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Recent experiments have revealed a vital nucleation process of iodic acid (HIO 3) and iodous acid (HIO 2) under marine boundary layer conditions. However, HIO 3 –HIO 2 nucleation may not effectively drive the observed rapid new particle formation (NPF) in certain coastal regions influenced by urban air masses. Dimethylamine (DMA) is a promising basic precursor to enhance nucleation considering its strong ability to stabilize acidic clusters and the wide distribution in marine atmosphere, while its role in HIO 3 –HIO 2 nucleation remains unrevealed. Hence, a method combining quantum chemical calculations and Atmospheric Cluster Dynamics Code (ACDC) simulations was utilized to study the HIO 3 –HIO 2 –DMA nucleation process. We found that DMA can preferentially accept the proton from HIO 3 as a basic precursor in the most stable configurations of HIO 3 –HIO 2 –DMA clusters. Kinetically, the participation of DMA in the cluster formation pathways of the iodine oxoacid system could be significant at the 10 -1 to 1 pptv level of [DMA]. Furthermore, DMA can enhance the cluster formation rates of the HIO 3 –HIO 2 system in marine and polar regions near DMA sources more than 10 3 -fold. Compared to the classical nucleation mechanism, the HIO 3 –HIO 2 –DMA mechanism exhibits strong nucleation ability, worthy of consideration as a promising mechanism in marine and polar regions rich in amine sources. The newly proposed HIO 3 –HIO 2 –DMA ternary mechanism might provide an explanation for some missing fluxes of atmospheric iodine particles. [ABSTRACT FROM AUTHOR]

Intense and frequent new particle formation (NPF) events have been observed in polluted urban environments, yet the dominant mechanisms are still under debate. To understand the key species and governing processes of NPF in polluted urban environments, we conducted comprehensive measurements in downtown Beijing during January–March, 2018. We performed detailed analyses on sulfuric acid cluster composition and budget, as well as the chemical and physical properties of oxidized organic molecules (OOMs). Our results demonstrate that the fast clustering of sulfuric acid (H2SO4) and base molecules triggered the NPF events, and OOMs further helped grow the newly formed particles toward climate‐ and health‐relevant sizes. This synergistic role of H2SO4, base species, and OOMs in NPF is likely representative of polluted urban environments where abundant H2SO4 and base species usually co‐exist, and OOMs are with moderately low volatility when produced under high NOx concentrations. Plain Language Summary: Atmospheric new particle formation (NPF) is a dominant source of atmospheric ultrafine particles worldwide. Those particles profoundly influence climate and human health. NPF includes two consecutive processes, that is, the formation of new particles (∼2 nm in diameter) and their subsequent growth to larger sizes. Extensive studies conducted in the laboratory and in forested areas have shown that many gaseous species can participate in NPF, such as sulfuric acid, ammonia, amines, and oxidize organic molecules. However, the actual roles of these vapors may vary significantly from location to location and are largely unclear in urban environments. Here, based on measurements of sulfuric acid, sulfuric acid clusters, and oxidize organic molecules, we demonstrate that sulfuric acid and base molecules were responsible for the initial formation of new particles during a wintertime field campaign in Beijing. The majority of oxidized organic molecules had a minor contribution to the formation of new particles but were crucial for particle growth above 2–3 nm to climate‐ and health‐relevant sizes. Key Points: Process‐level understanding of new particle formation (NPF) in wintertime Beijing was obtained based on measurement state‐of‐the‐art instrumentsThe analysis of sulfuric acid cluster composition and budget showed that sulfuric acid‐base clustering initiated NPFCondensable organic vapors were characterized and demonstrated to have a crucial influence on the growth of newly formed particles [ABSTRACT FROM AUTHOR]

The growth rate of atmospheric new particles is a key parameter that determines their survival probability to become cloud condensation nuclei and hence their impact on the climate. There have been several methods to estimate the new particle growth rate. However, due to the impact of coagulation and measurement uncertainties, it is still challenging to estimate the initial growth rate of sub-3 nm particles, especially in polluted environments with high background aerosol concentrations. In this study, we explore the feasibility of the appearance time method to estimate the growth rate of sub-3 nm particles. The principle of the appearance time method and the impacts of coagulation on the retrieved growth rate are clarified. New formulae in both discrete and continuous spaces are proposed to correct the impacts of coagulation. Aerosol dynamic models are used to test the new formulae. New particle formation in urban Beijing is used to illustrate the importance to consider the impacts of coagulation on sub-3 nm particle growth rate and its calculation. We show that the conventional appearance time method needs to be corrected when the impacts of coagulation sink, coagulation source, and particle coagulation growth are non-negligible compared to the condensation growth. Under the simulation conditions with a constant vapor concentration, the corrected growth rate agrees with the theoretical growth rates. The variation of vapor concentration is found to impact growth rate obtained with the appearance time method. Under the simulation conditions with a varying vapor concentration, the average bias of the corrected 1.5-3 nm particle growth rate range from 6-44 %. During the test new particle formation event in urban Beijing, the corrected condensation growth rate of sub-3 nm particles was in accordance with the growth rate contributed by sulfuric acid condensation, whereas the conventional appearance time method overestimated the condensation growth rate of 1.5 nm particles by 80 %. [ABSTRACT FROM AUTHOR]

New particle formation (NPF) and subsequent particle growth occur frequently in various atmospheric environments. Significant influence of transport on aerosol size distributions is commonly observed, especially for non-regional NPF events. With certain assumptions and approximations, a population balance method is proposed to examine the influence of transport on the temporal evolution of aerosol size distributions during NPF events. The method is derived from the aerosol general dynamic equation in the continuous form. Meteorological information (e.g., wind speed, wind direction, and water vapor mixing ratio) was used to complement the analysis. The NPF events observed in Southeast Tibet, Fukue Island, and urban Beijing were analyzed using the proposed method. Significant contribution of transport to the observed aerosol size distributions is found during the NPF events in both Southeast Tibet and Fukue Island. The changes in the contribution of transport have a good correlation with the changes in wind speed and direction. This correlation indicates that local mountain and valley breezes govern the observed new particles at the Southeast Tibet site. Most NPF events observed at Fukue Island are closely related to the long-range transport of aerosols and gaseous precursors due to the movement of air masses. Regional NPF events are typically observed in urban Beijing and the contribution of transport to the observed aerosol size distributions is negligible when compared to other processes such as condensational growth and coagulation scavenging. In a relatively clean atmospheric environment, the proposed method can be used to characterize the contribution of transport to particles in the size range from ~ 10 to ~ 50 nm. During intense NPF events in a relatively polluted atmospheric environment, however, the estimated contribution of transport is sensitive to the uncertainties in condensational growth and coagulation scavenging due to the dominance of their corresponding terms in the population balance equation. [ABSTRACT FROM AUTHOR]