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Abstract:
The interactions and effects of aerosols and clouds are among the largest uncertainties in the assessment and modeling of climate change. Due to the increasing human influence on planet Earth, remote locations that allow to probe pristine air approximating pre-industrial conditions are becoming progressively rare. Such conditions are, however, of particular scientific interest for understanding climate and global environmental change. This dissertation investigates the properties and interactions of aerosols serving as cloud condensation nuclei (CCN) under pristine and polluted conditions by aircraft and ground-based measurements. At the Amazon Tall Tower Observatory (ATTO), a robust experimental setup was developed and used to obtain the first size-resolved CCN measurements spanning a full seasonal cycle in the Amazonian rainforest. The unique data set comprises annual and seasonal averages of aerosol and CCN parameters as well as characteristic case studies including present-day pristine conditions, biomass burning aerosols, and long range transport effects. A novel parameterization of CCN efficiency spectra and concentrations as a function of water vapor supersaturation was developed for efficient representation of aerosol and CCN properties in model studies. This approach enhances the efficiency, accuracy, and reliability with which CCN concentrations can be taken into account in process models and large-scale models of clouds and precipitation, the atmosphere and climate. The parametrization shall serve as a basis for detailed microphysical studies on the formation and properties of clouds and precipitation in the Amazon region. The CCN observations at the ATTO site were also used to evaluate a new method to retrieve CCN concentrations from satellite data. Within the ACRIDICON project (Aerosol, Cloud, Precipitation, and Radiation Interactions and Dynamics of Convective Cloud Systems), aerosol-cloud interactions were investigated during two collaborative measurement campaigns. During the ACRIDICON-Zugspitze campaign, cloud supersaturations were derived from both aerosol size distribution and size-resolved CCN measurements. An instrument package for airborne measurements of CCN and aerosol properties was developed and implemented on the research aircraft HALO to probe the vertical structure and evolution of convective clouds in the Amazon Basin during the ACRIDICON-CHUVA campaign. The results indicate that new particle formation occurs in the outflow of deep convective clouds, which seems to be an important source of aerosol particles and CCN in the boundary layer and may be the dominant process of secondary particle formation in the pre-industrial atmosphere.
