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

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.