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

Abstract. Air pollution is particularly severe in developing megacities, such as Beijing, where pollutants from vehicles equipped with modern gasoline direct injection (GDI) engines must be paid enough attention. This study presents the characteristics of individual particles emitted by a GDI gasoline vehicle and their ageing in a smog chamber under the Beijing urban environment, as part of the Atmospheric Pollution & Human Health (APHH) research programme. Using electron microscopy, we identified the particles emitted from a commercial GDI-engine vehicle running under various conditions, namely cold start, hot start, hot stabilized running, idle, and acceleration states. Our results show that most of the particles were organic, soot and Ca-rich ones, with small quantities of S-rich and metal-containing particles. In terms of grain size, the particles exhibited a bimodal distribution in number vs size, with one mode at 800–900 nm, and the other at 140–240 nm. The amounts of organic particles emitted under hot start and hot stabilized states were higher than those emitted under other conditions. The amount of soot particles was higher under cold start and acceleration states. Under the idle state, the proportion of Ca-rich particles was highest, although their absolute number was low. In addition to quantifying the types of particles emitted by the engine, we studied the ageing of the particles during 3.5 hours of photochemical oxidation in an environmental chamber under the Beijing urban environment. Ageing transformed soot particles into core-shell structures, coated by secondary organic species, while the content of sulfur in Ca-rich and organic particles increased. Overall, the majority of particles from GDI-engine vehicles are organic and soot particles with submicron or nanometric size. The particles are highly reactive; they react in the atmosphere and change their morphology and composition within hours via catalyzed acidification that involves gaseous pollutants under high pollution levels in Beijing.