Role of Methanesulfonic Acid in Sulfuric Acid–Amine and Ammonia New Particle Formation

[Image: see text] Aerosol nucleation accounts for over half of all seed particles for cloud droplet formation. In the atmosphere, sulfuric acid (SA) nucleates with ammonia, amines, oxidized organics, and many more compounds to form particles. Studies have also shown that methanesulfonic acid (MSA) nucleates independently with amines and ammonia. MSA and SA are produced simultaneously via dimethyl sulfide oxidation in the marine atmosphere. However, limited knowledge exists on how MSA and SA nucleate together in the presence of various atmospherically relevant base compounds, which is critical to predicting marine nucleation rates accurately. This work provides experimental evidence that SA and MSA react to form particles with amines and that the SA-MSA-base nucleation has different reaction pathways than SA-base nucleation. Specifically, the formation of the SA-MSA heterodimer creates more energetically favorable pathways for SA-MSA-methylamine nucleation and an enhancement of nucleation rates. However, SA-trimethylamine nucleation is suppressed by MSA, likely due to the steric hindrance of the MSA and trimethylamine. These results display the importance of including nucleation reactions between SA, MSA, and various amines to predict particle nucleation rates in the marine atmosphere.