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

1. Scope -is the work directly or implicitly related to atmospheric composition? 2. Novelty -does the work provide a) a general and/or broader relevance (e.g. not a pure local study), b) new results or methods, and c) does it add significantly to the knowledge of atmospheric composition and its impacts?3. Quality -does the work contain high quality a) atmospheric observations, b) process studies, c) modeling exercises or d) data analysis?Will your paper be within the scope of Atmospheric Environment?We try to be flexible with novel scientific articles on issues of atmospheric composition even, if they are not directly related to atmospheric measurements (e.g. wind tunnel studies, dynamometer studies, remote sensing retrieval, etc). However, we are still cautious of purely mathematical derivations, preliminary results or insignificant case and local studies. The authors should make sure that the articles contain substantial contributions to the science of atmospheric composition before sending them for review.

New particle formation in the atmosphere is an important parameter in governing the radiative forcing of atmospheric aerosols. However, detailed nucleation mechanisms remain ambiguous, as laboratory data have so far not been successful in explaining atmospheric nucleation. We investigated the formation of new particles in a smog chamber simulating the photochemical formation of H 2 SO 4 and organic condensable species. Nucleation occurs at H 2 SO 4 concentrations similar to those found in the ambient atmosphere during nucleation events. The measured particle formation rates are proportional to the product of the concentrations of H 2 SO 4 and an organic molecule. This suggests that only one H 2 SO 4 molecule and one organic molecule are involved in the rate-limiting step of the observed nucleation process. Parameterizing this process in a global aerosol model results in substantially better agreement with ambient observations compared to control runs.aerosol particles | atmospheric nucleation | new particle formation | sulfuric acid A tmospheric aerosols affect the radiative balance in the Earth's atmosphere and influence cloud formation, thereby playing a central role in climate forcing. They also have an important impact on visibility and human health. Many of these effects depend on the particle size distribution, which is governed by the emission of primary particles on the one hand and formation of new particles on the other hand. New particle formation events have been observed frequently and worldwide, in boreal forests, coastal, rural, and urban regions, as well as the free troposphere (1). Their contribution to the regional and global budget of atmospheric particles is likely to be significant though it is still poorly constrained (2-5). A detailed understanding of atmospheric nucleation processes is therefore needed.Observations in the planetary boundary layer revealed a consistent correlation between sulfuric acid (H 2 SO 4 ) and the concentration of newly formed particles (6-9), where the particle formation rate can be described with a simple power law:The exponent m was found to consistently vary between 1 and 2. According to the nucleation theorem (10), this suggests that the critical cluster (the smallest stable "particle") contains only one or two H 2 SO 4 or sulfuric acid-containing molecules. Classical binary (H 2 SO 4 -water) and ternary (H 2 SO 4 -NH 3 -water) mechanisms predict much higher values of the exponent and fail to explain the ambient observations (11, 12). Therefore, new approaches such as H 2 SO 4 cluster activation (13) (for m ¼ 1) and kinetic nucleation (14) (for m ¼ 2) have been developed trying to explain the observed new particle formation events. Recently the formation of organosulfate clusters was suggested to explain the chemistry behind the cluster activation or kinetic mechanisms and thus atmospheric nucleation (15). From detailed analyses of nucleation and growth it was inferred that sesquiterpenes might be involved in new particle formation. On the other hand, a number...

Abstract.A hygroscopicity tandem differential mobility analyzer (HTDMA) was used to measure the water uptake (hygroscopicity) of secondary organic aerosol (SOA) formed during the chemical and photochemical oxidation of several organic precursors in a smog chamber. Electron ionization mass spectra of the non-refractory submicron aerosol were simultaneously determined with an aerosol mass spectrometer (AMS), and correlations between the two different signals were investigated. SOA hygroscopicity was found to strongly correlate with the relative abundance of the ion signal m/z 44 expressed as a fraction of total organic signal (f 44 ). m/z 44 is due mostly to the ion fragment CO + 2 for all types of SOA systems studied, and has been previously shown to strongly correlate with organic O/C for ambient and chamber OA. The analysis was also performed on ambient OA from two field experiments at the remote site Jungfraujoch, and the megacity Mexico City, where similar results were found. A simple empirical linear relation between the hygroscopicity of OA at subsaturated RH, as given by the hygroscopic growth factor (GF) or "κ org " parameter, and f 44 was determined and is given by κ org = 2.2× f 44 − 0.13. This approximation can be further verified and refined as the Correspondence to: J. Duplissy (jonathan.duplissy@cern.ch) database for AMS and HTDMA measurements is constantly being expanded around the world. The use of this approximation could introduce an important simplification in the parameterization of hygroscopicity of OA in atmospheric models, since f 44 is correlated with the photochemical age of an air mass.

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.

[1] Cloud droplets form by nucleation on atmospheric aerosol particles. Populations of such particles invariably contain organic material, a major source of which is thought to be condensation of photo-oxidation products of biogenic volatile organic compounds (VOCs). We demonstrate that smog chamber studies of the formation of such biogenic secondary organic aerosol (SOA) formed during photooxidation must be conducted at near atmospheric concentrations to yield atmospherically representative particle composition, hygroscopicity and cloud-forming potential. Under these conditions, the hygroscopicity measured at 95% relative humidity can be used reliably to predict the CCN activity of the SOA particles by assuming droplet surface tension of pure water. We also show that the supersaturation required to activate a given size of particle decreases with age. [2] Aerosol particles partially counteract the warming effect of greenhouse gases by modifying cloud radiative properties. Depending on their size and chemical composition, natural and anthropogenic aerosol particles may act as cloud condensation nuclei (CCN) and thereby change the cloud radiative properties and lifetime. The magnitude of this effect is the single biggest unknown in modeling radiative forcing (http://www.ipcc.ch.). Atmospheric aerosols often consist of (internal) mixtures of organic and inorganic substances. The cloud forming potential (i.e. the hygroscopic properties) of the inorganic constituents are generally well known, whereas not much is known on the hygroscopic properties associated with the organic matter, which typically contributes 20-50% to the mass of fine aerosol particles [Kanakidou et al., 2005]. Secondary organic aerosol (SOA), formed by photo-oxidation of volatile organic compounds (VOCs), is a major contribution to this organic fraction [Robinson et al., 2007]. In this study, we conducted smog chamber experiments on the SOA's hygroscopic properties and ability to act as CCN, using precursor concentrations approaching atmospheric levels. Hygroscopicity measurements in sub-and supersaturated conditions were compared in order to investigate potential surface tension reduction effects of SOA solution droplets. The surface tension may be important for models inferring the CCN potential from measured hygroscopic properties at sub-saturated conditions. It has been shown that organic material collected from Po Valley fog samples may lower the surface tension in bulk solution and it was inferred that they may similarly affect surface tension in growing cloud droplets [Facchini et al., 1999].[3] Experiments were performed at the PSI chamber [Paulsen et al., 2005] using varying initial concentrations of 183 to 10 ppbv a-pinene, a frequently used aerosol precursor representative of biogenic emissions (comparable results were found when using the anthropogenic organic aerosol precursor 1,3,5-trimethylbenzene). The chemical nature of the SOA particles was investigated using the Aerodyne aerosol mass spectrometer (AMS) [Jayne et al., 2000] (see...

[1] Compounds assigned to be oxidation products of isoprene (2-methyl-1,3-butadiene) have recently been observed in ambient aerosols, suggesting that isoprene might play an important role in secondary organic aerosol (SOA) formation due to its large global source strength. SOA yields from photooxidation of isoprene and NO x in a chamber agree fairly well with previous data. Matrix assisted laser desorption/ionization mass spectrometry showed the formation of high molecular weight compounds over the course of 15-hour experiments. Concurrently, the volatility of the SOA decreased markedly as observed by a tandem differential mobility analyzer. The volume fraction remaining of SOA at 150°C increased steadily from 5 to 25% during the same experiments. These observations are attributed to oligomerization reactions occurring in the aerosol phase. Under dry conditions a lower volatility was observed. Citation: Dommen, J