Mechanistic Insights into Nitric Sulfuric Acid Formation and Its Enhancement of Sulfuric Acid-Ammonia Nucleation under Severe Urban Pollution

Abstract. Organosulfates (OSs) constitute an important component of secondary organic aerosols (SOA). Although previous studies have investigated the aerosol nucleation of OSs such as glycolic acid sulfate and methyl hydrogen sulfate, exploring the aerosol nucleation behavior of additional OSs remains essential. Herein, the formation mechanism of nitric sulfuric acid (NSA) from the reaction of sulfur trioxide (SO₃) with nitric acid (NA)and its role in sulfuric acid (SA)-ammonia (A) aerosol nucleation was investigated using quantum chemical calculations combined with kinetic simulations. Our results demonstrate that NSA can be formed rapidly and stably in the gas phase with a low barrier of 3.61 kcal·mol^-1. The SO₃ + HNO₃ reaction remains competitive with major atmospheric SO₃ loss pathways, including reactions of SO₃ + HCOOH, SO₃ + C₆H₅COOH and NH₃-catalyzed reactions, and even H₂O-catalyzed hydrolysis under dry conditions. Kinetic simulations further demonstrate that NSA significantly accelerates SA-A nucleation, with rates reaching ~ 10^-3 cm^-3 s^-1 under heavily polluted urban conditions (e.g., Beijing), corresponding to a five-order-of-magnitude enhancement over binary clusters and a 10¹-10¹¹-fold increase relative to nitric acid. These findings underscore the importance of incorporating NSA into atmospheric aerosol models.