Comparison between idling and cruising gasoline vehicles in primary emissions and secondary organic aerosol formation during photochemical ageing

Driving conditions are among the important factors determining gasoline vehicle emissions, yet their relation with exhaust-derived secondary pollutants is poorly understood. Here, we introduced exhaust from a gasoline vehicle under hot idling and cruising conditions into an indoor smog chamber by using a chassis dynamometer and investigated the formation of secondary organic aerosols (SOA) during photochemical ageing under light after characterizing the primary emission of non-methane hydrocarbons (NMHCs), nitrogen oxide (NO_x) and primary organic aerosol (POA) in the dark. When compared to emission factors (EFs) at idling, during cruising at 20 km h⁻¹ or 40 km h⁻¹, the EFs of NMHCs decreased by more than an order of magnitude, while the EFs of NO_x were more than doubled, resulting in a large drop in the NMHC-to-NO_x ratios. The percentages of reactive alkenes and aromatic hydrocarbons also decreased from idling to cruising at 20 km h⁻¹ to that at 40 km h⁻¹. The emission factor of benzene, a carcinogenic compound, decreased more than 10 times from ~0.35 g kg-fuel⁻¹ at idling to ~0.03 g kg-fuel⁻¹ during cruising. During photochemical ageing of exhaust, substantial SOA was formed, and the SOA/POA ratios decreased from 52 to 92 at idling to 4–14 during cruising. Traditional aromatics could explain 30–64% of the measured SOA at idling but less than 15% of the measured SOA during cruising. Our results highlight that traffic congestion would greatly promote the emission of reactive volatile organic compounds and carcinogenic benzene from gasoline vehicles and also show that NMHCs as a target in gasoline vehicle emission tests cannot effectively represent the SOA and ozone formation potentials of the partially oxidized hydrocarbons from poorly functioning converters.