| Summary: | In their fourth assessment report, the Intergovernmental Panel on Climate Change points out the influence of particles suspended in air, known as an aerosol, as the main uncertainty in the understanding of the climate system. Aerosols affect climate in a number of ways. The so-called direct effect is due to scattering and absorption of light passing through the atmosphere. The indirect effects are caused by aerosol-cloud interactions. The formation of clouds requires pre-existing particles for the water vapor to condense onto. The number concentration, size distribution and chemical properties, as well as the atmospheric dynamics, determine the development of a cloud. Clouds reflect incoming solar radiation into space, but also trap heat radiation from the ground. Ice formation in clouds affects their interaction with radiation, their lifetime, and it may induce the formation of precipitation. Cloud droplets can be supercooled to very low temperatures, and droplets of pure water freeze by homogeneous nucleation at temperatures below -33°C. However, droplet freezing can be facilitated by solid particles, a process called heterogeneous freezing. This thesis presents results from laboratory studies of ice formation processes of atmospheric relevance. The overall aim has been to improve the understanding of ice cloud microphysics. An electrodynamic balance was developed and used to study freezing of single levitated droplets in the micrometer size range. Evaporation freezing of oxalic acid solution droplets was investigated at temperatures from 236 to 248 K. Freezing was observed in the whole temperature range, but at the higher temperatures freezing took place after a period of droplet evaporation. The process was explained by the formation of oxalic acid precipitates as the droplets evaporated, which in a subsequent step induced freezing. The potential importance of the process in atmosphere is currently unknown, but it may explain observed freezing in evaporating cloud and further studies should elucidate if the ...
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