Reactions of Methanesulfonic Acid with Amines and Ammonia as a Source of New Particles in Air

New particle formation (NPF) from gaseous precursors as a significant source of aerosol needs to be better understood to accurately predict the impacts on visibility, climate change, and human health. While ternary nucleation of sulfuric acid, amines/NH₃, and water is recognized as a significant driver for NPF, increasing evidence suggests a contribution from methanesulfonic acid (MSA) and amines under certain conditions. Here we report the formation of particles 2.5–10 nm in diameter from the reactions of MSA with methyl­amine (MA), dimethyl­amine (DMA), and NH₃ at reaction times of 2.3–7.8 s in a flow reactor and compare these particles with those previously reported to be formed from reaction with trimethyl­amine (TMA). The effects of water vapor and concentrations of gaseous precursors on the particle number concentration and particle size were studied. The presence of water significantly enhances particle formation and growth. Under similar experimental conditions, particle number concentrations decrease in the order MA ≫ TMA ≈ DMA ≫ NH₃, where NH₃ is 2–3 orders of magnitude less efficient than DMA. Quantum chemical calculations of likely intermediate clusters were carried out to provide insights into the role of water and the different capacities of amines/NH₃ in particle formation. Both gas-phase basicity and hydrogen-bonding capacity of amines/NH₃ contribute to the potential for particles to form and grow. Our results indicate that, although amines typically have concentrations 1–3 orders of magnitude lower than that of NH₃ in the atmosphere, they still play an important role in driving NPF