Abstract

We present a method to reduce the impact of source brightness fluctuations (SBFs) on spectra recorded by Fourier-transform spectrometry (FTS). Interferograms are recorded without AC coupling of the detector signal (DC mode). The SBF are determined by low-pass filtering of the DC interferograms, which are then reweighted by the low-pass, smoothed signal. Atmospheric solar absorption interferograms recorded in DC mode have been processed with and without this technique, and we demonstrate its efficacy in producing more consistent retrievals of atmospheric composition. We show that the reweighting algorithm improves retrievals from interferograms subject to both gray and nongray intensity fluctuations, making the algorithm applicable to atmospheric data contaminated by significant amounts of aerosol or cloud cover.

© 2007 Optical Society of America

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References

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  1. N. H. E. Ahlers and H. P. Freedman, "A simple ratio-recording spectrometer," J. Sci. Instrum. 32, 61-64 (1955).
    [CrossRef]
  2. L. Shao, M. J. Pollard, P. R. Griffiths, D. T. Westermann, and D. L. Bjorneberg, "Rejection criteria for open-path Fourier-transform infrared spectrometry during continuous atmospheric monitoring," Vib. Spectrosc. 43, 75-85 (2007).
    [CrossRef]
  3. J. W. Brault, "Fourier-transform spectrometry," in High Resolution in Astronomy, A.O.Benz, M.C. E.Huber, and M.Mayor, eds. (Swiss Society of Astrophysics and Astronomy, 1985), pp. 3-62.
  4. M. J. Kurylo and S. Solomon, Network for the Detection of Stratospheric Change, NASA report, Code EEU (NASA, 1990).
  5. R. A. Washenfelder, G. C. Toon, J.-F. Blavier, Z. Yang, N. T. Allen, P. O. Wennberg, S. A. Vay, D. M. Matross, and B. C. Daube, "Carbon dioxide column abundances at the Wisconsin Tall Tower site," J. Geophys. Res. 111, D22305 (2006).
    [CrossRef]
  6. "The Total Carbon Column Observing Network," http://tccon.caltech.edu.

2007

L. Shao, M. J. Pollard, P. R. Griffiths, D. T. Westermann, and D. L. Bjorneberg, "Rejection criteria for open-path Fourier-transform infrared spectrometry during continuous atmospheric monitoring," Vib. Spectrosc. 43, 75-85 (2007).
[CrossRef]

2006

R. A. Washenfelder, G. C. Toon, J.-F. Blavier, Z. Yang, N. T. Allen, P. O. Wennberg, S. A. Vay, D. M. Matross, and B. C. Daube, "Carbon dioxide column abundances at the Wisconsin Tall Tower site," J. Geophys. Res. 111, D22305 (2006).
[CrossRef]

1955

N. H. E. Ahlers and H. P. Freedman, "A simple ratio-recording spectrometer," J. Sci. Instrum. 32, 61-64 (1955).
[CrossRef]

J. Geophys. Res.

R. A. Washenfelder, G. C. Toon, J.-F. Blavier, Z. Yang, N. T. Allen, P. O. Wennberg, S. A. Vay, D. M. Matross, and B. C. Daube, "Carbon dioxide column abundances at the Wisconsin Tall Tower site," J. Geophys. Res. 111, D22305 (2006).
[CrossRef]

J. Sci. Instrum.

N. H. E. Ahlers and H. P. Freedman, "A simple ratio-recording spectrometer," J. Sci. Instrum. 32, 61-64 (1955).
[CrossRef]

Vib. Spectrosc.

L. Shao, M. J. Pollard, P. R. Griffiths, D. T. Westermann, and D. L. Bjorneberg, "Rejection criteria for open-path Fourier-transform infrared spectrometry during continuous atmospheric monitoring," Vib. Spectrosc. 43, 75-85 (2007).
[CrossRef]

Other

J. W. Brault, "Fourier-transform spectrometry," in High Resolution in Astronomy, A.O.Benz, M.C. E.Huber, and M.Mayor, eds. (Swiss Society of Astrophysics and Astronomy, 1985), pp. 3-62.

M. J. Kurylo and S. Solomon, Network for the Detection of Stratospheric Change, NASA report, Code EEU (NASA, 1990).

"The Total Carbon Column Observing Network," http://tccon.caltech.edu.

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Figures (6)

Fig. 1
Fig. 1

(a) Raw DC interferogram, I raw , with 14% solar intensity variation; (b) smoothed, low-pass signal, I smooth ; (c) corresponding reweighted interferogram, I corr .

Fig. 2
Fig. 2

Comparison between CO2 retrievals from reweighted (dark circles) and the raw control (light squares) spectra as a function of relative solar intensity variation during a 90 s scan. The lower panel zooms in on relative solar intensity variation below 10%.

Fig. 3
Fig. 3

Percent difference between CO 2 retrievals from spectra obtained from reweighted and control interferograms with solar intensity variation less than 3%.

Fig. 4
Fig. 4

Time series of the oxygen mixing ratio retrieved from spectra at Darwin, Australia. Prior to November 2005, AC data acquisition was used. Plotted here are data obtained between 20° and 70° solar zenith angles. Only spectra obtained with relative solar variation less than 20% are shown.

Fig. 5
Fig. 5

Spectrum distorted with nongray interference of optical depth ranging from 0.0 (original spectrum) to 1.25 at 15,750 cm 1 (panel A). The spectra are inverse Fourier transformed to yield the interferograms in panel B. We interpolate between these interferograms to generate the interferogram in panel C, which exhibits spectrally and temporally dependent attenuation compared with the original interferogram. The attenuated interferogram is reweighted using the DC correction to yield the interferogram in panel D. We then transform each interferogram to yield the control spectrum in panel E and the reweighted spectrum in panel F. Although the spectra in E and F appear identical, the line depths in the uncorrected spectrum (E) are more shallow than those in panel F.

Fig. 6
Fig. 6

(a) Center burst of raw interferogram. (b) Low-pass signal resulting from a filter with s = 3000 cm 1 . The observed dip at ZPD is likely due to detector nonlinearity. (c) Low-pass interferogram resulting from filter with s = 300 cm 1 . The dip at ZPD has been smoothed away in lowering the frequency cutoff.

Tables (1)

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Table 1 Retrieval Errors [%] from Spectra Distorted by Nongray SBF

Equations (2)

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F = { ( 1 + cos ( π ν / s ) 2 ) N , if   ν < s 0 , if   ν > s .
I c o r r ( x ) = I raw ( x ) I smooth ( x ) I smooth ( ZPD ) .

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