Secondary organic aerosol formation from idling gasoline passenger vehicle emissions investigated in a smog chamber

Gasoline vehicles have recently been pointed out as potentially the main source of anthropogenic secondary organic aerosol (SOA) in megacities. However, there is a lack of laboratory studies to systematically investigate SOA for- mation in real-world exhaust. In this study, SOA formation from pure aromatic precursors, idling and cold start gasoline exhaust from three passenger vehicles (EURO2-EURO4) were investigated with photo-oxidation experiments in a 6 m 3 smog chamber. The experiments were carried out down to at- mospherically relevant organic aerosol mass concentrations. The characterization instruments included a high-resolution aerosol mass spectrometer and a proton transfer mass spec- trometer. It was found that gasoline exhaust readily forms SOA with a signature aerosol mass spectrum similar to the oxidized organic aerosol that commonly dominates the or- ganic aerosol mass spectra downwind of urban areas. After a cumulative OH exposure of 5◊ 10 6 cm 3 h, the formed SOA was 1-2 orders of magnitude higher than the primary OA emissions. The SOA mass spectrum from a relevant mix- ture of traditional light aromatic precursors gave f43 (mass fraction at m/z = 43), approximately two times higher than to the gasoline SOA. However O : C and H : C ratios were similar for the two cases. Classical C6-C9 light aromatic precursors were responsible for up to 60 % of the formed SOA, which is significantly higher than for diesel exhaust. Important candidates for additional precursors are higher- order aromatic compounds such as C10 and C11 light aro- matics, naphthalene and methyl-naphthalenes. We conclude that approaches using only light aromatic precursors give an incomplete picture of the magnitude of SOA formation and the SOA composition from gasoline exhaust.