An advanced theory for multi-wave open path optical absorption spectroscopy measurements

Abstract

A new version of the multi-wave open path optical absorption spectroscopy method is herewith presented for continuous measurements of multiple airborne pollutants along an open light path. It involves a standard least squares procedure that minimizes deviations of the calculated optical thickness of the monitoring path from the measured thickness. A novel feature of the proposed method consists of interpreting the influence of error sources as a discrepancy between sounding spectra, emitting to the reference and working paths, produced by these very errors. It is shown that this discrepancy results in a systematic error in the calculated trace gas concentrations, and its elimination transforms the optical open path meter into an absolute meter. The advantages of the proposed approach are revealed when estimating the role of inaccuracies in signal measurements, wavelength scale lock-in, working wavelength setting, stability of the source of radiation, and inaccuracies in the parameters of the calculating method. The special benefit of the proposed multi-wave optical absorption spectroscopy method lies in using an excess of working wavelengths for reducing the influence of various error sources.