Transmission quantification of open path FTIR spectra with temperature compensation

摘要

A transmission quantification approach that is effective at quantifying the concentration of key atmospheric gases, including water vapor and methane, does not require a background spectrum and is immune to changes between background and absorbance spectra. By using local minima and maxima in transmission of a target gas, this approach creates two spectral arrays as long as a single beam input spectra. One of these spectral arrays represents the points in wave-number space that are less absorbing points, and the other represents the more absorbing points. A concentration for a given gas is calculated by determining what reference concentration creates a residual after division by a pure gas spectrum that forces these two arrays to converge.

主权项

1. A method for determining, using a computing device, an actual concentration of a gas in a path, the method comprising the steps of:
coupling an output from a spectrometer to the computing device; storing on the computing device, reference transmittance data for at least one concentration of the gas and a sample input signal corresponding to an analytical spectrum (IA) of light that has propagated along the path, the analytical spectrum corresponding to light absorbed along the path; dividing, using the computing device, the analytical spectrum by the reference transmittance data to determine a plurality of residual spectra, including a residual spectrum for each concentration in the reference transmittance data; separating, using the computing device, each of the residual spectra into absorbing and non-absorbing residual portions; and identifying, out of the plurality of residual spectra, a residual spectra with a minimum difference between absorbing and non-absorbing portions; and determining, from the residual spectra with the minimum difference and without a background spectrum I0, the actual concentration of the gas in the path.