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The free energy barrier is fundamental to nucleation studies. Most of the existing literature interprets the free energy barrier as the reaction free energy of the nucleating system, whereas a positive reaction free energy of a large nucleation system means that nucleation is not spontaneous according to the second law of thermodynamics. Here we clarify the meaning of the free energy barrier and demonstrate that it is not the reaction free energy for the nucleation process under realistic conditions. Instead, it implicitly assumes that the concentration of each cluster is equal to the monomer concentration. In terms of the proper reaction free energy of cluster formation, the dependency of the partial molar free energy of clusters on their concentration is demonstrated to be important. In fact, a positive reaction free energy when cluster concentrations equal the vapor concentration (i.e., the free energy barrier) indicates that clusters will spontaneously evaporate under those conditions. This dependency on cluster concentration is consistent with kinetic understandings that the concentration gradient of clusters in the size space drives cluster growth and the critical cluster size is not the only decisive factor for cluster growth. Compared to cluster evolutions simulated using nucleation kinetics, the proper reaction free energy is shown to be able to well explain the nucleation process. Copyright © 2022 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

New particle formation (NPF) is one of the major sources of atmospheric ultrafine particles. Due to the high aerosol and trace gas concentrations, the mechanism and governing factors for NPF in the polluted atmospheric boundary layer may be quite different from those in clean environments, which is however less understood. Herein, based on long-term atmospheric measurements from January 2018 to March 2019 in Beijing, the nucleation mechanism and the influences of H2SO4 concentration, amine concentrations, and aerosol concentration on NPF are quantified. The collision of H2SO4 –amine clusters is found to be the dominating mechanism to initialize NPF in urban Beijing. The coagulation scavenging due to the high aerosol concentration is a governing factor as it limits the concentration of H2SO4 –amine clusters and new particle formation rates. The formation of H2SO4 –amine clusters in Beijing is sometimes limited by low amine concentrations. Summarizing the synergistic effects of H2SO4 concentration, amine concentrations, and aerosol concentration, we elucidate the governing factors for H2SO4 –amine nucleation for various conditions. [ABSTRACT FROM AUTHOR]

Tetraheptylammonium bromide (THABr), tetrabutylammonium bromide (TBABr) and tetraethylammonium bromide (TEABr) dissolved in methanol or water methanol mixtures (~1 mM) produce via positive electrospray atomization and high resolution classification electrical classification standard clean ions (monomer and dier) which are singly charged. THABr is hydrophobic and insoluble in water, TBABr and TEABr are hygroscopic and water soluble (0.6 and 2.8 kg/L respectively). These ions are used to study the effect of hygroscopicity on the activation of aerosol particles in the sub 2 nm range via the detection efficiency measurement of a boosted ultrafine TSI condensation particle counter (3025A). Water solubility of particles seems to play a role in the activation and growth with butanol vapor in the CPC (condensation particle counter) independently of the size. [ABSTRACT FROM AUTHOR]

Oxidized organic molecules from ozonolysis of α-pinene are believed to participate in atmospheric new particle formation in forest regions. However, the detailed process of nucleation originated from α-pinene ozonolysis remains unclear. In this study, an atmospheric pressure interface time-of-flight mass spectrometer coupled with a planar differential mobility analyzer (DMA-MS) was applied to investigate the gas-phase reaction of α-pinene and ozone. Ion mobility and mass to charge ratio of the oxidized molecules were simultaneously measured. The ion mobility distribution of these oxidation products highlighted three mobility diameter ranges: 0.89–0.99 nm, 1.16–1.24 nm and 1.31–1.38 nm, where the dominating molecules were CH 2 O 2 , C 7-10 H 10-20 O 3-7 and C 19-20 H 30-32 O 6-9 , respectively. Combining the DMA-MS measurements, quantum chemistry calculations, and ion mobility modeling, the dominant monomer (C 10 H 16 O 3) was confirmed as pinonic acid while the primary dimer (C 20 H 32 O 6) was considered as pinonic acid cluster ((C 10 H 16 O 3) 2). Considering the yield of pinonic acid produced from α-pinene ozonolysis (2–4%), (C 10 H 16 O 3) 2 potentially accounts for part of the nocturnal C 20 dimers and ungrowable sub-2-nm particles in forests. Our work provides a new method to facilitate the understanding of atmospheric nucleation and initial growth. • A novel method to characterize molecular structures is introduced. • The dominating products of α-pinene ozonolysis are confirmed to be organic acid molecules and clusters. • Pinonic acid cluster potentially accounts for part of the nocturnal C 20 dimers and ungrowable sub-2-nm particles in forests. [ABSTRACT FROM AUTHOR]