Reactive Evolution of HMML from Gas Phase Formation to Interface Transformation on Sulfuric Acid Aerosols: A Multiscale Computational Study

Hydroxymethyl-methyl-α-lactone (HMML) is a key epoxide precursor in forming tracer compounds 2-methylglyceric acid (2-MG) or 2-methylglyceric acid sulfate (2-MGOS) from isoprene under high-NOx conditions. Despite its importance, the formation and transformation of HMML─particularly under acidic aerosol conditions─are still poorly understood, limiting comprehensive knowledge of secondary organic aerosol (SOA) formation. In this study, quantum chemical calculations, Born-Oppenheimer molecular dynamics (BOMD), and metadynamics (MTD) simulations are employed to investigate both the formation of HMML from methacryloyl peroxynitrate (MPAN) and its interfacial transformation mechanisms on sulfuric acid aerosols. Results show that OH radicals preferentially add to the β-carbon of MPAN, generating HMML via a concerted process involving C-O bond formation and O-O bond cleavage. At acidic aerosol interfaces, HMML stably adsorbs through hydrogen bonding and can convert to 2-MG or 2-MGOS through one-step or two-step pathways, depending on the local solvation environment. High sulfuric acid concentration (59 wt. %) promotes direct nucleophilic addition by interfacial HSO₄^-/SO₄^2- ions, favoring 2-MGOS formation, while lower concentration (30 wt. %) favors an H₂O-mediated 2-MG pathway. These findings underscore the decisive influence of aerosol interfacial microenvironments on HMML transformation pathways, product distribution, and their broader role in SOA formation and atmospheric multiphase chemistry.