The limits of signal averaging in atmospheric trace-gas monitoring by tunable diode-laser absorption spectroscopy (TDLAS)
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Modern research in atmospheric chemistry requires highly sensitive techniques for the measurement of concentrations of free radicals which determine the rate of photochemical destruction of most atmospheric pollutants. Tunable diode-laser absorption spectroscopy (TDLAS) has already been successfully used for measurements of very low concentrations of stable gases, but further improvement in its sensitivity by signal averaging has been limited by the stability of the instrument. In this paper the concept of the Allan variance is utilized to analyze the stability of an existing frequency-modulated (FM) TDLAS instrument leading to a detection limit for NO2 of 34 pptv at 6 Hz detection bandwidth. The stability of the instrument allows averaging over 60 s. Taking into account the measuring cycle consisting of the determination of the sample spectra and zero air spectra as well as gas exchange in the absorption cell, the detection limit achievable with this particular instrument was 10 pptv within 25 s under laboratory conditions. Possibilities of further improvement of the detection limit are discussed.
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