EUCAARI was a EU 6th Framework Programme Integrated Project on aerosol, cloud, climate and air quality interactions
The secondary aerosol formation (nucleation) in atmosphere has a major influence on the CCN number concentrations in the atmosphere, and thus on the aerosol-cloud interactions. However, the mechanisms of new particle formation were not well known before EUCAARI project.
Ion spectrometers, new instruments developed with EUCAARI, were continuously operated for a year at 13 field sites during the EUCAARI Intensive Observation Period and an airborne version was applied in the EUCAARI long range experiment. Based on these measurements, the first quantitative estimate on the concentrations of freshly nucleated particles were obtained for the continental boundary layer and the free troposphere. The concentrations of neutral sub-3 nm particles seem to exceed those of similar-size charged particles in the lower troposphere. Overall, the observations show that ion-induced nucleation usually outweighed by much stronger neutral nucleation events in the continental lower troposphere.
All the results obtained during EUCAARI indicate that sulphuric acid plays a central role in atmospheric nucleation. However, also vapours other than sulphuric acid are needed to explain the nucleation and the subsequent growth particle processes. Under conditions typical for continental boundary layers, such vapours include various organic compounds and very likely also ammonia or amines. Field and laboratory data demonstrate that the nucleation rate scales to the first or second power of the nucleating vapour concentration(s). This finding agrees with the few earlier field observations, but is in stark contrast with classical thermodynamic nucleation theories. Quantum chemical calculations showed confirmed the potential of ammonia, dimethylamine, organic acids resulting from monoterpene oxidiation and organo-sulphates can be involved in nucleation.
The most concrete outcome of the EUCAARI nucleation studies are the new semi-empirical nucleation rate parameterizations for neutral and ion-induced nucleation based on field observations.