Abstract. We present comprehensive results on continuous atmospheric cluster and particle measurements in the size range ∼1-42 nm within the European Integrated project on Aerosol Cloud Climate and Air Quality interactions (EU-CAARI) project. We focused on characterizing the spatial and temporal variation of new particle formation events and Correspondence to: H. E. Manninen (hanna.manninen@helsinki.fi) relevant particle formation parameters across Europe. Different types of air ion and cluster mobility spectrometers were deployed at 12 field sites across Europe from March 2008 to May 2009. The measurements were conducted in a wide variety of environments, including coastal and continental locations as well as sites at different altitudes (both in the boundary layer and the free troposphere). New particle formation events were detected at all of the 12 field sites during the year-long measurement period. From the data, Published by Copernicus Publications on behalf of the European Geosciences Union. 7908H. E. Manninen et al.: EUCAARI ion spectrometer measurements nucleation and growth rates of newly formed particles were determined for each environment. In a case of parallel ion and neutral cluster measurements, we could also estimate the relative contribution of ion-induced and neutral nucleation to the total particle formation. The formation rates of charged particles at 2 nm accounted for 1-30% of the corresponding total particle formation rates. As a significant new result, we found out that the total particle formation rate varied much more between the different sites than the formation rate of charged particles. This work presents, so far, the most comprehensive effort to experimentally characterize nucleation and growth of atmospheric molecular clusters and nanoparticles at ground-based observation sites on a continental scale.

Abstract. In this paper we describe and summarize the main achievements of the European Aerosol Cloud Climate and Air Quality Interactions project (EUCAARI). EUCAARI started on 1 January 2007 and ended on 31 December 2010 leaving a rich legacy including: (a) a comprehensive database with a year of observations of the physical, chemical and optical properties of aerosol particles over Europe, (b) comprehensive aerosol measurements in four developing countries, (c) a database of airborne measurements of aerosols and clouds over Europe during May 2008, (d) comprehensive modeling tools to study aerosol processes fron nano to global scale and their effects on climate and air quality. In addition a new Pan-European aerosol emissions inventory was developed and evaluated, a new cluster spectrometer was built and tested in the field and several new aerosol parameterizations and computations modules for chemical transport and global climate models were developed and evaluated. These achievements and related studies have substantially improved our understanding and reduced the uncertainties of aerosol radiative forcing and air quality-climate interactions. The EUCAARI results can be utilized in European and global environmental policy to assess the aerosol impacts and the corresponding abatement strategies.

This review is based on ca. 260 publications, 93 of which included data on the temporal and spatial variation of the concentration of small ions (<1.6 nm in diameter) especially in the lower troposphere, chemical composition, or formation and growth rates of sub-3 nm ions. This information was collected on tables and figures. The small ions exist all the time in the atmosphere, and the average concentrations of positive and negative small ions are typically 200–2500 cm⁻³. However, concentrations up to 5000 cm⁻³ have been observed. The results are in agreement with observations of ion production rates in the atmosphere. We also summarised observations on the conversion of small ions to intermediate ions, which can act as embryos for new atmospheric aerosol particles. Those observations include the formation rates (<i>J</i>₂[ion]) of 2-nm intermediate ions, growth rates (GR[ion]) of sub-3 nm ions, and information on the chemical composition of the ions. Unfortunately, there were only a few studies which presented <i>J</i>₂[ion] and GR[ion]. Based on the publications, the formation rates of 2-nm ions were 0–1.1 cm⁻³ s⁻¹, while the total 2-nm particle formation rates varied between 0.001 and 60 cm⁻³ s⁻¹. Due to small changes in <i>J</i>₂[ion], the relative importance of ions in 2-nm particle formation was determined by the large changes in <i>J</i>₂[tot], and, accordingly the contribution of ions increased with decreasing <i>J</i>₂[tot]. Furthermore, small ions were observed to activate for growth earlier than neutral nanometer-sized particles and at lower saturation ratio of condensing vapours

Abstract. The paper describes the Neutral cluster and Air Ion Spectrometer (NAIS) – a multichannel aerosol instrument capable of measuring the distribution of ions (charged particles and cluster ions) of both polarities in the electric mobility range from 3.2 to 0.0013 cm2 V−1 s−1 and the distribution of aerosol particles in the size range from 2.0 to 40 nm. We introduce the principles of design, data processing and spectrum deconvolution of the instrument.

Abstract. Within the project EUCAARI (European Integrated project on Aerosol Cloud Climate and Air Quality interactions), atmospheric nucleation was studied by (i) developing and testing new air ion and cluster spectrometers, (ii) conducting homogeneous nucleation experiments for sulphate and organic systems in the laboratory, (iii) investigating atmospheric nucleation mechanism under field conditions, and (iv) applying new theoretical and modelling tools for data interpretation and development of parameterisations. The current paper provides a synthesis of the obtained results and identifies the remaining major knowledge gaps related to atmospheric nucleation. The most important technical achievement of the project was the development of new in-struments for measuring sub-3 nm particle populations, along with the extensive application of these instruments in both the laboratory and the field. 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 processes, at least in continental boundary layers. Candidate vapours in this respect are some organic compounds, ammonia, and especially amines. Both our field and laboratory data demonstrate that the nucleation rate scales to the first or second power of the nucleating vapour concentration(s). This agrees with the few earlier field observations, but is in stark contrast with classical thermodynamic nucleation theories. The average formation rates of 2-nm particles were found to vary by almost two orders of magnitude between the different EUCAARI sites, whereas the formation rates of charged 2-nm particles varied very Published by Copernicus Publications on behalf of the European Geosciences Union. 10830V.-M. Kerminen et al.: Highlights of the EUCAARI project and future directions little between the sites. Overall, our observations are indicative of frequent, yet moderate, ion-induced nucleation usually outweighed by much stronger neutral nucleation events in the continental lower troposphere. The most concrete outcome of the EUCAARI nucleation studies are the new semiempirical nucleation rate parameterizations based on field observations, along with updated aerosol formation parameterizations.