Daniel Knopf

Aerosol-Cloud interactions are known to account for the largest uncertainties in predicting future climate changes. However, the role of ice clouds has yet not been assessed due to our insufficient understanding of the formation process of ice in the atmosphere. Experimentally the effect of organics containing particles on the homogeneous and heterogeneous nucleation of ice is investigated. Homemade aerosol nucleation cells attached to optical microscopes are employed to study the formation of ice from aerosol particles as a function of temperature and relative humidity. Homogeneous and heterogeneous ice nucleation rate coefficients are derived to yield atmospherically relevant ice particle production rates. Heterogeneous chemistry. A larde fraction of particulate matter contains organic compounds. Heterogeneous reactions of particulate organic matter and trace gas species such as ozone, NO3, and OH can lead to the chemical modification of the particle. This can change the particle's composition and thus its CCN and IN activity and toxicity with subsequent effects on the radiative budget and air quality. The chemical kinetics of heterogeneous reactions are studied in the laboratory and by application of models which account for the peculiar properties of the gas-particle interfaces. The importance of heterogeneous reactions for particle source apportionment, climate change, and air quality is assessed.