Secondary Organic Aerosol Formation of Fleet Vehicle Emissions in China: Potential Seasonality of Spatial Distributions.

Vehicle emissions are an important source of urban particular matter. To investigate the secondary organic aerosol (SOA) formation potential of real-world vehicle emissions, we exposed on-road air in Beijing to hydroxyl radicals generated in an oxidation flow reactor (OFR) under high-NOx conditions on-board a mobile laboratory and characterized SOA and their precursors with a suite of state-of-the-art instrumentation. The OFR produced 10-170 μg m-3 of SOA with a maximum SOA formation potential of 39-50 μg m-3 ppmv-1 CO that occurred following an integrated OH exposure of (1.3-2.0) × 1011 molecules cm-3 s. The results indicate relatively shorter photochemical ages for maximum SOA production than previous OFR results obtained under low-NOx conditions. Such timescales represent the balance of functionalization and fragmentation, possibly resulting in different spatial distributions of SOA in different seasons as the oxidant level changes. The detected precursors may explain as much as 13% of the observed SOA with the remaining plausibly contributed by the oxidation of undetected intermediate-volatility organic compounds. Extrapolation of the results suggests an annual SOA production rate of 0.78 Tg yr-1 from mobile gasoline sources in China, highlighting the importance of effective regulation of gaseous vehicular precursors to improve air quality in the future.