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  1. null (Ed.)
    Abstract. Acid–base clusters and stable salt formation are critical drivers of new particle formation events in the atmosphere. In this study, we explore salt heterodimer (a cluster of one acid and one base) stability as a function of gas-phase acidity, aqueous-phase acidity, heterodimer proton transference, vapor pressure, dipole moment and polarizability for salts comprised of sulfuric acid, methanesulfonic acid and nitric acid with nine bases. The best predictor of heterodimer stability was found to be gas-phase acidity. We then analyzed the relationship between heterodimer stability and J4×4, the theoretically predicted formation rate of a four-acid, four-base cluster, for sulfuric acid salts over a range of monomer concentrations from 105 to 109 molec cm−3 and temperatures from 248 to 348 K and found that heterodimer stability forms a lognormal relationship with J4×4. However, temperature and concentration effects made it difficult to form a predictive expression of J4×4. In order to reduce those effects, heterodimer concentration was calculated from heterodimer stability and yielded an expression for predicting J4×4 for any salt, given approximately equal acid and base monomer concentrations and knowledge of monomer concentration and temperature. This parameterization was tested for the sulfuric acid–ammonia system by comparing the predicted values to experimental data and was found to be accurate within 2 orders of magnitude. We show that one can create a simple parameterization that incorporates the dependence on temperature and monomer concentration on J4×4 by defining a new term that we call the normalized heterodimer concentration, Φ. A plot of J4×4 vs. Φ collapses to a single monotonic curve for weak sulfate salts (difference in gas-phase acidity >95 kcal mol−1) and can be used to accurately estimate J4×4 within 2 orders of magnitude in atmospheric models. 
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  2. In the atmosphere, water in all phases is ubiquitous and plays important roles in catalyzing atmospheric chemical reactions, participating in cluster formation and affecting the composition of aerosol particles. Direct measurements of water-containing clusters are limited because water is likely to evaporate before detection, and therefore, theoretical tools are needed to study hydration in the atmosphere. We have studied thermodynamics and population dynamics of the hydration of different atmospherically relevant base monomers as well as sulfuric acid–base pairs. The hydration ability of a base seems to follow in the order of gas-phase base strength whereas hydration ability of acid–base pairs, and thus clusters, is related to the number of hydrogen binding sites. Proton transfer reactions at water–air interfaces are important in many environmental and biological systems, but a deeper understanding of their mechanisms remain elusive. By studying thermodynamics of proton transfer reactions in clusters containing up to 20 water molecules and a base molecule, we found that that the ability of a base to accept a proton in a water cluster is related to the aqueous-phase basicity. We also studied the second deprotonation reaction of a sulfuric acid in hydrated acid–base clusters and found that sulfate formation is most favorable in the presence of dimethylamine. Molecular properties related to the proton transfer ability in water clusters are discussed. 
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  3. null (Ed.)
  4. The role of an oxidation product of trimethylamine, trimethylamine oxide, in atmospheric particle formation is studied using quantum chemical methods and cluster formation simulations. Molecular-level cluster formation mechanisms are resolved, and theoretical results on particle formation are confirmed with mass spectrometer measurements. Trimethylamine oxide is capable of forming only one hydrogen bond with sulfuric acid, but unlike amines, trimethylamine oxide can form stable clusters via ion–dipole interactions. That is because of its zwitterionic structure, which causes a high dipole moment. Cluster growth occurs close to the acid:base ratio of 1:1, which is the same as for other monoprotic bases. Enhancement potential of trimethylamine oxide in particle formation is much higher than that of dimethylamine, but lower compared to guanidine. Therefore, at relatively low concentrations and high temperatures, guanidine and trimethylamine oxide may dominate particle formation events over amines. 
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  5. Abstract. It has been widely observed around the world that the frequency and intensityof new particle formation (NPF) events are reduced during periods of highrelative humidity (RH). The current study focuses on how RH affects theformation of highly oxidized molecules (HOMs), which are key components ofNPF and initial growth caused by oxidized organics. The ozonolysis ofα-pinene, limonene, and Δ3-carene, with and without OHscavengers, were carried out under low NOx conditions undera range of RH (from ∼3 % to ∼92 %) in atemperature-controlled flow tube to generate secondary organic aerosol (SOA).A Scanning Mobility Particle Sizer (SMPS) was used to measure the sizedistribution of generated particles, and a novel transverse ionizationchemical ionization inlet with a high-resolution time-of-fight massspectrometer detected HOMs. A major finding from this work is that neitherthe detected HOMs nor their abundance changed significantly with RH, whichindicates that the detected HOMs must be formed from water-independentpathways. In fact, the distinguished OH- and O3-derived peroxyradicals (RO2), HOM monomers, and HOM dimers could mostly beexplained by the autoxidation of RO2 followed by bimolecularreactions with other RO2 or hydroperoxy radicals (HO2),rather than from a water-influenced pathway like through the formation of astabilized Criegee intermediate (sCI). However, as RH increased from ∼3 % to ∼92 %, the total SOA number concentrations decreased bya factor of 2–3 while SOA mass concentrations remained relatively constant. These observations show that, whilehigh RH appears to inhibit NPF as evident by the decreasing numberconcentration, this reduction is not caused by a decrease inRO2-derived HOM formation. Possible explanations for these phenomenawere discussed.

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