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The mechanism of new particle formation (NPF) events is still poorly understood. The CLOUD experiment is studying at which conditions this process occurs. During the CLOUD 7 campaign (fall 2012) the evolution of the nucleation rate with different reagents (sulphuric acid, ammonia, dimethylamine and α-pinene) was tested. Here we investigate the composition of freshly formed positive ions during the nucleation process, which allows us to understand the formation mechanisms of these clusters. The time evolution of the clusters is shown demonstrating the correlation between the formation of these clusters and the nucleation process.

The role of α-pinene in aerosol nucleation and growth was investigated using the CERN CLOUD chamber, a nano scanning mobility particle sizer (nanoSMPS) and several condensation particle counters (CPCs) with different diameter cut-offs. Different oxidation conditions for α-pinene - OH⋅ vs. ozone oxidation - were considered to investigate their contributions to particle nucleation and growth. Results from the latest CERN experiment from fall 2012 (CLOUD 7) are presented.

The mechanisms behind the nucleation of vapors forming new particles in the atmosphere had been proven difficult to establish. One main aim of the CLOUD experiment was to explore in detail these first steps of atmospheric new particle formation by performing extremely well controlled laboratory experiments. We examined nucleation and growth in the presence of different mixtures of vapors, including sulfuric acid, ammonia, dimethylamine, and oxidation products of pinanediol or α-pinene. Among the employed state-of-the-art instrumentation, a high-resolution mass spectrometer that directly sampled negatively charged ions and clusters proved particularly useful. We were able to resolve most of the chemical compositions found for charged sub-2nm clusters and to observe their growth in time. These compositions reflected the mixture of condensable vapors in the chamber and the role of each individual vapor in forming sub-2nm clusters could be explored. By inter-comparing between individual experiments and ambient observations, we try to establish which vapors participate in nucleation in the actual atmosphere, and how.

In this work we present a study that aims to understand the new particle formation events and their underlying mechanism. Measurements were conducted during January-February 2013 at the Swiss high alpine research station Jungfraujoch which is predominantly located in the lower free troposphere. With the atmospheric pressure interface Time-of-Flight mass spectrometer (APi-TOF) the chemical composition of the ions (positive and negative) was measured during the nucleation process. It was observed that nucleation is correlated with a rise in the sulphuric acid concentration.

The role of oxidized organic compounds in the process of new particle formation in the atmosphere is poorly known. Here we used the ultraclean and most sophisticated CLOUD chamber to investigate systematically particle formation in the presence of sulfuric acid and oxidized organics. We varied independently the concentrations of both of these components. In addition, nucleation was observed without and in the presence of ionic compounds. From the results a new parameterized description of nucleation was derived for global climate model simulations.