Rejection criteria for open-path Fourier transform infrared spectrometry during continuous atmospheric monitoring

Abstract

Over 32,000 interferograms measured during open-path Fourier transform infrared (OP/FT-1R) measurements at dairy and hog farms were evaluated for anomalies. Five types of anomalies could be distinguished: a reduction in the interferogram intensity because of weather-related optical misalignment; an increase in the amplitude of interferograms measured with too short a path-length that leads to a non-linear detector response; a periodic interference caused by wind-induced vibrations; the presence of spikes in the interferogram; and an increase in the noise level of the interferogram (and hence of the spectrum) because of the effect of electrical interference. Prior to testing for the presence of anomalous data, each interferogram is subjected to a high-pass filter. A noise level index is then calculated from the wings of the interferogram and interferograms are rejected if the value of this parameter is too high. When the criteria developed in this project are applied, OP/FT-1R spectra may be measured at 1-min intervals over a period of several days.

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

“…For the full data set of 227 control scans, the column median is 8.24 ϫ 10 21 molecules, with 75% of the retrieved columns falling between Ϫ1.2% and 0.9% of the median value. In contrast, the reweighted data have a median column retrieval of 8.25 ϫ 10 21 molecules͞cm 2 , with 75% of the data falling within Ϫ0.3% and 0.4% of the median. For scans taken with solar intensity variation less than 10%, the control data show a spread between Ϫ0.6% and 0.7% of the median, while for the reweighted spectra, the spread is between Ϫ0.03% and 0.4% of the median.…”

“…Knowledge of this DC intensity can then be used to correct for slow variations in source intensity as an interferogram is obtained. This is a fundamentally different approach than that of Shao et al, who attempt to identify and reject interferograms affected by SBFs [2].…”

Application of open-path Fourier transform infrared spectroscopy (OP-FTIR) to measure greenhouse gas concentrations from agricultural fields

“…The IFGs and corresponding SB spectra were influenced by ambient factors that included wind-derived vibrations, scintillation induced by air mixing, water vapour content, dust accumulation, and condensation on the retroreflector. Criteria of quality assurance were based on the inspection of the IFG and SB spectra, following the standard guideline in the MIDAC instrumentation manual and the FTIR open-path monitoring guidance documents (Russwurm and Childers, 1996) with the supplement criteria published by Childers et al (2001b) and Shao et al (2007) to acquire high-quality spectra. The maximum and minimum of the IFG centre bursts were controlled between approximately 0.61 and 1.14 V of the analogue-to-digital converter based on the physical path length of 150 m. Any IFG centre-burst signals > 2.25 V were rejected to avoid a non-linear response of the MCT detector.…”

“…Also, soils play the role of a sink or a source for atmospheric CO 2 Smith et al, 2008). Changing land use of crop production, especially agriculture-related uses such as tile drainage and tillage man-agement, and agricultural lime application (e,g., CaCO 3 and MgCa(CO 3 ) 2 ) potentially become a large source of CO 2 released to the atmosphere via microbial decomposition of soil organic carbon (Smith, 2004;IPCC, 2007;Cole et al, 1997;West et al, 2005).…”

Completely automated open-path FT-IR spectrometry

“…Any interferogram for which the ratio of the signal at the centerburst to the root-mean-square noise level in the wings of the interferogram is less than a certain threshold value is rejected. The percentage of interferograms that are rejected is usually small except under very adverse weather conditions but the confidence in the analytical data in those interferograms that are retained is significantly increased [16].…”