Rapid formation of hydroxymethyl hydroperoxide and its vital role in methanesulfonic acid-methylamine nucleation: impacts of urban industrial and forested areas

Abstract. Organic peroxides are widely recognized as important contributors to secondary organic aerosols formation. Among these, hydroxymethyl hydroperoxide (HMHP) is a common species found in both the gas phase and fine aerosols. Despite its abundance, the molecular-level formation of HMHP through methanesulfonic acid (MSA)-catalyzed hydrolysis of CH₂OO, particularly in the gas phase and at the air-water interface, remains insufficiently examined. Moreover, the role of HMHP in new particle formation (NPF) has not been fully elucidated. Herein, we employ quantum chemical calculations together with Born-Oppenheimer molecular dynamics simulations to investigate HMHP formation from CH₂OO hydrolysis with MSA under both gas phase and interfacial conditions. Our results show that HMHP forms rapidly and stably in both environments. Further analysis using the atmospheric cluster dynamics code reveals that HMHP not only enhances the clustering stability of MSA-methylamine (MA) clusters, but also exerts a direct role in promoting MSA-MA nucleation. Importantly, in regions with elevated HMHP concentrations (3.00 × 10¹⁰ - 1.25 × 10¹¹ molecules · cm^-3), such as Niwot Ridge and Southeastern United States, the HMHP-involved pathways contribute unexpectedly up to 42 % and 59 % of total nucleation flux at 258.15 K, respectively. These findings provide new insights into HMHP formation pathways and the efficient MSA-MA-HMHP nucleation mechanism, offering a plausible explanation for the frequent and intense NPF events observed in continental regions.