uncertain altitudes or temperatures).

UOW Authors


  •   Smith, T E L. (external author)
  •   Wooster, Martin (external author)
  •   Tattaris, M (external author)
  •   Griffith, David

Publication Date


  • 2011

Citation


  • Smith, T. E.L., Wooster, M. J., Tattaris, M. & Griffith, D. W.T. (2011). Absolute accuracy and sensitivity analysis of OP-FTIR retrievals of CO2, CH4 and CO over concentrations representative of "clean air" and "polluted plumes". Atmospheric Measurement Techniques, 4 (1), 97-116.

Scopus Eid


  • 2-s2.0-79551653299

Ro Metadata Url


  • http://ro.uow.edu.au/scipapers/3061

Has Global Citation Frequency


Number Of Pages


  • 19

Start Page


  • 97

End Page


  • 116

Volume


  • 4

Issue


  • 1