Unraveling Rapid Mechanism and Nucleation Potential of Hydrogen Peroxymethyl Formate under Atmospheric Conditions

Products from the reaction between Criegee intermediates (CH₂OO) and organic acids significantly contribute to the formation and growth of secondary organic aerosol (SOA). However, the reaction between CH₂OO and the simplest organic acid, formic acid (HCOOH), is still not well understood. Herein, quantum chemical calculations and Born-Oppenheimer molecular dynamics (BOMD) simulations were employed to investigate the CH₂OO + HCOOH reaction both in the gas phase and on the water microdroplet. The results show that HOOCH₂OCHO (HPMF) formation from the gaseous CH₂OO + HCOOH reaction is a barrierless process and can compete with HOCH₂OOH (HMHP) formation from H₂O-catalyzed CH₂OO hydrolysis, with a rate of 439 times higher at 15 km. We also find that both H₂O-mediated HPMF formation and HCOOH-mediated HMHP formation occur rapidly on the water microdroplet, with their rate 3 orders of magnitude faster than the gas-phase reaction. Notably, the produced HPMF was stable and can form larger clusters with sulfuric acid, ammonia, and water molecules, potentially facilitating atmospheric new particle formation (NPF). These findings will not only elucidate the potential loss route of CH₂OO in rainforest and urban polluted regions but will also help us better understand the role of Criegee intermediates in the SOA formation.