The impact of vehicle emissions on secondary aerosol formation: a laboratory study using an oxidation flow reactor

Atmospheric aerosols play a critical role in global warming, climate change, and human health. A deeper understanding of secondary aerosol formation, that is the generation of particles in the atmosphere through the interaction of solar radiation with pollutants directly emitted from various sources, is essential for accurately assessing their impacts on air quality. In this study, we examine the role of potential gaseous and particulate precursors of secondary organic and inorganic aerosols that are directly emitted into the atmosphere by Euro6D gasoline and diesel vehicle exhausts. The strong link between secondary aerosol formation and primary vehicle emissions highlights the crucial influence of fuel composition on emission behaviours, emphasizing the need for a comprehensive identification of the key contributing factors. Emissions from various gasoline and diesel fuels were evaluated at the exhaust of two vehicles during a worldwide harmonized light vehicle test cycle. After dilution, the primary emissions were introduced into an oxidation flow reactor to replicate atmospheric oxidation conditions, achieving an equivalent atmospheric aging time of approximately five days. The resulting secondary aerosols were analysed. Our results indicate that Euto6D gasoline vehicles emit the highest levels of total hydrocarbons, especially during the initial minutes of the driving cycle under cold start conditions. Although a similar pattern was observed for Euro6D diesel vehicles, their total hydrocarbon emissions were at least 40 percent lower than those of gasoline vehicles. Moreover, diesel-powered vehicles emitted higher levels of nitrogen oxides and no detectable ammonia, whereas gasoline-powered vehicles produced significant ammonia as a by-product of the three-way catalyst. Quantification of the secondary aerosol emission factors further revealed that gasoline-powered vehicles have a greater propensity to form secondary aerosols, yielding higher concentrations of primary particulate matter and potential gaseous precursors entering the reactor. In contrast, diesel vehicles equipped with advanced after-treatment systems effectively reduced both the particle concentration and the level of gaseous precursors. Our findings demonstrate the role of fuel composition in the formation of primary and secondary aerosols. Reducing aromatic compounds in gasoline and introducing oxygenated components such as methanol and ethanol can lower hydrocarbon emissions and hence secondary organic aerosol formation. Diesel fuels without aromatic compounds in the mixture exhibit superior performance in minimizing secondary aerosol production. These results underscore the importance of investigating fuel reformulation as a strategy to mitigate the formation of secondary aerosols.