From real-driving emissions to urban air quality: composition of aged PM from modern diesel, gasoline, and CNG fueled cars and plug-in hybrid electric vehicles

Abstract

Secondary aerosol emissions from vehicle exhaust often surpass primary particle emissions, yet they are not currently regulated, as they remain difficult to constrain. Here we investigate the factors driving the formation and chemical composition of secondary aerosol from light-duty vehicle exhaust emissions, focusing on the most recent Euro emission standard (Euro 6d), and including hybrid and natural gas cars. Seven modern cars were driven through a real-driving emission simulation cycle in a chassis dynamometer. The exhaust emissions were aged in a PAM chamber and their chemical and physical properties measured with an aerosol mass spectrometer and state-of-the-art instrumentation. Results indicate that secondary aerosol emissions surpassed fresh aerosol emissions for all cars, except for old Euro 4 diesel. While on average, Euro 6d gasoline and diesel cars aged PM emissions were about 90 % lower than emissions from older cars, their cold start emissions were still significant. Hybrid cars also emitted considerably when switching to combustion engine, which, depending on the length and style of the driving, could be comparable to non-hybrid vehicles emissions. Aged organic aerosol was dominated by oxidized compounds typical of ambient secondary organic aerosol, with unique compositions across vehicle types and fuels. Notably, the CNG vehicle emitted hydrocarbon-like organics, likely originating from less reactive species from lubricant oil, and the Euro 4 diesel exhibited organic nitrate formation, an underreported component in vehicle exhaust with atmospheric implications. Secondary aerosol and its precursors should be regulated and considered in reduction technologies, to best mitigate atmospheric PM in urban traffic-influenced areas.