Atmospheric nucleation is the dominant source of aerosol particles in the global atmosphere and an important player in aerosol climatic effects. The key steps of this process occur in the sub-2-nanometer (nm) size range, in which direct size-segregated observations have not been possible until very recently. Here, we present detailed observations of atmospheric nanoparticles and clusters down to 1-nm mobility diameter. We identified three separate size regimes below 2-nm diameter that build up a physically, chemically, and dynamically consistent framework on atmospheric nucleation--more specifically, aerosol formation via neutral pathways. Our findings emphasize the important role of organic compounds in atmospheric aerosol formation, subsequent aerosol growth, radiative forcing and associated feedbacks between biogenic emissions, clouds, and climate.

The formation of new atmospheric aerosol particles and their subsequent growth have been observed frequently at various locations all over the world. The atmospheric nucleation rate (or formation rate) and growth rate (GR) are key parameters to characterize the phenomenon. Recent progress in measurement techniques enables us to measure atmospheric nucleation at the size (mobility diameter) of 1.5 (±0.4) nm. The detection limit has decreased from 3 to 1 nm within the past 10 years. In this protocol, we describe the procedures for identifying new-particle-formation (NPF) events, and for determining the nucleation, formation and growth rates during such events under atmospheric conditions. We describe the present instrumentation, best practices and other tools used to investigate atmospheric nucleation and NPF at a certain mobility diameter (1.5, 2.0 or 3.0 nm). The key instruments comprise devices capable of measuring the number concentration of the formed nanoparticles and their size, such as a suite of modern condensation particle counters (CPCs) and air ion spectrometers, and devices for characterizing the pre-existing particle number concentration distribution, such as a differential mobility particle sizer (DMPS). We also discuss the reliability of the methods used and requirements for proper measurements and data analysis. The time scale for realizing this procedure is 1 year.

Atmospheric aerosol formation is known to occur almost all over the world, and the importance of these particles to climate and air quality has been recognized. Although almost all of the processes driving aerosol formation take place below a particle diameter of 3 nanometers, observations cover only larger particles. We introduce an instrumental setup to measure atmospheric concentrations of both neutral and charged nanometer-sized clusters. By applying the instruments in the field, we come to three important conclusions: (i) A pool of numerous neutral clusters in the sub-3 nanometer size range is continuously present; (ii) the processes initiating atmospheric aerosol formation start from particle sizes of approximately 1.5 nanometers; and (iii) neutral nucleation dominates over the ion-induced mechanism, at least in boreal forest conditions.

New particle formation (NPF) is the source of over half of the atmosphere's cloud condensation nuclei, thus influencing cloud properties and Earth's energy balance. Unlike in the planetary boundary layer, few observations of NPF in the free troposphere exist. We provide observational evidence that at high altitudes, NPF occurs mainly through condensation of highly oxygenated molecules (HOMs), in addition to taking place through sulfuric acid-ammonia nucleation. Neutral nucleation is more than 10 times faster than ion-induced nucleation, and growth rates are size-dependent. NPF is restricted to a time window of 1 to 2 days after contact of the air masses with the planetary boundary layer; this is related to the time needed for oxidation of organic compounds to form HOMs. These findings require improved NPF parameterization in atmospheric models.

Abstract. Sulphuric acid and organic vapours have been identified as the key components in the ubiquitous secondary new particle formation in the atmosphere. In order to assess their relative contribution and spatial variability, we analysed altogether 36 new particle formation events observed at four European measurement sites during EUCAARI campaigns in [2007][2008][2009]. We tested models of several different nucleation mechanisms coupling the formation rate of neutral particles (J ) with the concentration of sulphuric acid ([H 2 SO 4 ]) or low-volatility organic vapours ([org]) condensing on sub-4 nm particles, or with a combination of both concentrations. Furthermore, we determined the related nucleation coefficients connecting the neutral nucleation rate J with the vapour concentrations in each mechanism. The main goal of the study was to identify the mechanism of new particle formation and subsequent growth that minimizes the difference between the modelled and measured nucleation rates. At three out of four measurement sites -Hyytiälä (Finland), Melpitz (Germany) and San Pietro Capofiume (Italy) -the nucleation rate was closely connected to squared sulphuric acid concentration, whereas in Hohenpeissenberg (Germany) the low-volatility organic vapours were observed Correspondence to: P. Paasonen (pauli.paasonen@helsinki.fi) to be dominant. However, the nucleation rate at the sulphuric acid dominant sites could not be described with sulphuric acid concentration and a single value of the nucleation coefficient, as K in J =K [H 2 SO 4 ] 2 , but the median coefficients for different sites varied over an order of magnitude. This inter-site variation was substantially smaller when the heteromolecular homogenous nucleation between H 2 SO 4 and organic vapours was assumed to take place in addition to homogenous nucleation of H 2 SO 4 alone, i.e., In general, our results suggest that organic vapours do play a role, not only in the condensational growth of the particles, but also in the nucleation process, with a site-specific degree.

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.

A new particle size magnifier (PSM) for detection of nano-CN as small as ∼1 nm in mobility diameter was developed, calibrated and tested in atmospheric measurements. The working principle of a PSM is to mix turbulently cooled sample flow with heated clean air flow saturated by the working fluid. This provides a high saturation ratio for the working fluid and activates the seed particles and grows them by condensation of the working fluid. In order to reach high saturation ratios, and thus to activate nano-CN without homogeneous nucleation, diethylene glycol was chosen as the working fluid. The PSM was able to grow nano-CN to mean diameter of 90 nm, after which an ordinary condensation particle counter was used to count the grown particles (TSI 3010). The stability of the PSM was found to be good making it suitable for stand-alone field measurements. Calibration results show that the detection efficiency of the prototype PSM + TSI 3010 for charged tetra-alkyl ammonium salt molecules having mobility equivalent diameters of 1.05, 1.47, 1.78, and 2.57 nm are 25, 32, 46, and 70%, respectively. The commercial version of the PSM (Airmodus A09) performed even better in the smallest sizes the detection efficiency being 51% for 1.47 nm and 67% for 1.78 nm.

A recently developed atmospheric pressure interface mass spectrometer (APi-TOF) measured the negative and positive ambient ion composition at a boreal forest site. As observed in previous studies, the negative ions were dominated by strong organic and inorganic acids (e.g. malonic, nitric and sulfuric acid), whereas the positive ions consisted of strong bases (e.g. alkyl pyridines and quinolines). Several new ions and clusters of ions were identified based on their exact masses, made possible by the high resolution, mass accuracy and sensitivity of the APi-TOF. Time series correlograms aided in peak identification and assigning the atomic compositions to molecules. Quantum chemical calculations of proton affinities and cluster stabilities were also used to confirm the plausibility of the assignments. Acids in the gas phase are predominantly formed by oxidation in the gas phase, and thus the concentrations are expected to vary strongly between day and night. This was also the case in this study, where the negative ions showed strong diurnal behavior, whereas the daily changes in the positive ions were considerably smaller. A special focus in this work was the changes in the ion distributions occurring during new particle formation events. We found that sulfuric acid, together with its clusters, dominated the negative ion spectrum during these events. The monomer (HSO₄^−) was the largest peak, together with the dimer (H₂SO₄ · HSO₄^−) and trimer ((H₂SO₄)₂ · HSO₄^−). SO₅^− also tracked HSO₄^− at around 20% of the HSO₄^− concentration at all times. During the strongest events, the tetramer and a cluster with the tetramer and ammonia were also detected. Quantum chemical calculations predict that sulfuric acid clusters containing ammonia are much more stable when neutral, thus the detection of a single ion cluster implies that ammonia can be an important compound in the nucleation process. We also believe to have made the first observations of an organosulfate (glycolic acid sulfate) in the gas phase. This ion, and its cluster with sulfuric acid, correlates with the HSO₄^−, but peaks in the early afternoon, some hours later than HSO₄^− itself. A list of all identified ions is presented in the supplementary material, and also a list of all detected masses not yet identified