A possible unaccounted source of atmospheric sulfate formation: amine-promoted hydrolysis and non-radical oxidation of sulfur dioxide

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.

A Computational Perspective on the Chemical Reaction of HFO-1234zc with the OH Radical in the Gas Phase and in the Presence of Mineral Dust

The gas phase and heterogeneous reaction on mineral dust aerosols of trace gases could significantly affect the tropospheric oxidation capacity and aerosol composition of the atmosphere. In this work, the OH radical-initiated oxidation of a hydrofluoroolefin, HFO-1234zc, and subsequent reaction of favorable intermediates with other reactive species, such as O₂, HO₂, and NO_x (x = 1-2) radicals, were studied, and the role of mineral dust in the form of silicate clusters on the reaction mechanism and rate constant was studied. In the gas phase, OH radical addition to HFO-1234zc is kinetically more favorable than the H-atom abstraction reaction. The calculated reaction energy barrier and thermochemical parameters show that both the initial reactions are more feasible on silicate clusters. Thus, silicates can act as chemical sinks for trapping of hydrofluoroolefins (HFOs). It is found that both gas-phase and heterogeneous reactions are responsible for the transformation of HFOs into fluorinated compounds in the atmosphere. Further, the results show that the ozone creation potential of HFO-1234zc is low, and few of the products are harmful to aquatic organisms. This study provides new insights on the formation of toxic pollutants from the oxidation of HFO-1234zc, which may have significant implications in the troposphere.

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.

New Reactions for the Formation of Organic Nitrate in the Atmosphere

Organic nitrates make an important contribution to the formation of secondary organic aerosols, but the formation mechanisms of organic nitrates are not fully understood at the molecular level. In the present work, we explore a new route for the formation of organic nitrates in the reaction of formaldehyde (HCHO) with nitric acid (HNO₃) catalyzed by water (H₂O), ammonia (NH₃), and dimethylamine ((CH₃)₂NH) using theoretical methods. The present results using CCSD(T)-F12a/cc-pVTZ-F12//M06-2X/MG3S unravel that dimethylamine has a stronger catalytic ability in the reaction of HCHO with HNO₃, reducing the barrier by 21.97 kcal/mol, while water and ammonia only decrease the energy barrier by 7.35 and 13.56 kcal/mol, respectively. In addition, the calculated kinetics combined with the corresponding concentrations of these species show that the HCHO + HNO₃ + (CH₃)₂NH reaction can compete well with the naked HCHO + HNO₃ reaction at 200-240 K, which may make certain contributions to the formation of organic nitrates under some atmospheric conditions.

Resolving the amine-promoted hydrolysis mechanism of N2O5 under tropospheric conditions

Hydrolysis of N₂O₅ under tropospheric conditions plays a critical role in assessing the fate of O₃, OH, and NO_x in the atmosphere. However, its removal mechanism has not been fully understood, and little is known about the role of entropy. Herein, we propose a removal path of N₂O₅ on the water clusters/droplet with the existence of amine, which entails a low free-energy barrier of 4.46 and 3.76 kcal/mol on a water trimer and droplet, respectively, at room temperature. The free-energy barrier exhibits strong temperature dependence; a barrierless hydrolysis process of N₂O₅ at low temperature (≤150 K) is observed. By coupling constrained ab initio molecular dynamics (constrained AIMD) simulations with thermodynamic integration methods, we quantitively evaluated the entropic contributions to the free energy and compared NH₃-, methylamine (MA)-, and dimethylamine (DMA)-promoted hydrolysis of N₂O₅ on water clusters and droplet. Our results demonstrate that methylation of NH₃ stabilizes the product state and promotes hydrolysis of N₂O₅ by reducing the free-energy barriers. Furthermore, a quantitative analysis of the internal coordinate distribution of the reaction center and the relative position of surrounding species reveals that the significant entropic contribution primarily results from the ensemble effect of configurations observed in the AIMD simulations. Such an ensemble effect becomes more significant with more water molecules included. Lowering the temperature effectively minimizes the entropic contribution, making the hydrolysis more exothermic and barrierless. This study sheds light on the importance of the promoting effect of amines and the entropic effect on gas-phase hydrolysis reactions, which may have far-reaching implications in atmospheric chemistry.

Anharmonic IR spectra of solvated ammonium and aminium ions: resemblance between water and bisulfate solvations

In this work, we analyze the vibrational spectra of ammonium, methylammonium, and dimethylammonium ions solvated by either water molecules or bisulfate anions using anharmonic vibrational algorithms. Rich and complicated spectral features in the 2700-3200 cm^-1 region of the experimental spectra of these clusters are attributed to originate from strong Fermi resonance between hydrogen-bonded NH stretching fundamentals and NH bending overtones. Additional weaker bands around 2500-2600 cm^-1 in solvated aminium ions are assigned to the combination tones involving the CH-NH (methyl-amino) rocking modes. Furthermore, the qualitative resemblance in band positions and spectral patterns between two-water-solvated and two-bisulfate-solvated cations suggest a common vibrational coupling scheme beneath the two seemingly different micro-solvation environments.

NO3·-Initiated Gas-Phase Formation of Nitrated Phenolic Compounds in Polluted Atmosphere

Organic nitrogen (ON) compounds are key contents of particulate matter in the megacities of Asia. As a series of important ON, nitrated phenolic compounds (NPs) are of high concentration in the atmosphere, although their formation mechanism and role in particulate nucleation and growth are not fully understood. Herein, using a high level of quantum mechanical calculations, we explore the formation paths of NPs initiated by NO₃· radicals, where some common atmospheric species, such as H₂O, (H₂O)₂, NH₃, and dimethylamine (DMA), can act as molecular catalysts. The kinetic study predicts that the formation rate of methyl nitrophenols with the assistance of DMA and (H₂O)₂ can reach ∼10³ molecules·cm^-3·s^-1 in a polluted and humid atmosphere. The volatilities obtained from the empirical model show the formed NPs mainly belong to the intermediate and semivolatile organic compounds, which can participate in the growth process of aerosols rather than the early stage of cluster nucleation. Moreover, some NPs can be salified with atmospheric bases to further increase their contributions to the particulate formation.