Reducing the impact of source brightness fluctuations on spectra obtained by Fourier-transform spectrometry

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.

The Total Carbon Column Observing Network

“…Room-temperature InGaAs and Si detectors are available with the 125HR that cover the entire 3900-15 500 cm −1 spectral region simultaneously. These detectors are supported by custom electronics that allow for solar intensity variations during data acquisition to be corrected using the method described by Keppel-Aleks et al [16], resulting in higher-quality spectra and less data loss during partly cloudy conditions (see appendix A(b) for further details). The spectra are recorded with 45 cm maximum optical path difference (OPD) (approx.…”

“…The 24 bit ADC has sufficient dynamic range to allow the interferogram to be digitized without gain switching. The electronics also allow for DC recording of the detector signals, so that the Keppel-Aleks et al [16] algorithm can be applied (appendix A(b)). The signal-to-noise ratio (SNR) for a single spectrum (75 s acquisition) is approximately 750 near 5000 cm −1 .…”

“…Keppel-Aleks et al [16] developed and implemented a method of correcting for solar intensity variations. They divide the interferogram by the unmodulated DC detector signal to restore the interferogram fringes to their correct amplitudes at each OPD.…”

“…In the case of TCCON data, the unmodulated DC detector signal comes from smoothing the DC-coupled detector signals. Keppel-Aleks et al [16] show that, on partly cloudy days, this scheme results in a substantial reduction in the scatter of the retrieval results. For interferograms containing less than 10 per cent solar intensity variation, the standard deviation of the retrieved O 2 columns between scans is reduced from 2.5 to 0.25 per cent.…”

Addition of a channel for XCO observations to a portable FTIR spectrometer for greenhouse gas measurements

“…This process takes advantage of our accurate knowledge of the dry-air mole fraction of molecular oxygen and helps to reduce various error sources . In addition, the TCCON uses DC-coupled interferograms for improving the precision of the measurements and for quality flagging (Keppel-Aleks et al, 2007), which is also used for the portable spectrometers. The main difference is that the spectral resolution of the portable spectrometer is much lower: it provides double-sided interferograms with a maximum optical path difference (OPDmax) of 1.8 cm, while the TCCON applies 45 cm or higher OPDmax.…”