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Secondary organic aerosol (SOA) is ubiquitous in the atmosphere and plays a pivotal role in climate, air quality, and health. The production of low-volatility dimeric compounds through accretion reactions is a key aspect of SOA formation. However, despite extensive study, the structures and thus the formation mechanisms of dimers in SOA remain largely uncharacterized. In this work, we elucidate the structures of several major dimer esters in SOA from ozonolysis of α-pinene and β-pinene—substantial global SOA sources—through independent synthesis of authentic standards. We show that these dimer esters are formed in the particle phase and propose a mechanism of nucleophilic addition of alcohols to a cyclic acylperoxyhemiacetal. This chemistry likely represents a general pathway to dimeric compounds in ambient SOA. 

In this study we revisit one of the simplest RO2 + RO2 reactions: the self-reaction of the ethene derived hydroxyperoxy radical formed via sequential addition of ·OH and O2 to ethene. Previous studies of this reaction suggested that the branching to ‘accretion products,’ compounds containing the carbon backbone of both reactants, was minimal. Here, CF3O− GC-CIMS is used to quantify the yields of ethylene glycol, glycolaldehyde, a hydroxy hydroperoxide produced from RO2 + HO2, and a C4O4H10 accretion product. These experiments were performed in an environmental chamber at 993 hPa and 294 K. We provide evidence that the accretion product is likely dihydroxy diethyl peroxide (HOC2H4OOC2H4OH = ROOR) and forms in the gas-phase with a branching fraction of 23 ± 5%. We suggest a new channel in the RO2+RO2 chemistry leading directly to the formation of HO2 (together with glycolaldehyde and an alkoxy radical). Finally, by varying the ratio of the formation rate of RO2 and HO2 in our chamber, we constrain the ratio of the rate coefficient for the reaction of RO2 + RO2 to that of RO2 + HO2 and find that this ratio is .22±.07, consistent with previous flash photolysis studies.