2-Methyltetrol sulfate ester-initiated nucleation mechanism enhanced by common nucleation precursors: A theory study

Barrierless reactions of C2 Criegee intermediates with H2SO4 and their implication to oligomers and new particle formation

The formation of oligomeric hydrogen peroxide triggered by Criegee intermediate maybe contributes significantly to the formation and growth of secondary organic aerosol (SOA). However, to date, the reactivity of C2 Criegee intermediates (CH3CHOO) in areas contaminated with acidic gas remains poorly understood. Herein, high-level quantum chemical calculations and Born-Oppenheimer molecular dynamics (BOMD) simulations are used to explore the reaction of CH3CHOO and H2SO4 both in the gas phase and at the air-water interface. In the gas phase, the addition reaction of CH3CHOO with H2SO4 to generate CH3HC(OOH)OSO3H (HPES) is near-barrierless, regardless of the presence of water molecules. BOMD simulations show that the reaction at the air-water interface is even faster than that in the gas phase. Further calculations reveal that the HPES has a tendency to aggregate with sulfuric acids, ammonias, and water molecules to form stable clusters, meanwhile the oligomerization reaction of CH3CHOO with HPES in the gas phase is both thermochemically and kinetically favored. Also, it is noted that the interfacial HPES- ion can attract H2SO4, NH3, (COOH)2 and HNO3 for particle formation from the gas phase to the water surface. Thus, the results of this work not only elucidate the high atmospheric reactivity of C2 Criegee intermediates in polluted regions, but also deepen our understanding of the formation process of atmospheric SOA induced by Criegee intermediates.

Ammonolysis of Glyoxal at the Air-Water Nanodroplet Interface

The reactions of glyoxal (CHO)2 ) with amines in cloud processes contribute to the formation of brown carbon and oligomer particles in the atmosphere. However, their molecular mechanisms remain unknown. Herein, we investigate the ammonolysis mechanisms of glyoxal with amines at the air-water nanodroplet interface. We identified three and two distinct pathways for the ammonolysis of glyoxal with dimethylamine and methylamine by using metadynamics simulations at the air-water nanodroplet interface, respectively. Notably, the stepwise pathways mediated by the water dimer for the reactions of glyoxal with dimethylamine and methylamine display the lowest free energy barriers of 3.6 and 4.9 kcal ⋅ mol-1 , respectively. These results showed that the air-water nanodroplet ammonolysis reactions of glyoxal with dimethylamine and methylamine were more feasible and occurred at faster rates than the corresponding gas phase ammonolysis, the OH+(CHO)2 reaction, and the aqueous phase reaction of glyoxal, leading to the dominant removal of glyoxal. Our results provide new and important insight into the reactions between carbonyl compounds and amines, which are crucial in forming nitrogen-containing aerosol particles.

Significant influence of water molecules on the SO3 + HCl reaction in the gas phase and at the air-water interface

The products resulting from the reactions between atmospheric acids and SO3 have a catalytic effect on the formation of new particles in aerosols. However, the SO3 + HCl reaction in the gas-phase and at the air-water interface has not been considered. Herein, this reaction was explored exhaustively by using high-level quantum chemical calculations and Born Oppenheimer molecular dynamics (BOMD) simulations. The quantum calculations show that the gas-phase reaction of SO3 + HCl is highly unlikely to occur under atmospheric conditions with a high energy barrier of 22.6 kcal mol-1. H2O and (H2O)2 play obvious catalytic roles in reducing the energy barrier of the SO3 + HCl reaction by over 18.2 kcal mol-1. The atmospheric lifetimes of SO3 show that the (H2O)2-assisted reaction dominates over the H2O-assisted reaction within the altitude range of 0-5 km, whereas the H2O-assisted reaction is more favorable within an altitude range of 10-50 km. BOMD simulations show that H2O-induced formation of the ClSO3-⋯H3O+ ion pair and HCl-assisted formation of the HSO4-⋯H3O+ ion pair were identified at the air-water interface. These routes followed a stepwise reaction mechanism and proceeded at a picosecond time scale. Interestingly, the formed ClSO3H in the gas phase has a tendency to aggregate with sulfuric acids, ammonias, and water molecules to form stable clusters within 40 ns simulation time, while the interfacial ClSO3- and H3O+ can attract H2SO4, NH3, and HNO3 for particle formation from the gas phase to the water surface. Thus, this work will not only help in understanding the SO3 + HCl reaction driven by water molecules in the gas-phase and at the air-water interface, but it will also provide some potential routes of aerosol formation from the reaction between SO3 and inorganic acids.

Computational chemistry of cluster: Understanding the mechanism of atmospheric new particle formation at the molecular level

New particle formation (NPF), which exerts significant influence over human health and global climate, has been a hot topic and rapidly expands field of research in the environmental and atmospheric chemistry recent years. Generally, NPF contains two processes: formation of critical nucleus and further growth of the nucleus. However, due to the complexity of the atmospheric nucleation, which is a multicomponent process, formation of critical clusters as well as their growth is still connected to large uncertainties. Detection limits of instruments in measuring specific gaseous aerosol precursors and chemical compositions at the molecular level call for computational studies. Computational chemistry could effectively compensate the deficiency of laboratory experiments as well as observations and predict the nucleation mechanisms. We review the present theoretical literatures that discuss nucleation mechanism of atmospheric clusters. Focus of this review is on different nucleation systems involving sulfur-containing species, nitrogen-containing species and iodine-containing species. We hope this review will provide a deep insight for the molecular interaction of nucleation precursors and reveal nucleation mechanism at the molecular level.

The reaction of Criegee intermediates with formamide and its implication to atmospheric aerosols

The reactions of Criegee intermediates (CIs) play an important role in the formation of secondary organic aerosols that have negative effect on visibility, human health, and global climate. New particle formation (NPF) can contribute to more than half of the aerosols in terms of their number concentration. Here, the reactions of CIs with formamide (FA) in the gas-phase and at the air/water interface were investigated using quantum chemistry calculation and Born-Oppenheimer molecular dynamic simulations. The results show that the reaction mechanism of CIs with FA is similar to that with formic acid, and the formation of hydroperoxymethyl formimidate (P4) is the most favorable pathway both in the gas-phase and at the air/water interface. Moreover, the potential contribution of the products to NPF was also evaluated by means of the molecular dynamic simulations. The results indicate that the product (P4) can participate in the SA-based NPF and water molecules are beneficial to enhance the NPF. The exploration will provide insight into the reaction of CIs with amide and the effect of the Criegee chemistry on the atmospheric aerosols.

In vitro toxic synergistic effects of exogenous pollutants-trimethylamine and its metabolites on human respiratory tract cells

The wide presence of volatile organic amines in atmosphere has been clarified to relate to adverse effects on human respiratory health. However, toxic effects of them on human respiratory tract and their metabiotic mechanism of in vivo transformation have not been elucidated yet. Herein, cell viability and production of reactive oxygen species (ROSs) were first investigated during acute exposure of trimethylamine (TMA) to bronchial epithelial cells (16HBE), along with identification of toxic metabolites and metabolic mechanisms of TMA from headspace atmosphere and cell culture. Results showed that cell activity decreased and ROS production increased with raising exposure TMA concentration. Toxic effects may be induced not only by TMA itself, but also more likely by its cellular metabolites. Increased dimethylamine identified in headspace atmosphere and cell solution was the main metabolite of TMA, and methylamine was also confirmed to be a further metabolite. In addition, TMA can also be oxygenated to generate N,N-dimethylformamide and N,N'-Bis(2-hydroxyethyl)-1,2-ethanediaminium by N-formylation or hydroxylation, which was considered to be the participation of cytochrome P450 (CYP) enzymes. Overall, we can conclude that respiratory tract cells may produce more toxic metabolites during exposure of toxic organic amines, which together further induce cellular oxidative stress and necrosis. Hence, the environment and health impact of metabolites as well as original parent atmospheric organic amines should be paid more attention in further researches and disease risk assessments.

Theoretical study of the formation and nucleation mechanism of highly oxygenated multi-functional organic compounds produced by α-pinene

In recent years, highly oxygenated organic molecules (HOMs) derived from photochemical reactions of α-pinene, the most abundant monoterpene, have been considered as important precursors of biogenic particles. However, the specific reactions of HOMs remain largely unknown, especially the corresponding formation and nucleation mechanism in the nanoscale. In this study, we implemented quantum chemical calculations and molecular dynamics (MD) simulations to explore the mechanism of the formation of HOM monomers/dimers by ozonolysis and autoxidation of α-pinene. Furthermore, we investigated the mechanisms of HOMs with different oxygen-to‑carbon (O/C) ratios and functional groups participating in neutral and ion-induced nucleation. The results show that the formation of HOMs is hardly affected by water, sulfuric acid and ions. In the ion-induced nucleation, HOM can dominate the initial nucleation steps; however, in the neutral nucleation, HOMs are more likely to participate in the growth stage. In addition, the nucleation ability of HOM has a bearing on the O/C ratio and the types of the functional groups. The current calculations provide valuable insight into the formation mechanism of the pure organic particles at low sulfuric acid concentrations.