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 for