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Peroxide-containing highly oxygenated molecules (HOMs) are formed upon ozonolysis of terpenes emitted from the biosphere and are expected to be a major driving force for the formation of new particles and secondary organic aerosol (SOA) in the atmosphere. We evaluate and model the contribution of organic peroxides to α-pinene SOA and their evolution under different conditions. We determine a HOM molar yield of ∼5%, contributing 30% to the initial SOA mass. Although the formation of these compounds is kinetically favored, we demonstrate that they are thermodynamically unstable with half-lives shorter than 1 hr under dark conditions. Their decomposition significantly alters SOA chemical composition, volatility, and oxidation state. We show that photolysis of carbonyls occurring within a timescale of hours is an efficient but largely overlooked mechanism by which SOA may evolve in the atmosphere. Both …