[1] The low-temperature aerosol and cloud chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere) of Forschungszentrum Karlsruhe was used to investigate the effect of sulfuric acid coating on the ice nucleation efficiency of soot aerosol particles from a spark discharge generator. The uncoated (sulfuric acid-coated) soot aerosol showed a nearly lognormal size distribution with number concentrations of 300-5000 cm À3 (2500-56,000 cm À3 ), count median diameters of 70-140 nm (90-200 nm), and geometric standard deviation of 1.3-1.4 (1.5-1.6). The volume fraction of the sulfuric acid coating to the total aerosol volume concentration ranged from 21 to 81%. Ice activation was investigated in dynamic expansion experiments simulating cloud cooling rates between about À0.6 and À3.5 K min À1. At temperatures between 186 and $235 K, uncoated soot particles acted as deposition nuclei at very low ice saturation ratios between 1.1 and 1.3. Above 235 K, ice nucleation only occurred after approaching liquid saturation. Coating with sulfuric acid significantly increased the ice nucleation thresholds of soot aerosol to saturation ratios increasing from $1.3 at 230 K to $1.5 at 185 K. This immersion mode of freezing nucleates ice well below the thresholds for homogeneous freezing of pure sulfuric acid solution droplets measured in previous AIDA experiments. A case study indicated that in contrast to the homogeneous freezing the nucleation rate of the immersion freezing mechanism depends only weakly on relative humidity and thereby the solute concentration. These results show that it is important to know the mixing state of soot and sulfuric acid aerosol particles in order to properly assess their role in cirrus formation. Citation: Möhler, O., et al. (2005), Effect of sulfuric acid coating on heterogeneous ice nucleation by soot aerosol particles,

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.

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&deg; and 178.2&deg; directions. At 178.2&deg;, the scattered light is analysed for the linear polarisation state to deduce the linear depolarisation ratio &delta;_l 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. <br><br> 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 &delta;_l. 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. <br><br> 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&deg;. <br><br> 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

No abstract