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

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, i...