Influence of particle size and shape on the backscattering linear depolarisation ratio of small ice crystals – cloud chamber measurements in the context of contrail and cirrus microphysics

Abstract

Abstract. The article presents the laser scattering and depolarisation instrument SIMONE that is installed at the large aerosol and cloud chamber facility AIDA of the Karlsruhe Institute of Technology. SIMONE uses a 488 nm cw laser to probe simulated atmospheric clouds by measuring the scattered light from the 1.8 • and 178.2 • directions. At 178.2 • , the scattered light is analysed for the linear polarisation state to deduce the particle linear depolarisation ratio δ p which is a common measurement parameter of atmospheric lidar applications. The optical setup and the mathematical formalism of the depolarisation detection concept are given. SIMONE depolarisation measurements in spheroidal hematite aerosol and supercooled liquid clouds are used to validate the instrument.SIMONE data from a series of AIDA ice nucleation experiments at temperatures between 195 and 225 K were analysed in terms of the impact of the ice particle microphysics on δ p . We found strong depolarisation values of up to 0.4 in case of small growing and sublimating ice particles with volume equivalent diameters of only a few micrometers.Modelling runs with the T-matrix method showed that the measured depolarisation ratios can be accurately reproduced assuming spheroidal and cylindrical particles with a size distribution that has been constrained by IR extinction spectroscopy. Based on the T-matrix modelling runs, we demonstrate that in case of small ice crystals the SIMONE depolarisation results are representative for the lidar depolarisation ratio which is measured at exact backscattering direction of 180 • .The relevance of our results for the interpretation of recent lidar observations in cirrus and contrails is discussed. In view of our results, the high depolarisation ratios observed by the spaceborne lidar CALIOP in the tropical upper troposphere might be a hint for the presence of small (sublimating) ice particles in the outflows of deep convective systems.

A new temperature- and humidity-dependent surface site density approach for deposition ice nucleation

“…The distinction between aerosol particles and ice crystals is made by selecting a suitable size threshold. The formation of small ice crystals is also indicated by the change in depolarization detected by SIMONE (Schnaiter et al, 2012). SIMONE is used for observing scattering signals from particles crossing the pathway of a polarized laser beam (λ = 488 nm) which horizontally traverses the cloud chamber.…”

Aqueous phase oxidation of sulphur dioxide by ozone in cloud droplets

“…An optical particle sizer (WELAS Promo 2000, Palas GmbH) measured the droplet size distributions during the cloud periods. An in situ particle backscatter instrument (the SIMONE, Schnaiter et al, 2012) detected the presence of droplets and phase transitions within the chamber. In addition, the Particle Phase Discriminator mark 2, Karlsruhe edition (PPD-2K) was used to monitor the phase of particles in the size range of 6 to 60 µm, and thus to detect ice formation.…”

Laser filament-induced aerosol formation

“…1, the aerosol and cloud chamber AIDA features 4 m diameter and 7.5 m height (84.5 m 3 ) (Möhler et al, 2003;Schnaiter et al, 2012). The beam of the Teramobile mobile femtosecond terawatt laser system (Wille et al, 2002) was expanded to 10 cm diameter and focused by an f = 4 m lens into the centre of the AIDA chamber.…”

“…The experiments were performed in the large-scale aerosol and cloud chamber AIDA (Aerosol Interaction and Dynamics in the Atmosphere) (Möhler et al, 2003;Schnaiter et al, 2012) under illumination by the TW-class mobile laser system Teramobile (Wille et al, 2002). Temperature, relative humidity, trace gas levels, and laser power were varied systematically.…”