An Open Path, Fast Response IR Spectrometer for Simultaneous Detection of C02 and Water Vapor Fluctuations

Abstract

A fast response C02 and water vapor (H2O) analyzer was developed in this study for the measurement of atmospheric turbulence fluctuations and, in conjunction with a fast response anemometer, transport of these entities. High speed and high resolution detection of C02 and H2O was accomplished simultaneously for the same air sample with open path IR absorption techniques. The miniature size sensor features a once folded 12.5 cm pathlength. The sensor structure and sampling technique provided direct compensation for undesirable factors (e.g., dust) affecting transmission of IR-radiation in the optical path and for the changes in source intensity and detector response due to sensor temperature fluctuations. The rms-noise level of the C02 and H2O outputs corresponded to 0.3 ppm and 0.02 g m?2 fluctuations respectively, when absorbing gases were removed from the path. Spectral analysis showed that the noise was well below the signal level for both gases within the frequency range from 0.002 to 5 Hz. In comparison with the commercially available Lyman-α hygrometer the H2O output noise level of the IR-sensor was a factor of two larger, but the simultaneously measured signal spectra and also the latent heat flux values obtained with the two sensors in the field were practically identical. The IR-sensor produced C02 flux values that were comparable to those reported in the literature.