Improved Lyman-alpha hygrometer for small-scale atmospheric turbulence measurements. Part II: The O2 filtering technique

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Abstract
The detector of the IMST/SA-CNRS improved Lyman-alpha hygrometer is a solar-blind photomultiplier tube that ensures large signal-to-noise ratio and allows a miniaturization of the sampling volume. This PM tube has a relatively large spectral response and can detect, in addition to the Lyman-alpha line, hydrogen molecular transitions emitted by the discharge source between 120 and 180 nm. In order to avoid a spurious sensitivity to oxygen density fluctuations, an active filtering cell is added to absorb the undesired lines. The instrument sensitivity is calculated with the help of a two-gas, three-spectral-line UV-beam absorption model, fitted to calibration measurements. The technique of the active filtering associated with the three-line absorption model produces the following advantages: (1) reduced sensitivity to oxygen fluctuations, (2) a clear physical meaning to the model terms allowing the sensitivity computation in most conditions, (3) increased signal-to-noise ratio, and (4) in most conditions, improved sensitivity to water vapor compared to the monochromatic Lyman-alpha line.

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