Improved Lyman-alpha hygrometer for small-scale atmospheric turbulence measurements. Part I: Miniaturizing the sampling volume

Abstract

An improved Lyman‐alpha hygrometer has been developed in order to measure high‐frequency small‐scale fluctuations of water vapor concentration in the lower atmosphere and in tunnels (e.g., IMST large air–sea interaction simulation tunnel). The sampling volume is reduced to 3 mm in diameter by aerodynamic shaping; the signal‐to‐noise ratio is largely increased by designing special UV sources and using a photomultiplier connected to a low‐noise electrometer; parasitic sensitivities are reduced by a precise control of the variable path length and by filtering the UV beam with an oxygen cell. This article describes the problems and improvements related to reducing the sampling volume and presents some characteristic spectral functions obtained in IMST tunnel, up to 200 Hz. A further article will describe the O2 filtering technique, the control of sensitivity, and the data reduction procedures.