Source brightness fluctuation correction of solar absorption fourier transform mid infrared spectra

Abstract. The precision and accuracy of trace gas observations using solar absorption Fourier Transform infrared spectrometry depend on the stability of the light source. Fluctuations in the source brightness, however, cannot always be avoided. Current correction schemes, which calculate a corrected interferogram as the ratio of the raw DC interferogram and a smoothed DC interferogram, are applicable only to near infrared measurements. Spectra in the mid infrared spectral region below 2000 cm−1 are generally considered uncorrectable, if they are measured with a MCT detector. Such measurements introduce an unknown offset to MCT interferograms, which prevents the established source brightness fluctuation correction. This problem can be overcome by a determination of the offset using the modulation efficiency of the instrument. With known modulation efficiency the offset can be calculated, and the source brightness correction can be performed on the basis of offset-corrected interferograms. We present a source brightness fluctuation correction method which performs the smoothing of the raw DC interferogram in the interferogram domain by an application of a running mean instead of high-pass filtering the corresponding spectrum after Fourier transformation of the raw DC interferogram. This smoothing can be performed with the onboard software of commercial instruments. The improvement of MCT spectra and subsequent ozone profile and total column retrievals is demonstrated. Application to InSb interferograms in the near infrared spectral region proves the equivalence with the established correction scheme.

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Source brightness fluctuation correction of solar absorption Fourier Transform mid infrared spectra

Atmospheric Measurement Techniques Discussions ◽

10.5194/amtd-4-443-2011 ◽

2011 ◽

Vol 4 (1) ◽

pp. 443-459

Author(s):

T. Ridder ◽

T. Warneke ◽

J. Notholt

Keyword(s):

Spectral Region ◽

Near Infrared ◽

Trace Gas ◽

Solar Absorption ◽

Mid Infrared ◽

Infrared Spectral Region ◽

Infrared Spectral ◽

Brightness Fluctuation ◽

Precision And Accuracy ◽

Total Column

Abstract. Solar absorption Fourier Transform infrared spectrometry is considered a precise and accurate method for the observation of trace gases in the atmosphere. The precision and accuracy of such measurements are dependent on the stability of the light source. Fluctuations in the source brightness reduce the precision and accuracy of the trace gas concentrations, but cannot always be avoided. Thus, a strong effort is made within the community to reduce the impact of source brightness fluctuations by applying a correction on the spectra following the measurements. So far, it could be shown that the precision and accuracy of CO2 total column concentrations could be improved by applying a source brightness fluctuation correction to spectra in the near infrared spectral region. The analysis of trace gas concentrations obtained from spectra in the mid infrared spectral region is fundamental. However, spectra below 2000 cm−1 are generally considered uncorrectable, if they are measured with a MCT detector. Such measurements introduce an unknown offset to MCT interferograms, which prevents a source brightness fluctuation correction. Here, we show a method of source brightness fluctuation correction, which can be applied on spectra in the whole infrared spectral region including spectra measured with a MCT detector. We present a solution to remove the unknown offset in MCT interferograms allowing MCT spectra for an application of source brightness fluctuation correction. This gives an improvement in the quality of MCT spectra and we demonstrate an improvement in the retrieval of O3 profiles and total column concentrations. For a comparison with previous studies, we apply our source brightness fluctuation correction method on spectra in the near infrared spectral region and show an improvement in the retrieval of CO2 total column concentrations.

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Determination of Wavelength Accuracy in the Near-Infrared Spectral Region Based on NIST's Infrared Transmission Wavelength Standard SRM 1921

Applied Spectroscopy ◽

10.1366/0003702001949573 ◽

2000 ◽

Vol 54 (3) ◽

pp. 450-455 ◽

Cited By ~ 5

Author(s):

Stephen R. Lowry ◽

Jim Hyatt ◽

William J. McCarthy

Keyword(s):

Reference Material ◽

Standard Reference Material ◽

Spectral Region ◽

Near Infrared ◽

Cost Effective ◽

Infrared Transmission ◽

Internal Reference ◽

Mid Infrared ◽

Infrared Spectral Region ◽

Infrared Spectral

A major concern with the use of near-infrared (NIR) spectroscopy in many QA/QC laboratories is the need for a simple reliable method of verifying the wavelength accuracy of the instrument. This requirement is particularly important in near-infrared spectroscopy because of the heavy reliance on sophisticated statistical vector analysis techniques to extract the desired information from the spectra. These techniques require precise alignment of the data points between the vectors corresponding to the standard and sample spectra. The National Institute of Standards and Technology (NIST) offers a Standard Reference Material (SRM 1921) for the verification and calibration of mid-infrared spectrometers in the transmittance mode. This standard consists of a 38 μm-thick film of polystyrene plastic. While SRM 1921 works well as a mid-infrared standard, a thicker sample is required for use as a routine standard in the near-infrared spectral region. The general acceptance and proven reliability of polystyrene as a standard reference material make it a very good candidate for a cost-effective NIR standard that could be offered as an internal reference for every instrument. In this paper we discuss a number of the parameters in a Fourier transform (FT)-NIR instrument that can affect wavelength accuracy. We also report a number of experiments designed to determine the effects of resolution, sample position, and optics on the wavelength accuracy of the system. In almost all cases the spectral reproducibility was better than one wavenumber of the values extrapolated from the NIST reference material. This finding suggests that a thicker sample of polystyrene plastic that has been validated with the SRM 1921 standard would make a cost-effective reference material for verifying wavelength accuracy in a medium-resolution FT-NIR spectrometer.

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Long-term intercomparison of MIPAS additional species ClONO2, N2O5, CFC-11, and CFC-12 with MIPAS-B measurements

Annals of Geophysics ◽

10.4401/ag-6329 ◽

2014 ◽

Author(s):

Gerald Wetzel ◽

Hermann Oelhaf ◽

Felix Friedl-Vallon ◽

Anne Kleinert ◽

Guido Maucher ◽

...

Keyword(s):

Fourier Transform ◽

Spectral Region ◽

Michelson Interferometer ◽

Additional Species ◽

Mid Infrared ◽

Infrared Spectral Region ◽

Atmospheric Sounding ◽

Infrared Spectral

The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) aboard the environmental satellite ENVISAT is a limb-viewing Fourier-transform emission spectrometer working in the mid-infrared spectral region between 685 cm-1 and 2410 cm-1 [...]

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