Much of our understanding of atmospheric secondary organic aerosol (SOA) formation from volatile organic compounds derives from laboratory chamber measurements, including mass yield and elemental composition. These measurements alone are insufficient to identify the chemical mechanisms of SOA production. We present here a comprehensive dataset on the molecular identity, abundance, and kinetics of α-pinene SOA, a canonical system that has received much attention owing to its importance as an organic aerosol source in the pristine atmosphere. Identified organic species account for ∼58-72% of the α-pinene SOA mass, and are characterized as semivolatile/low-volatility monomers and extremely low volatility dimers, which exhibit comparable oxidation states yet different functionalities. Features of the α-pinene SOA formation process are revealed for the first time, to our knowledge, from the dynamics of individual particle-phase components. Although monomeric products dominate the overall aerosol mass, rapid production of dimers plays a key role in initiating particle growth. Continuous production of monomers is observed after the parent α-pinene is consumed, which cannot be explained solely by gasphase photochemical production. Additionally, distinct responses of monomers and dimers to α-pinene oxidation by ozone vs. hydroxyl radicals, temperature, and relative humidity are observed. Gas-phase radical combination reactions together with condensed phase rearrangement of labile molecules potentially explain the newly characterized SOA features, thereby opening up further avenues for understanding formation and evolution mechanisms of α-pinene SOA.secondary organic aerosol | particulate matter | air quality | climate S econdary organic aerosol (SOA), comprising a large number of structurally different organic oxygenates, is a dominant constituent of submicrometer atmospheric particulate matter (1). Molecular characterization of SOA has been a major research goal in atmospheric chemistry for several decades (2), owing to the importance of organic aerosol in air quality and Earth's energy budget. Both biogenic (e.g., isoprene, monoterpenes) and anthropogenic (e.g., aromatics, large alkanes) organic compounds are well-established precursors to SOA. Knowledge of the SOA molecular composition is crucial for elucidation of its underlying formation mechanisms.The most abundant monoterpene in the troposphere is α-pinene (3). The oxidation of α-pinene by ozone has become a canonical SOA system (4-12). Identification of multifunctional particlephase products has been reported, including monomers with carboxylic acid moieties (4, 6) and high-molecular-weight compounds (7,8,12), although molecular structures and formation pathways of oligomers remain uncertain (5). Recently, a class of extremely low-volatility gas-phase organic compounds (ELVOCs) has been identified as an important component in the α-pinene ozonolysis chemistry (13). Identification of the ELVOCs in the particle phase and elucidation of the mechanism of their format...
