Contactless optical hygrometry in LACIS-T

Abstract

Abstract. The Fast Infrared Hygrometer (FIRH), employing open-path tunable diode laser absorption spectroscopy at the wavelengths near the 1364.6896 nm line, was adapted to perform contactless humidity measurements at the Turbulent Leipzig Aerosol Cloud Interaction Simulator (LACIS-T), a unique turbulent moist-air wind tunnel. The configuration of the setup allows for scanning from outside the walls of the wind tunnel and at various positions without the need for repeated optics adjustments. We identified three factors which significantly influence the measurement – self-broadening of the absorption line, interference in the glass windows and parasitic absorption in the ambient air outside the wind tunnel – and developed correction methods which satisfactorily account for these effects. The comparison between FIRH and a reference hygrometer (dew-point mirror MBW 973) indicated a good agreement within the expected errors across the wide range of water vapour concentration 1.0–6.1×1017 cm−3 (equivalent to dew-point temperature of −5.4 to +21 ∘C at the temperature of 23 ∘C). High temporal resolution (∼2 kHz) allowed for studying turbulent fluctuations in the course of intensive mixing of two air streams which had the same mean velocity but differed in temperature and humidity, also including the settings for which the mixture can be supersaturated. The obtained results contribute to improved understanding and interpretation of cloud formation studies conducted in LACIS-T by complementing the previous characterizations of turbulent velocity and temperature fields inside the wind tunnel.

Aerosol, Clouds and Trace Gases Research Infrastructure (ACTRIS): The European Research Infrastructure Supporting Atmospheric Science

“…Many have also been performed in simulation chambers under well-characterized and controllable conditions. This was the case for ACTRIS variables, such as organic compounds (Apel et al, 2008;Wisthaler et al, 2008;Glowania et al, 2021), NO2 (Fuchs et al, 2010a), ice nucleating particles (DeMott et al, 2018), and more specialized instruments for the detection of radicals (Schlosser et al, 2009;Fuchs et al, 2010b;Dorn et al, 2013), glyoxal (Pang et al, 2014), OH reactivity (Fuchs et al, 2017), water vapor (Fahey et al, 2014;Nowak et al, 2022) or ultrafast temperature sensors (Doussin et al, 2023).…”

Reduction of optical noise in near-infrared range laser hygrometry

“…Anti-reflection coatings on the glass surfaces or the application of wedge or thick optical windows that are tilted in respect to the incident light beam belong to them. All these solutions are useful for small cuvettes, but they are not applicable for apparatuses like the atmospheric tunnels with large windows, where the hygrometer position is scanned over distances of tens of centimetres in respect to the air flux [8,9]. In this case, the laser beam passes through two similar glass plates (we denote them by the indexes 𝑎 and 𝑏 -see Fig.…”

“…Previously, the hygrometer was used for free-air surrounding. Recently, the sensor was applied for humidity analysis in an atmospheric tunnel [7,8]. We stated that in this case the recorded H 2 O absorption spectrum is affected by optical interference which occurs when the laser light is transmitted through the tunnel windows.…”