Modeling approaches for atmospheric ion-dipole collisions : all-atom trajectory simulations and central field methods

Ion-dipole collisions can facilitate the formation of atmospheric aerosol particles and play an important role in their detection in chemical ionization mass spectrometers. Conventionally, analytical models, or simple parametrizations, have been used to calculate the rate coefficients of ion-dipole collisions in the gas phase. Such models, however, neglect the atomistic structure and charge distribution of the collision partners. To determine the accuracy and applicability of these approaches under atmospheric conditions, we calculated collision cross sections and rate coefficients from all-atom molecular dynamics collision trajectories, sampling the relevant range of impact parameters and relative velocities, and from a central field model using an effective attractive interaction fitted to the long-range potential of mean force between the collision partners. We considered collisions between various atmospherically relevant molecular ions and dipoles and charged and neutral dipolar clusters. Based on the good agreement between collision cross sections and rate coefficients obtained from molecular dynamics trajectories and a generalized central field model, we conclude that the effective interactions between the collision partners are isotropic to a high degree, and the model is able to capture the relevant physicochemical properties of the systems. In addition, when the potential of mean force is recalculated at the respective temperatures, the central field model exhibits the correct temperature dependence of the collision process. The classical parametrization by Su and Chesnavich (1982), which combines a central field model with simplified trajectory simulations, is able to predict the collision rate coefficients and their temperature dependence quite well for molecular systems, but the agreement worsens for systems containing clusters. Based on our results, we propose the combination of potential of mean force calculation and a central field model as a viable and elegant alternative to the brute force sampling of individual collision trajectories over a large range of impact parameters and relative velocities.Peer reviewe

Nonisothermal nucleation in the gas phase is driven by cool subcritical clusters

Nucleation of clusters from the gas phase is a widely encountered phenomenon, yet rather little is understood about the underlying out-of-equilibrium dynamics of this process. The classical view of nucleation assumes isothermal conditions where the nucleating clusters are in thermal equilibrium with their surroundings. However, in all first-order phase transitions, latent heat is released, potentially heating the clusters and suppressing the nucleation. The question of how the released energy affects cluster temperatures during nucleation as well as the growth rate remains controversial. To investigate the nonisothermal dynamics and energetics of homogeneous nucleation, we have performed molecular dynamics simulations of a supersaturated vapor in the presence of thermalizing carrier gas. The results obtained from these simulations are compared against kinetic modeling of isothermal nucleation and classical nonisothermal theory. For the studied systems, we find that nucleation rates are suppressed by two orders of magnitude at most, despite substantial release of latent heat. Our analyses further reveal that while the temperatures of the entire cluster size populations are elevated, the temperatures of the specific clusters driving the nucleation flux evolve from cold to hot when growing from subcritical to supercritical sizes and resolve the apparent contradictions regarding cluster temperatures. Our findings provide unprecedented insight into realistic nucleation events and allow us to directly assess earlier theoretical considerations of nonisothermal nucleation.Peer reviewe

Diamines Can Initiate New Particle Formation in the Atmosphere

Recent experimental evidence suggests that diamines can enhance atmospheric new particle formation more efficiently compared to monoamines such as dimethylamine Here we investigate the molecular interactions between sulfuric acid (sa) and the diamine putrescine (put) using computational methods. The molecular structure of up to four sulfuric acid molecules and up to four putrescine molecules were obtained, at the omega B97X-D/6-31++G(d,p) level of theory. We utilized a domain local pair natural orbital coupled cluster method (DLPNO-CCSD(T)/aug-cc-pVTZ) to obtain highly accurate binding energies of the clusters. We find that the (sa)(1-4)(put)(1-4) clusters show more ionic character than clusters consisting of sulfuric acid and dimethylamine (dma) by readily forming several sulfate ions in the cluster. To estimate the stability of the clusters, we calculate the evaporation rates and compare them to ESI-APi-TOF measurements. Using the atmospheric cluster dynamics code (ACDC), we simulate and compare the new particle formation rates between the (sa)(1-4)(put)(1-4) and (sa),(1-4)(dma)(1-4) cluster systems. We find that putrescine significantly enhances the formation of new particles compared to dimethylamine. Our findings suggest that a large range of amines with different basicity is capable of explaining various regions of the observed new particle formation events. These results indicate that diamines, or related compounds with high basicity, might be important species in forming the initial cluster with sulfuric acid and subsequently more abundant amines with lower basicity can assist in the new particle formation process by attaching to the sulfuric acid-diamine nucleus.Peer reviewe

An Exploratory Study of the Learning of Transferable Skills in a Research-Oriented Intensive Course in Atmospheric Sciences

Transferable skills, such as learning skills as well as oral and written communication skills, are needed by today’s experts. The learning of transferable skills was studied during a multidisciplinary two-week, research-oriented intensive course in atmospheric sciences. Students were assessed on their experience of learning data analysis, writing reports and articles, oral presentation, learning and teaching, as well as project and time management skills and the importance of learning these transferable skills in the beginning and at the end of the course. The learning outcomes were constructively aligned with the course and it supported the learning of transferable skills needed by researchers working with multidisciplinary research questions. The methods of teaching were group work, data analysis of real scientific questions and real scientific data, a few expert lectures, discussions with experts and peer-support, and the course evaluation that was based on the groups’ oral presentations and a written report. The groups consisted of seven to eight students and four to six assistants who were working side-by-side for the period of the course. Students considered data analysis, including the formulation of research questions, as the most important transferable skill of the course and stated that it was also what they learned the most. We conclude that the students felt that working with real scientific questions and data in multidisciplinary groups supports the learning of transferable skills. The findings suggest that the students may have learned transferable skills from peers, assistants, and teachers while working in small groups of students in different stages of their studies. The study was conducted from student feedback from one course only, but we have observed while organizing over 50 similar courses that working on real scientific questions and data in a multidisciplinary and multicultural course has been motivating for both the teachers and the students. We recommend this method to be used by research groups who are training the future generation of researchers and experts in atmospheric sciences and other fields.Peer reviewe

Impact of Quantum Chemistry Parameter Choices and Cluster Distribution Model Settings on Modeled Atmospheric Particle Formation Rates

We tested the influence of various parameters on the new particle formation rate predicted for the sulfuric acid–ammonia system using quantum chemistry and cluster distribution dynamics simulations, in our case, Atmospheric Cluster Dynamics Code (ACDC). We found that consistent consideration of the rotational symmetry number of monomers (sulfuric acid and ammonia molecules, and bisulfate and ammonium ions) leads to a significant rise in the predicted particle formation rate, whereas inclusion of the rotational symmetry number of the clusters only changes the results slightly, and only in conditions where charged clusters dominate the particle formation rate. This is because most of the clusters stable enough to participate in new particle formation have a rotational symmetry number of 1, and few exceptions to this rule are positively charged clusters. In contrast, the application of the quasi-harmonic correction for low-frequency vibrational modes tends to generally decrease predicted new particle formation rates and also significantly alters the slope of the formation rate curve plotted against the sulfuric acid concentration, which is a typical convention in atmospheric aerosol science. The impact of the maximum size of the clusters explicitly included in the simulations depends on the simulated conditions. The errors arising from a limited set of clusters are higher for higher evaporation rates, and thus tend to increase with temperature. Similarly, the errors tend to be higher for lower vapor concentrations. The boundary conditions for outgrowing clusters (that are counted as formed particles) have only a small influence on the results, provided that the definition is chemically reasonable and that the set of simulated clusters is sufficiently large. A comparison with data from the Cosmics Leaving OUtdoor Droplets (CLOUD) chamber and a cluster distribution dynamics model using older quantum chemistry input data shows improved agreement when using our new input data and the proposed combination of symmetry and quasi-harmonic corrections.Peer reviewe

Atomistic Simulation of Ice Nucleation on Silver Iodide (0001) Surfaces with Defects

Small particles of silver iodide (AgI) are known to have excellent ice nucleating capabilities and have been used in rain seeding applications. It is widely believed that the silver-terminated (0001) surface of beta-AgI acts as a template for the basal plane of hexagonal ice. However, the (0001) surface of ionic crystals with the wurtzite structure is polar and will therefore exhibit reconstructions and defects. Here, we use atomistic molecular dynamics simulations to study how the presence of defects on AgI(0001) affects the rates and mechanism of heterogeneous ice nucleation at moderate supercooling at -10 degrees C. We consider AgI(0001) surfaces exhibiting vacancies, step edges, terraces, and pits and compare them to simulations of the corresponding ideal surface. We find that, while point defects have no significant effect on ice nucleation rates, step edges, terraces, and pits reduce both the nucleation and growth rates by up to an order of magnitude. The reduction of the ice nucleation rate correlates well with the fraction of the surface area around the defects where perturbations of the hydration layer hinder the formation of a critical ice nucleus.Peer reviewe

Computational Study of the Effect of Mineral Dust on Secondary Organic Aerosol Formation by Accretion Reactions of Closed-Shell Organic Compounds

The effect of dust aerosols on accretion reactions of water, formaldehyde, and formic acid was studied in the conditions of earth's troposphere at the DLPNO-CCSD(T)/aug-cc-pVTZ//omega B97X-D/6-31++G** level of theory. A detailed analysis of the reaction mechanisms in the gas phase and on the surface of mineral dust, represented by mono- and trisilicic acid, revealed that mineral dust has the potential of decreasing reaction barrier heights. Specifically, at 0 K, mineral dust can lower the apparent energy barrier of the reaction of formaldehyde with formic acid to zero. However, when the entropic contributions to the reaction free energies were accounted for, mineral dust was found to selectively enhance the reaction of water with formaldehyde, while inhibiting the reaction of formaldehyde and formic acid, in the lower parts of the troposphere (with temperatures around 298 K). In the upper troposphere (with temperatures closer to 198 K), mineral dust catalyzes both reactions and also the reaction of methanol with formic acid. Despite the intrinsic potential of mineral dust, calculation of the catalytic enhancement parameter for a likely range of dust aerosol concentrations suggested that dust aerosols will not contribute to secondary organic aerosol formation via dimerization of closed-shell organic compounds. The main reason for this is the relatively low absolute concentratign of tropospheric dust aerosol and its inefficiency in increasing the effective reaction rate coefficients.Peer reviewe