SOA formation from the photooxidation of α-pinene in the presence of freshly emitted diesel soot exhaust

Outdoor smog chamber experiments were performed to investigate the effects of the preexisting diesel soot particles on secondary aerosol formation. Diesel exhaust (50–240 μg m−3 of diesel soot particles) was added to the outdoor chambers without or with α-pinene (0.14 ppmV). The increase of medium polarity and organic composition change of diesel soot particles were tracked by analyzing deuterated alkanes, which were dosed to the system. Changes due to photooxidation of diesel soot exhaust were accelerated when α-pinene was added. Major α-pinene oxidation products were also analyzed from gas and particle samples and they were mostly carbonyl compounds such as cis-pinonaldehyde, cis-pinonic acid, pinalic-4-acid and 10-hydroxy pinonaldehyde. The partitioning coefficients of the major carbonyl products were much higher in diesel soot particles than predicted by theory. The high particle concentration of carbonyl products on diesel soot particles could be explained by heterogeneous acid-catalyzed reactions.