Probing ice clouds by broadband mid-infrared extinction spectroscopy: case studies from ice nucleation experiments in the AIDA aerosol and cloud chamber R. Wagner, S. Benz, O. Möhler, H. Saathoff, and U. Schurath Forschungszentrum Karlsruhe, Institute of Meteorology and Climate Research, Karlsruhe, Germany
Abstract. Series of infrared extinction spectra of ice crystals were recorded in the
6000–800 cm−1 wavenumber regime during expansion cooling experiments
in the large aerosol and cloud chamber AIDA of Forschungszentrum Karlsruhe.
Either supercooled sulphuric acid solution droplets or dry mineral dust
particles were added as seed aerosols to initiate ice formation after having
established ice supersaturated conditions inside the chamber. The various
ice 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
T-matrix calculations to retrieve the number concentration as well as the
number size distribution of the generated ice clouds. The precise evaluation
of the time-dependent ice particle number concentrations, i.e., the rates of
new ice particle formation, is of particular importance to quantitatively
analyse the ice nucleation experiments in terms of nucleation rates and ice activation spectra. The ice particles were
modelled as finite circular cylinders with aspect ratios ranging from 0.5 to
3.0. Benefiting from the comprehensive diagnostic tools for the
characterisation of ice clouds which are available at the AIDA facility, the
infrared retrieval results with regard to the ice particle number
concentration could be compared to independent measurements with various
optical particle counters. This provided a unique chance to quantitatively
assess potential errors or solution ambiguities in the retrieval procedure
which mainly originate from the difficulty to find an appropriate shape
representation for the aspherical particle habits of the ice crystals. Based
on these inter-comparisons, we demonstrate that there is no standard
retrieval approach which can be routinely applied to all different
experimental scenarios. In particular, the concept to account for the
asphericity of the ice crystals, the a priori constraints which might be imposed on
the unknown number size distribution of the ice crystals (like employing an
analytical distribution function), and the wavenumber range which is
included in the fitting algorithm should be carefully adjusted to each
single retrieval problem.
Citation: Wagner, R., Benz, S., Möhler, O., Saathoff, H., and Schurath, U.: Probing ice clouds by broadband mid-infrared extinction spectroscopy: case studies from ice nucleation experiments in the AIDA aerosol and cloud chamber, Atmos. Chem. Phys., 6, 4775-4800, doi:10.5194/acp-6-4775-2006, 2006.