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. Air pollution is particularly severe in developing megacities, such asBeijing, where vehicles equipped with modern gasoline-direct-injection (GDI)engines are becoming one of major sources of the pollution. This studypresents the characteristics of individual particles emitted by a GDIvehicle and their ageing in a smog chamber under the Beijing urbanenvironment, as part of the Atmospheric Pollution u0026 Human Health (APHH)research programme. Using transmission electron microscopy, we identifiedthe particles emitted from a commercial GDI-engine vehicle running undervarious conditions, namely cold-start, hot-start, hot stabilized running,idle, and acceleration states. Our results showed that most of the particleswere organic, soot, and Ca-rich ones, with small quantities of S-rich andmetal-containing particles. In terms of particle size, the particlesexhibited a bimodal distribution in number vs size, with one mode at 800–900 nm and the other at 140–240 nm. The numbers of organic particles emittedunder hot-start and hot stabilized states were higher than those emittedunder other conditions. The number of soot particles was higher under cold-start and acceleration states. Under the idle state, the proportion ofCa-rich particles was highest, although their absolute number was low. Inaddition to quantifying the types of particles emitted by the engine, westudied the ageing of the particles during 3.5 h of photochemicaloxidation in an environmental chamber under the Beijing urban environment.Ageing transformed soot particles into core–shell structures, coated bysecondary organic species, while the content of sulfur in Ca-rich andorganic particles increased. Overall, the majority of particles fromGDI-engine vehicles were organic and soot particles with submicron ornanometric size. The particles were highly reactive; they reacted in theatmosphere and changed their morphology and composition within hours viacatalysed acidification that involved gaseous pollutants at high pollutionlevels in Beijing.  Highlights.GDI-engine vehicles emitted a large amount of both primary and secondaryorganic aerosol (SOA).Higher numbers of organic particles were emitted under hot stabilized running and hot-start states.Sulfate and secondary organic aerosol formed on the surface of primaryparticles after ageing.Particles aged rapidly by catalysed acidification under high pollutionlevels in Beijing.

Morphology and size of the particles emitted from a gasoline-direct-injection-engine vehicle and their ageing in an environmental chamber