Nanoparticles grown from methanesulfonic acid and methylamine: microscopic structures and formation mechanism

Amine-Enhanced Methanesulfonic Acid-Driven Nucleation: Predictive Model and Cluster Formation Mechanism

Atmospheric amines are considered to be an effective enhancer for methanesulfonic acid (MSA)-driven nucleation. However, out of the 195 detected atmospheric amines, the enhancing potential (EP) has so far only been studied for five amines. This severely hinders the understanding of the contribution of amines to MSA-driven nucleation. Herein, a two-step procedure was employed to probe the EP of various amines on MSA-driven nucleation. Initially, the formation free energies (ΔG) of 50 MSA-amine dimer clusters were calculated. Based on the calculated ΔG values, a robust quantitative structure-activity relationship (QSAR) model was built and utilized to predict the ΔG values of the remaining 145 amines. The QSAR model identified two guanidino-containing compounds as the potentially strongest enhancer for MSA-driven nucleation. Second, the EP of guanidino-containing compounds was studied by employing larger clusters and selecting guanidine (Gud) as a representative. The results indicate that Gud indeed has the strongest EP. The Gud-MSA system presents a unique clustering mechanism, proceeding via the initial formation of the (Gud)₁(MSA)₁ cluster, and subsequently by cluster collisions with either a (Gud)₁(MSA)₁ or (Gud)₂(MSA)₂ cluster. The developed QSAR model and the identification of amines with the strongest EP provide a foundation for comprehensively evaluating the contribution of atmospheric amines to MSA-driven nucleation.

Microscopic Insights Into the Formation of Methanesulfonic Acid–Methylamine–Ammonia Particles Under Acid-Rich Conditions

Understanding the microscopic mechanisms of new particle formation under acid-rich conditions is of significance in atmospheric science. Using quantum chemistry calculations, we investigated the microscopic formation mechanism of methanesulfonic acid (MSA)–methylamine (MA)–ammonia (NH3) clusters. We focused on the binary (MSA)2n-(MA)n and ternary (MSA)3n-(MA)n-(NH3)n, (n = 1–4) systems which contain more acid than base molecules. We found that the lowest-energy isomers in each system possess considerable thermodynamic and dynamic stabilities. In studied cluster structures, all bases are protonated, and they form stable ion pairs with MSA, which contribute to the charge transfer and the stability of clusters. MA and NH3 have a synergistic effect on NPF under acid-rich conditions, and the role of NH3 becomes more remarkable as cluster size increases. The excess of MSA molecules does not only enhance the stability of clusters, but provides potential sites for further growth.

Structural Effects of Amines in Enhancing Methanesulfonic Acid-Driven New Particle Formation

Atmospheric amines can enhance methanesulfonic acid (MSA)-driven new particle formation (NPF), but the mechanism is fundamentally different compared to that of the extensively studied sulfuric acid (SA)-driven process. Generally, the enhancing potentials of amines in SA-driven NPF follow the basicity, while this is not the case for MSA-driven NPF, where structural effects dominate, making MSA-driven NPF more prominent for methylamine (MA) compared to dimethylamine (DMA). Therefore, probing structural factors determining the enhancing potentials of amines on MSA-driven NPF is key to fully understanding the contribution of MSA to NPF. Here, we performed a comparative study on DMA and MA enhancing MSA-driven NPF by examining cluster formation using computational methods. The results indicate that DMA-MSA clusters are more stable than the corresponding MA-MSA clusters for cluster sizes up to (DMA)₂(MSA)₂, indicating that the basicity of amines dominates the initial cluster formation. The methyl groups of DMA were found to present significant steric hindrance beyond the (DMA)₂(MSA)₂ cluster and this adds to the lower hydrogen bonding capacity of DMA, making the cluster growth less favorable compared to MA. This study implies that several amines could synergistically enhance MSA-driven NPF by maximizing the advantage of different amines in different amine-MSA cluster growth stages.

Integrated experimental and theoretical approach to probe the synergistic effect of ammonia in methanesulfonic acid reactions with small alkylamines

While new particle formation events have been observed worldwide, our fundamental understanding of the precursors remains uncertain. It has been previously shown that small alkylamines and ammonia (NH3) are key actors in sub-3 nm particle formation through reactions with acids such as sulfuric acid (H2SO4) and methanesulfonic acid (CH3S(O)(O)OH, MSA), and that water also plays a role. Because NH3 and amines co-exist in air, we carried out combined experimental and theoretical studies examining the influence of the addition of NH3 on particle formation from the reactions of MSA with methylamine (MA) and trimethylamine (TMA). Experiments were performed in a 1 m flow reactor at 1 atm and 296 K. Measurements using an ultrafine condensation particle counter (CPC) and a scanning mobility particle sizer (SMPS) show that new particle formation was systematically enhanced upon simultaneous addition of NH3 to the MSA + amine binary system, with the magnitude depending on the amine investigated. For the MSA + TMA reaction system, the addition of NH3 at ppb concentrations produced a much greater effect (i.e. order of magnitude more particles) than the addition of ∼12 000 ppm water (corresponding to ∼45-50% relative humidity). The effect of NH3 on the MSA + MA system, which is already very efficient in forming particles on its own, was present but modest. Calculations of energies, partial charges and structures of small cluster models of the multi-component particles likewise suggest synergistic effects due to NH3 in the presence of MSA and amine. The local minimum structures and the interactions involved suggest mechanisms for this effect.

Atmospheric implication of synergy in methanesulfonic acid–base trimers: a theoretical investigation

Synergy between molecules is ubiquitous in atmospheric clusters and significantly affects new particle formation (NPF). Herein, the effects of the synergy between base molecules on the stability and evaporation of MSA–X–Y (MSA = methanesulfonic acid; X, Y = ammonia (A), methylamine (M), or dimethylamine (D)) trimers were investigated via density functional theory (DFT) and the atmospheric clusters dynamic code (ACDC) method. The results show that proton transfer from MSA to X is exothermal and barrierless due to the synergy between X and Y molecules in MSA–X–Y trimers compared with MSA-X dimers. Cyclic hydrogen bonds are a typical characteristic of the stable trimers. Topological analysis using atoms in molecules (AIM) theory indicates that the electron density (ρ) and Laplacian of the electron density (∇²ρ) at the bond critical points (BCPs) in the trimers exceed the standard range of hydrogen bonds. The affinity for attaching a Y molecule to the MSA–X dimers and the substitution of Y₁ (Y = A and MA) by Y₂ (Y₂ = MA and DMA) in the MSA–X–Y trimers are thermodynamically spontaneous. In addition, the cyclic stabilization energy of the MSA–X–Y trimers increased as the alkalinities of X and Y increased. The total evaporation rate of the trimers decreased as the alkalinities of X and Y increased. Low temperature and high pressure significantly facilitate the formation of trimers. It is further confirmed that synergy plays an important role in atmospheric NPF events.

The effects of synergy of between X and Y on the stability of MSA–X–Y trimers were investigated via quantum chemical and kinetics simulation method.

Modelling consortium for chemistry of indoor environments (MOCCIE): integrating chemical processes from molecular to room scales

We report on the development of a modelling consortium for chemistry in indoor environments that connects models over a range of spatial and temporal scales, from molecular to room scales and from sub-nanosecond to days, respectively. Our modeling approaches include molecular dynamics (MD) simulations, kinetic process modeling, gas-phase chemistry modeling, organic aerosol modeling, and computational fluid dynamics (CFD) simulations. These models are applied to investigate ozone reactions with skin and clothing, oxidation of volatile organic compounds and formation of secondary organic aerosols, and mass transport and partitioning of indoor species to surfaces. MD simulations provide molecular pictures of limonene adsorption on SiO2 and ozone interactions with the skin lipid squalene, providing kinetic parameters such as surface accommodation coefficient, desorption lifetime, and bulk diffusivity. These parameters then constrain kinetic process models, which resolve mass transport and chemical reactions in gas and condensed phases for analysis of experimental data. A detailed indoor chemical box model is applied to simulate α-pinene ozonolysis with improved representation of gas-particle partitioning. Application of 2D-volatility basis set reveals that OH-induced aging sometimes drives increases in indoor organic aerosol concentrations, due to organic mass functionalization and enhanced partitioning. CFD simulations show that concentrations of ozone and primary product change near the human surface rapidly, indicating non-uniform spatial distributions from the occupant surface to ambient air, while secondary ozone product is relatively well-mixed throughout the room. This development establishes a framework to integrate different modeling tools and experimental measurements, opening up an avenue for development of comprehensive and integrated models with representations of various chemistry in indoor environments.

Uptake of water by an acid–base nanoparticle: theoretical and experimental studies of the methanesulfonic acid–methylamine system

Uptake of water by nanoparticles composed by methanesulfonic acid and methylamine using a combination of theoretical calculations and laboratory experiments.