Formation Mechanism of Atmospheric Ammonium Bisulfate: Hydrogen-Bond-Promoted Nearly Barrierless Reactions of SO₃ with NH₃ and H₂ O

Particulate matter (PM) air pollution threatens the health of people and ecosystems worldwide. As the key component of PM, ammonium sulfate plays a critical role in the formation of aerosol particles; thus, there is an urgent need to know the detailed mechanisms for its formation in the atmosphere. Through a quantum chemistry study, we reveal a series of nearly barrierless reactions that may occur in clusters/droplets in the atmosphere leading to the formation of ammonium bisulfate (NH₄ HSO₄ ), the precursor of ammonium sulfate. In this mechanism, NH₄ HSO₄ is directly formed through one-step reactions of SO₃ with H₂ O and NH₃ promoted by surrounding molecule(s) that substantially lower the reaction activation barrier to ≈0 kcal mol^-1 . The promoters of these reactions are found to be various common atmospheric molecules, such as water, ammonia, and sulfuric acid, which can form relatively strong hydrogen bonds with the reaction center. Our results suggest many more similar pathways that can be facilitated by other ambient molecules. Due to its one-step and barrierless reaction characteristics and the great abundance of potential reactions, this mechanism has great implications on the formation of atmospheric ammonium sulfate as well as on the growth of aerosol particles.