Abstract. Secondary organic aerosol (SOA) accounts for a significant fraction of ambient tropospheric aerosol and a detailed knowledge of the formation, properties and transformation of SOA is therefore required to evaluate its impact on atmospheric processes, climate and human health. The chemical and physical processes associated with SOA formation are complex and varied, and, despite considerable progress in recent years, a quantitative and predictive understanding of SOA formation does not exist and therefore represents a major research challenge in atmospheric science. This review begins with an update on the current state of knowledge on the global SOA budget and is followed by an overview of the atmospheric degradation mechanisms for SOA precursors, gas-particle partitioning theory and the analytical techniques used to determine the chemical composition of SOA. A survey of recent laboratory, field and modeling studies is also presented. The following topical and emerging issues are highlighted and discussed in detail: molecular characterization of biogenic SOA constituents, condensed phase reactions and oligomerization, the interaction of atmospheric organic components with sulfuric acid, the chemical and photochemical processing of organics in the atmospheric aqueous phase, aerosol formation from real plant emissions, interaction of atmospheric organic components with water, thermodynamics and mixtures in atmospheric models. Finally, the major challenges ahead in laboratory, field and modeling studies of SOA are discussed and recommendations for future research directions are proposed.

A recently developed method to rapidly quantify the elemental composition of bulk organic aerosols (OA) using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) is improved and applied to ambient measurements. Atomic oxygen-to-carbon (O/C) ratios characterize the oxidation state of OA, and O/C from ambient urban OA ranges from 0.2 to 0.8 with a diurnal cycle that decreases with primary emissions and increases because of photochemical processing and secondary OA (SOA) production. Regional O/C approaches approximately 0.9. The hydrogen-to-carbon (H/C, 1.4--1.9) urban diurnal profile increases with primary OA (POA) as does the nitrogen-to-carbon (N/C, approximately 0.02). Ambient organic-mass-to-organic-carbon ratios (OM/OC) are directly quantified and correlate well with O/C (R2 = 0.997) for ambient OA because of low N/C. Ambient O/C and OM/OC have values consistent with those recently reported from other techniques. Positive matrix factorization applied to ambient OA identifies factors with distinct O/C and OM/OC trends. The highest O/C and OM/OC (1.0 and 2.5, respectively) are observed for aged ambient oxygenated OA, significantly exceeding values for traditional chamber SOA,while laboratory-produced primary biomass burning OA (BBOA) is similar to ambient BBOA, O/C of 0.3--0.4. Hydrocarbon-like OA (HOA), a surrogate for urban combustion POA, has the lowest O/C (0.06--0.10), similar to vehicle exhaust. An approximation for predicting O/C from unit mass resolution data is also presented.

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 ...

Results from photooxidation of aromatic compounds in a reaction chamber show that a substantial fraction of the organic aerosol mass is composed of polymers. This polymerization results from reactions of carbonyls and their hydrates. After aging for more than 20 hours, about 50% of the particle mass consists of polymers with a molecular mass up to 1000 daltons. This results in a lower volatility of this secondary organic aerosol and a higher aerosol yield than a model using vapor pressures of individual organic species would predict.

About half of present-day cloud condensation nuclei originate from atmospheric nucleation, frequently appearing as a burst of new particles near midday 1 . Atmospheric observations show that the growth rate of new particles often accelerates when the diameter of the particles is between one and ten nanometres 2,3 . In this critical size range, new particles are most likely to be lost by coagulation with pre-existing particles 4 , thereby failing to form new cloud condensation nuclei that are typically 50 to 100 nanometres across. Sulfuric acid vapour is often involved in nucleation but is too scarce to explain most subsequent growth 5,6 , leaving organic vapours as the most plausible alternative, at least in the planetary boundary layer 7-10 . Although recent studies [11][12][13] predict that low-volatility organic vapours contribute during initial growth, direct evidence has been lacking. The accelerating growth may result from increased photolytic production of condensable organic species in the afternoon 2 , and the presence of a possible Kelvin (curvature) effect, which inhibits organic vapour condensation on the smallest particles (the nano-Köhler theory) 2,14 , has so far remained ambiguous. Here we present experiments performed in a large chamber under atmospheric conditions that investigate the role of organic vapours in the initial growth of nucleated organic particles in the absence of inorganic acids and bases such as sulfuric acid or ammonia and amines, respectively. Using data from the same set of experiments, it has been shown 15 that organic vapours alone can drive nucleation. We focus on the growth of nucleated particles and find that the organic vapours that drive initial growth have extremely low volatilities (saturation concentration less than 10 −4.5 micrograms per cubic metre). As the particles increase in size and the Kelvin barrier falls, subsequent growth is primarily due to more abundant organic vapours of slightly higher volatility (saturation concentrations of 10 −4.5 to 10 −0.5 micrograms per cubic metre). We present a particle growth model that quantitatively reproduces our measurements. Furthermore, we implement a parameterization of the first steps of growth in a global aerosol model and find that concentrations of atmospheric cloud concentration nuclei can change substantially in response, that is, by up to 50 per cent in comparison with previously assumed growth rate parameterizations.Two measurement campaigns at the CERN CLOUD (Cosmics Leaving OUtdoor Droplets) chamber (Methods) focused on aerosol growth with different levels of sulfuric acid and α-pinene oxidation products. With the chamber at 278 K and 38% relative humidity, tropospheric concentrations of α-pinene, ozone (O 3 ) and SO 2 were introduced (see Extended Data Table 1). Using various instruments (Methods and Extended Data Fig. 1) we measured the behaviour of freshly nucleated particles of 1-2 nm diameter and their subsequent growth up to 80 nm. Two chemical ionization mass spectrometers (Methods) using nitrate as th...

Atmospheric aerosols and their effect on clouds are thought to be important for anthropogenic radiative forcing of the climate, yet remain poorly understood 1 . Globally, around half of cloud condensation nuclei originate from nucleation of atmospheric vapours 2 . It is thought that sulfuric acid is essential to initiate most particle formation in the atmosphere 3,4 , and that ions have a relatively minor role 5 . Some laboratory studies, however, have reported organic particle formation without the intentional addition of sulfuric acid, although contamination could not be excluded 6,7 . Here we present evidence for the formation of aerosol particles from highly oxidized biogenic vapours in the absence of sulfuric acid in a large chamber under atmospheric conditions. The highly oxygenated molecules (HOMs) are produced by ozonolysis of α-pinene. We find that ions from Galactic cosmic rays increase the nucleation rate by one to two orders of magnitude compared with neutral nucleation. Our experimental findings are supported by quantum chemical calculations of the cluster binding energies of representative HOMs. Ion-induced nucleation of pure organic particles constitutes a potentially widespread source of aerosol particles in terrestrial environments with low sulfuric acid pollution.It is thought that aerosol particles rarely form in the atmosphere without sulfuric acid 3,4 , except in certain coastal regions where iodine oxides are involved 8 . Furthermore, ions are thought to be relatively unimportant in the continental boundary layer, accounting for only around 10% of particle formation 5 . Sulfuric acid derives from anthropogenic and volcanic sulfur dioxide emissions as well as dimethyl sulfide from marine biota. However, typical daytime sulfuric acid concentrations (10 5 -10 7 cm −3, or 0.004-0.4 parts per trillion by volume (p.p.t.v.) at standard conditions) are too low for sulfuric acid and water alone to account for the particle formation rates observed in the lower atmosphere 9 , so additional vapours are required to stabilize any embryonic sulfuric acid clusters against evaporation. Base species such as amines can do this and can explain part of atmospheric particle nucleation 10 . It is well established that oxidation products of volatile organic compounds (VOCs) are important for particle growth 11, but whether their role in the smallest particles is in nucleation or growth alone has remained ambiguous 4,12,13 . Recently, however, it has been shown that oxidized organic compounds do indeed help to stabilize sulfuric acid clusters and probably play a major role in atmospheric particle nucleation 6,14,15 . We refer to these compounds as HOMs (highly oxygenated molecules) rather than ELVOCs (extremely low-volatility organic compounds) 16 because the measured compounds span a wide range of low volatilities.Here we report atmospheric particle formation solely from biogenic vapours. The data were obtained at the CERN CLOUD chamber (Cosmics Leaving OUtdoor Droplets; see Methods for experimental details) betw...

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 ...