| Abstract
| - Biogenic hydrocarbons emitted by vegetation are importantcontributors to secondary organic aerosol (SOA), but theaerosol formation mechanisms are incompletely understood.In this study, the formation of aerosols and gas-phaseproducts from the ozonolysis and photooxidation of a seriesof biogenic hydrocarbons (isoprene, 8 monoterpenes, 4sesquiterpenes, and 3 oxygenated terpenes) are examined.By comparing aerosol growth (measured by DifferentialMobility Analyzers, DMAs) and gas-phase concentrations(monitored by a Proton Transfer Reaction Mass Spectrometer, PTR-MS), we study the general mechanisms ofSOA formation. Aerosol growth data are presented in termsof a “growth curve”, a plot of aerosol mass formedversus the amount of hydrocarbon reacted. From theshapes of the growth curves, it is found that all thehydrocarbons studied can be classified into two groupsbased entirely on the number of double bonds of thehydrocarbon, regardless of the reaction systems (ozonolysisor photooxidation) and the types of hydrocarbons studied: compounds with only one double bond and compoundswith more than one double bond. For compounds with onlyone double bond, the first oxidation step is rate-limiting,and aerosols are formed mainly from low volatility first-generation oxidation products; whereas for compoundswith more than one double bond, the second oxidation stepmay also be rate-limiting and second-generation productscontribute substantially to SOA growth. This behavior ischaracterized by a vertical section in the growth curve, inwhich continued aerosol growth is observed even afterall the parent hydrocarbon is consumed.
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