Characterization of photochemical aerosols from biogenic hydrocarbons

Abstract

An extensive set of outdoor smog chamber experiments were carried out to study aerosol formation by two representative biogenic hydrocarbons; isoprene and β-pinene. The hydrocarbons, at concentrations ranging from a few ppb to a few ppm, were photooxidized in the presence of NOₓ. Isoprene was found to produce negligible aerosol at ambient conditions, whereas β-pinene aerosol carbon yields were as high as 8%, depending strongly on the hydrocarbon to NOₓ ratio. Aerosol samples subjected to infrared absorption spectroscopy revealed that the dominant aerosol products for both isoprene and β-pinene are organic nitrates, organic acids, as well as other carbonyls and hydroxy compounds. GCMS of the neutral fraction of the β-pinene aerosol revealed nopinone and several other compounds with molecular weights ranging from 138–200 amu, indicating mainly mono- and dioxygenated products. The average vapor pressure of the β-pinene aerosol was estimated to be 37 ± 24 ppt at 31°C. Scanning electron micrographs showed that the particles consist of both liquid droplets and agglomerates of small (40–60 nm) solid particles.

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