Self-Catalytic Reaction of SO₃ and NH₃ to Produce Sulfamic Acid and Its Implication to Atmospheric Particle Formation

Sulfur trioxide (SO₃) is one of the most active chemical species in the atmosphere, and its atmospheric fate has profound implications to air quality and human health. The dominant gas-phase loss pathway for SO₃ is generally believed to be the reaction with water molecules, resulting in sulfuric acid. The latter is viewed as a critical component in the new particle formation (NPF). Herein, a new and competitive loss pathway for SO₃ in the presence of abundant gas-phase ammonia (NH₃) species is identified. Specifically, the reaction between SO₃ and NH₃, which produces sulfamic acid, can be self-catalyzed by the reactant (NH₃). In dry and heavily polluted areas with relatively high concentrations of NH₃, the effective rate constant for the bimolecular SO₃-NH₃ reaction can be sufficiently fast through this new loss pathway for SO₃ to become competitive with the conventional loss pathway for SO₃ with water. Furthermore, this study shows that the final product of the reaction, namely, sulfamic acid, can enhance the fastest possible rate of NPF from sulfuric acid and dimethylamine (DMA) by about a factor of 2. An alternative source of stabilizer for acid-base clustering in the atmosphere is suggested, and this new mechanism for NPF has potential to improve atmospheric modeling in highly polluted regions.