On ion-induced new-particle formation : from well-defined laboratory experiments to atmospheric observations

Aerosol particles can be directly emitted to the atmosphere in many ways, such as sea spray, volcanoes, wildfires or industrial combustion processes. However, aerosol particles also form in the atmosphere, when precursor gases nucleate and new-particle formation occurs. This phenomenon is frequently observed in the atmosphere, and studies suggest that approximately half of the global budget of cloud condensation nuclei results from secondary particle formation. Nevertheless, we do not yet understand atmospheric new-particle formation in detail. This thesis addresses underlying processes of new-particle formation in the atmosphere, with focus on the role of ions in those processes. The aim is to increase the understanding on atmospheric processes and the interpretation of observations, which are crucial to improve climate change predictions. Organic vapors with very low volatilities were found to play a significant role in both nucleation and growth of molecular clusters. These vapors can form through oxidation of volatile organic compounds such as monoterpenes (e.g. alpha-pinene or delta-3-carene, C10H16). While for a long time the prevailing opinion was that nucleation does not occur without sulfuric acid as a precursor, we found that organic vapors with very low volatilities can form particles even in the absence of sulfuric acid. Moreover, the early growth of freshly formed clusters can also be governed by low volatility organics. In cases where conditions are unfavorable for neutral clusters to form, ions can help by stabilizing nuclei, thus facilitating new-particle formation (ion-induced nucleation). To be able to study ion-related effects in detail, accurate ion concentration and size measurements are vital. For this thesis, we performed calibration measurements to verify the output of the neutral cluster and air ion spectrometer. We identified deviations in ion concentrations of up to 30%. By applying the correction terms that we derived from our laboratory results, the uncertainty of measured ion concentrations can be reduced to 10%. Studying the effect of ions on new-particle formation, we found that the contribution of ion-induced nucleation is sensitive to the concentration of cluster ions, decreasing to-wards low concentrations. High concentrations of cluster ions indicate small sinks of ions and charged clusters, as well as high recombination rates. The difference in cluster ion concentrations between the boreal forest in Hyytiälä, Finland, and the laboratory environment at CERN, Switzerland, can, at least partly, explain the different contributions of ion-induced nucleation when comparing results from the two environments. This thesis addresses new-particle formation, especially nucleation and growth process-es, and provides new insights to the participating precursor gases and the role of ions. Linking laboratory studies to ambient observations is challenging, and further investigation is needed to confirm implications for the Earth’s atmosphere.N/