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Series of infrared extinction spectra of ice crystals were recorded in the6000–800 cm<sup>−1</sup> wavenumber regime during expansion cooling experimentsin the large aerosol and cloud chamber AIDA of Forschungszentrum Karlsruhe.Either supercooled sulphuric acid solution droplets or dry mineral dustparticles were added as seed aerosols to initiate ice formation after havingestablished ice supersaturated conditions inside the chamber. The variousice nucleation runs were conducted at temperatures between 237 and 195 K,leading to median sizes of the nucleated ice particles of 1–15 µm.The measured infrared spectra were fitted with reference spectra from<b>T</b>-matrix calculations to retrieve the number concentration as well as thenumber size distribution of the generated ice clouds. The precise evaluationof the time-dependent ice particle number concentrations, i.e., the rates ofnew ice particle formation, is of particular importance to quantitativelyanalyse the ice nucleation experiments in terms of nucleation rates and ice activation spectra. The ice particles weremodelled as finite circular cylinders with aspect ratios ranging from 0.5 to3.0. Benefiting from the comprehensive diagnostic tools for thecharacterisation of ice clouds which are available at the AIDA facility, theinfrared retrieval results with regard to the ice particle numberconcentration could be compared to independent measurements with variousoptical particle counters. This provided a unique chance to quantitativelyassess potential errors or solution ambiguities in the retrieval procedurewhich mainly originate from the difficulty to find an appropriate shaperepresentation for the aspherical particle habits of the ice crystals. Basedon these inter-comparisons, we demonstrate that there is no standardretrieval approach which can be routinely applied to all differentexperimental scenarios. In particular, the concept to account for theasphericity of the ice crystals, the a priori constraints which might be imposed onthe unknown number size distribution of the ice crystals (like employing ananalytical distribution function), and the wavenumber range which isincluded in the fitting algorithm should be carefully adjusted to eachsingle retrieval problem

atmospheric chemistry and physics

Probing ice clouds by broadband mid-infrared extinction spectroscopy: case studies from ice nucleation experiments in the AIDA aerosol and cloud chamber