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<p>Organic aerosol (OA) constitutes a significant fraction of the atmospheric fine particulate matter that influences both air quality and climate. Secondary organic aerosol (SOA), which is formed through photo-oxidation of organic vapors in the atmosphere, is a major component of OA. There are some studies indicating the major role of vehicles emissions in SOA formation in urban cities of China. However, SOA formation is complex and uncertain.</p><p>Historically, the China fleet has been dominated by vehicles equipped with port-fuel injected (PFI), but the market share of vehicles equipped with gasoline direct injection engines (GDI) has increased dramatically. And 10% of renewable energy ethanol (E10) may be added to the gasoline of China market in the future. Go-PAM is one kind of potential aerosol mass for simulating SOA formation, which is designed and made by the University of Gothenburg.</p><p>In this study, we focus on the influence of ethanol content (0% or 10%), engine types (GDI or PFI) and different engine loads (idling or constant velocity) to the SOA formation potential from gasoline motor cars emissions. We exposed the diluted emissions to a range of oxidation (O<sub>3 </sub>and OH) concentrations in the Go-PAM, resulting different OH exposures. We observed variations of different cases in SOA formation.</p><p>Firstly, compared to PFI engine, GDI engine at idling loading has larger SOA mass concentrations. The peak SOA production of PFI engine at idling load occurred at equivalent photochemical age (EPA) of 3.8 days, which peak point occurred at larger EPA (4.8 days) for GDI engines. Secondly, there is no large difference between E10 and gasoline. Thirdly, OA enhancement is more obvious at idling (about 30-180 times) than at constant velocity (about 3-4 times) whatever engine is used. Generally, densities of particles at size of 70nm,140nm and 200nm keep growing from about 1.25 up to 1.45 g/cm<sup>3</sup>.</p><p>The results of this study highlight the utility of Go-PAM for studying SOA formation potential from vehicle exhaust, and provide indications of the influence of ethanol content and different engines to SOA formation in China.</p>

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