Isomer-Resolved Reactivity of Organic Peroxides in Monoterpene-Derived Secondary Organic Aerosol

Organic peroxides play a vital role in the formation, evolution, and healthimpacts of atmospheric aerosols, yet their molecular composition and fate in the particle phaseremain poorly understood. Here, we identified, using iodometry-assisted liquid chromatog-raphy mass spectrometry, a large suite of isomer-resolved peroxide monomers(C8-10H12-18O5-8) and dimers (C15-20H22-34O5-14) in secondary organic aerosol formedfrom ozonolysis of the most abundant monoterpene (alpha-pinene). Combining aerosolisothermal evaporation experiments and multilayer kinetic modeling, bulk peroxides werefound to undergo rapid particle-phase chemical transformation with an average lifetime ofseveral hours under humid conditions, while the individual peroxides decompose on timescalesof half an hour to a few days. Meanwhile, the majority of isomeric peroxides exhibit distinct particle-phase behaviors, highlighting theimportance of the characterization of isomer-resolved peroxide reactivity. Furthermore, the reactivity of most peroxides increaseswith aerosol water content faster in a low relative humidity (RH) range than in a high RH range. Such non-uniform water effectsimply a more important role of water as a plasticizer than as a reactant in influencing the peroxide reactivity. The high particle-phasereactivity of organic peroxides and its striking dependence on RH should be considered in atmospheric modeling of their fate andimpacts on aerosol chemistry and health effects.