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Field observations over the past decade indicate that a significant fractionof organic aerosol in remote areas may contain highly oxidized molecules.Aerosol processing or further oxidation (aging) of organic aerosol has beensuggested to be responsible for their formation through heterogeneous reaction with oxidants and multigenerational oxidation of vapours by OH radicals. Inthis study we investigated the influence of several aging processes on themolecular composition of secondary organic aerosols (SOA) using directinfusion and liquid chromatography high-resolution mass spectrometry. SOAwas formed in simulation chamber experiments from ozonolysis of a mixture offour biogenic volatile organic compounds (BVOC): α-pinene, β-pinene, Δ3-carene and isoprene. The SOA was subsequently agedunder three different sets of conditions: in the dark in the presence ofresidual ozone, with UV irradiation and OH radicals, and using UV lightonly. Among all studied conditions, only OH radical-initiated aging wasfound to influence the molecular composition of the aerosol and showed anincrease in carbon oxidation state (OSC) and elemental O / C ratios ofthe SOA components. None of the aging processes produced an observableeffect on the oligomers formed from ozonolysis of the BVOC mixture, whichwere found to be equally abundant in both "fresh" and "aged" SOA. Additionalexperiments using α-pinene as the sole precursor demonstrated thatoligomers are an important group of compounds in SOA produced from bothozonolysis and OH radical-initiated oxidation processes; however, acompletely different set of oligomers is formed under these two oxidationregimes. SOA from the OH-initiated oxidation of α-pinene had asignificantly higher overall OSC and O / C compared to that from pureozonolysis experiments confirming that the OH radical reaction is morelikely to be responsible for the occurrence of highly oxidized species inambient biogenic SOA

atmospheric chemistry and physics

Molecular composition of fresh and aged secondary organic aerosol from a mixture of biogenic volatile compounds: a high-resolution mass spectrometry study