Simplified mechanism for new particle formation from methanesulfonic acid, amines, and water via experiments and ab initio calculations

Abstract

Airborne particles affect human health and significantly influence visibility and climate. A major fraction of these particles result from the reactions of gaseous precursors to generate low-volatility products such as sulfuric acid and high-molecular weight organics that nucleate to form new particles. Ammonia and, more recently, amines, both of which are ubiquitous in the environment, have also been recognized as important contributors. However, accurately predicting new particle formation in both laboratory systems and in air has been problematic. During the oxidation of organosulfur compounds, gas-phase methanesulfonic acid is formed simultaneously with sulfuric acid, and both are found in particles in coastal regions as well as inland. We show here that: (i) Amines form particles on reaction with methanesulfonic acid, (ii) water vapor is required, and (iii) particle formation can be quantitatively reproduced by a semiempirical kinetics model supported by insights from quantum chemical calculations of likely intermediate clusters.Such an approach may be more broadly applicable in models of outdoor, indoor, and industrial settings where particles are formed, and where accurate modeling is essential for predicting their impact on health, visibility, and climate.kinetics modeling | multi-component nucleation | cluster enthalpy | flow tube reactor | atmospheric nanoparticles U nderstanding how gas phase precursors lead to the formation and growth of new particles that are important for scattering light, for serving as cloud condensation or ice nuclei, and for transport deep into the lung, is one of the most pressing scientific problems (1-5). The most studied system is the conversion of gas-phase SO 2 to sulfuric acid and sulfate particles, but even in this case, models typically underestimate particle formation by an order of magnitude or more (3, 4, 6). However, an accurate predictive capability based on molecular-level understanding is critical for projecting the impacts of particles and developing optimal control strategies.Classical nucleation theory (CNT) has been used for almost a century (7, 8) to predict new particle formation. At its heart, CNT is a thermodynamics approach that assumes that the precursor clusters have bulk liquid properties such as surface tension, and that addition to and evaporation from the clusters occurs via monomers. Modifications to CNT using kinetics approaches have been described (9, 10). More recently, dynamical nucleation theory (11-13) examined intermolecular interactions and used them to obtain rate constants for the individual steps through variational transition-state theory. This theory has been applied to particle formation in relatively simple systems and clusters of relatively few molecules. Recent data from field and laboratory studies, however, suggest that multicomponent systems with multiple reaction steps are likely involved in new particle formation in the atmosphere (14-22).We report here a combination of experimental and theoretical studies of new particle fo...

New particle formation and growth from methanesulfonic acid, trimethylamine and water

“…† As in the earlier model, the first few steps (reactions (S1)-(S8) in Table S1, ESI †) were assumed to be diffusion-controlled in the forward direction, and rate constants were calculated using hard-sphere collision theory. 53 Rate constants in the reverse direction were calculated using forward rate constants and DG of formation obtained from quantum chemical calculations. 53 For the subsequent steps, rate constants were adjusted to fit particle number concentrations obtained from experiments under different conditions.…”

“…53 Rate constants in the reverse direction were calculated using forward rate constants and DG of formation obtained from quantum chemical calculations. 53 For the subsequent steps, rate constants were adjusted to fit particle number concentrations obtained from experiments under different conditions. It should be noted that this multistep mechanism and the rate constants are not unique, but simply used to demonstrate that a box model kinetics approach can capture the fundamental behavior of the system.…”

“…52 The monohydrate and dihydrate (n = 1 or 2) are the two most prominent hydrates at all RH so are included in the model. Both unhydrated and hydrated MSA can react with TMA: 53 MSA + TMA " MSAÁTMA (1) …”

“…59,60 Although MSA was shown to be much less efficient than H 2 SO 4 in forming particles with water, [50][51][52]61 a recent study from this laboratory found that MSA does form particles with dimethylamine (DMA, (CH 3 ) 2 NH) and TMA, but only in the presence of water vapor. 53 However, this study used a large volume flow system with reaction times from 4.2 min to B1 h, so that early stages of particle formation and growth could not be investigated.…”

“…A kinetics scheme for particle formation from MSA, TMA and water that reproduces the experimental data reasonably well is developed based in part on previous quantum chemical calculations of likely early intermediates. 53 This approach is sufficiently simple that it could be incorporated into atmospheric models of new particle formation in a relatively straightforward manner.…”