SUPPLEMENTARY INFORMATION Typical measurement sequenceThe nucleation rates (J cm −3 s −1 ) are measured under neutral (J n ), galactic cosmic ray (J gcr ) or charged pion beam (J ch ) conditions. For J gcr a beam stopper blocks the pions and the chamber is irradiated by GCRs together with a small parasitic component of penetrating beam muons, whereas, for J ch , the beam stopper is opened and the pion beam is normally set to a time-averaged rate of (5 − 6) · 10 4 s −1 . Neutral nucleation rates are measured

Nucleation of aerosol particles from trace atmospheric vapours is thought to provide up to half of global cloud condensation nuclei 1 . Aerosols can cause a net cooling of climate by scattering sunlight and by leading to smaller but more numerous cloud droplets, which makes clouds brighter and extends their lifetimes 2 . Atmospheric aerosols derived from human activities are thought to have compensated for a large fraction of the warming caused by greenhouse gases 2 . However, despite its importance for climate, atmospheric nucleation is poorly understood. Recently, it has been shown that sulphuric acid and ammonia cannot explain particle formation rates observed in the lower atmosphere 3 . It is thought that amines may enhance nucleation 4-16 , but until now there has been no direct evidence for amine ternary nucleation under atmospheric conditions. Here we use the CLOUD (Cosmics Leaving OUtdoor Droplets) chamber at CERN and find that dimethylamine above three parts per trillion by volume can enhance particle formation rates more than 1,000-fold compared with ammonia, sufficient to account for the particle formation rates observed in the atmosphere. Molecular analysis of the clusters reveals that the faster nucleation is explained by a base-stabilization mechanism involving acid-amine pairs, which strongly decrease evaporation. The ion-induced contribution is generally small, reflecting the high stability of sulphuric acid-dimethylamine clusters and indicating that galactic cosmic rays exert only a small influence on their formation, except at low overall formation rates. Our experimental measurements are well reproduced by a dynamical model based on quantum chemical calculations of binding energies of molecular clusters, without any fitted parameters. These results show that, in regions of the atmosphere near amine sources, both amines and sulphur dioxide should be considered when assessing the impact of anthropogenic activities on particle formation.The primary vapour responsible for atmospheric nucleation is thought to be sulphuric acid (H 2 SO 4 ), derived from the oxidation of sulphur dioxide. However, peak daytime H 2 SO 4 concentrations in the atmospheric boundary layer are about 10 6 to 3 3 10 7 cm 23 (0.04-1.2 parts per trillion by volume (p.p.t.v.)), which results in negligible binary homogeneous nucleation of H 2 SO 4 -H 2 O (ref. 3). Additional species such as ammonia or amines 4,5 are therefore necessary to stabilize the embryonic clusters and decrease evaporation. However, ammonia cannot account for particle formation rates observed in the boundary layer 3 and, despite numerous field and laboratory studies [6][7][8][9][10][11][12][13][14][15][16] , amine ternary nucleation has not yet been observed under atmospheric conditions. Amine emissions are dominated by anthropogenic activities (mainly animal husbandry), but about 30% of emissions are thought to arise from the breakdown of organic matter in the oceans, and 20% from biomass burning and soil 8,17 . Atmospheric measurements of gasphase amines ...

In the CLOUD (Cosmics Leaving OUtdoor Droplets) chamber the sulfuric acid concentration is precisely controlled via OH oxidation of SO 2 , while BioOxOrg is produced via OH oxidation of pinanediol (PD, C 10 H 18 O 2 ). PD is a model compound for monoterpene oxidation products, which have recently been proposed as key mediators of new-particle formation (41) via terpene secondary organic aerosol (42). Pinanediol is added to the chamber by flushing clean air through an evaporator containing PD (Sigma Aldrich, 99%) at 69°C, just above its melting point. OH is generated by ozone photolysis driven by uniform UV illumination from a fiber-optic system. All experiments were performed at 278 (±0.01) K and 38% (±2%) relative humidity.Extreme care was applied to minimize possible contamination to the highest possible extent. After a full cleaning cycle the chamber (including flushing the chamber with water and baking it at 100°C), the contamination by NH 3 and dimethylamine was <2 and <0.1 pptv, respectively. Organic contamination was present, however on a very low level: reported that the total volatile organic compound (VOC) contamination was usually below 1 ppbv (43). On average more than 80% of the total VOCs was coming from only 5 exact masses (tentatively assigned as formaldehyde, acetaldehyde, acetone, formic acid, and acetic acid), which have a rather high vapor pressure and are therefore not important for nucleation and growth of particles. Some additional contamination by dimethylamine was present in these experiments, due to intentional injection of this compound in experiments immediately preceding those described here. This contamination is described in detail below, and it is shown that it is negligible for the determination of the nucleation rates described here. The gas and the particle phases were monitored by an SO 2 monitor (Enhanced Trace Level SO 2 Analyzer, Model 43i-TLE, Thermo Scientific, USA), an O 3 monitor (TEI 49C, Thermo Environmental Instruments, USA), a dew point mirror hygrometer (DewMaster Chilled Mirror Hygrometer, EdgeTech, USA), a chemical ionization mass spectrometer (CIMS) to measure H 2 SO 4 concentration (44), a proton transfer reaction time of flight (PTR-TOF) mass spectrometer to measure organic vapor concentrations such as [PD] (45), an ion chromatograph (IC) to measure ammonia (NH 3 ) and dimethylamine (DMA, C 2 H 7 N) (46), two atmospheric pressure interface time of flight (APi-TOF) mass spectrometers to measure the composition of positively and negatively charged clusters (47), and a wide array of condensation particle counters (CPC), including a particle size magnifier (PSM; Airmodus 09) (48) which was operated in a scanning mode to measure the growth rate of particles smaller than 2.5 nm, two diethylene glycol (DEG) CPCs (49), and a butanol CPC (TSI 3776). J 1.7 data were calculated on the one hand directly from the PSM data and on the other hand from the formation rate (dN 2 /dt) measured by the DEG CPC with a 50% efficiency (D 50 ) at 2 nm (50). In the latter case, the ...

Atmospheric aerosols formed by nucleation of vapors affect radiative forcing and therefore climate. However, the underlying mechanisms of nucleation remain unclear, particularly the involvement of organic compounds. Here, we present high-resolution mass spectra of ion clusters observed during new particle formation experiments performed at the Cosmics Leaving Outdoor Droplets chamber at the European Organization for Nuclear Research. The experiments involved sulfuric acid vapor and different stabilizing species, including ammonia and dimethylamine, as well as oxidation products of pinanediol, a surrogate for organic vapors formed from monoterpenes. A striking resemblance is revealed between the mass spectra from the chamber experiments with oxidized organics and ambient data obtained during new particle formation events at the Hyytiälä boreal forest research station. We observe that large oxidized organic compounds, arising from the oxidation of monoterpenes, cluster directly with single sulfuric acid molecules and then form growing clusters of one to three sulfuric acid molecules plus one to four oxidized organics. Most of these organic compounds retain 10 carbon atoms, and some of them are remarkably highly oxidized (oxygen-to-carbon ratios up to 1.2). The average degree of oxygenation of the organic compounds decreases while the clusters are growing. Our measurements therefore connect oxidized organics directly, and in detail, with the very first steps of new particle formation and their growth between 1 and 2 nm in a controlled environment. Thus, they confirm that oxidized organics are involved in both the formation and growth of particles under ambient conditions. Changes to CCN number concentrations from preindustrial times constitute a major uncertainty in estimates of anthropogenic climate forcing (1). New particle formation dominates the total number concentrations of atmospheric aerosol particles; however, newly formed particles must grow from D p ∼ 1.5 to D p ∼ 50-100 nm to be able to act as CCN, and the vast majority are SignificanceThe formation of nanoparticles by condensable vapors in the atmosphere influences radiative forcing and therefore climate. We explored the detailed mechanism of particle formation, in particular the role of oxidized organic molecules that arise from the oxidation of monoterpenes, a class of volatile organic compounds emitted from plants. We mimicked atmospheric conditions in a well-controlled laboratory setup and found that these oxidized organics form initial clusters directly with single sulfuric acid molecules. The clusters then grow by the further addition of both sulfuric acid and organic molecules. Some of the organics are remarkably highly oxidized, a critical feature that enables them to participate in forming initial stable molecular clusters and to facilitate the first steps of atmospheric nanoparticle formation.

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