Secondary Organic Aerosol Formation from Emissions of On- and Off-Road Vehicles

Tailpipe emissions from on- and off-road vehicles can be a significant source of secondary organic aerosol (SOA) both in urban areas and regionally. However, major uncertainties remain in understanding SOA formation from vehicle exhaust in different oxidation timescales.

Individual on- and off-road vehicles were tested on a dynamometer, with their exhaust introduced into an oxidation flow reactor (OFR) and an atmospheric simulation chamber to investigate SOA formation across a range of OH exposures. Exposures approximately ranged from very short durations of 0.1 up to 9 equivalent days, with the smog chamber experiments corresponding on average to 2 equivalent days.  In total, emissions from 14 vehicles, including 4 gasoline cars, 4 diesel cars, 4 scooters, and 2 tractors, covering a range of engine types and control technologies, were examined. Standardized driving cycles were followed, including WLTC for passenger cars, WMTC for scooters, and NRSC for tractors. The on-road cycles, carried out on a chassis dynamometer, simulated urban-speed conditions and included cold starts, while the off-road cycle consisted of steady-state engine operation at multiple loads and speeds, controlled by an EGGERS dynamometer.

Scanning mobility particle sizers (SMPS) were used to measure particle size distributions, while the chemical composition was characterized by a high-resolution aerosol mass spectrometer (HR-ToF-AMS) for the aerosol and by a VOCUS Proton-Transfer-Reaction-Mass-Spectrometer (VOCUS PTR-MS) for the gas phase. Black carbon and trace gases were monitored using an aethalometer (AE33) and online gas analyzers, respectively. The volatility distribution of SOA in the 1-D volatility basis set (VBS) was also characterized using a combination of thermodenuder measurements and TD-GCMS analysis of Tenax sorbent tubes sampled from the smog chamber after the oxidation.

Two-wheelers, especially a pre-Euro 2-stroke scooter, had the highest SOA formation potential (SOA_FP) across the full range of OH exposure, exceeding all other on-road vehicle types by more than an order of magnitude, where SOA_FP is defined as the amount of SOA formed per kg of fuel burned. A modern off-road tractor (Stage V) also showed substantial SOA_FP surpassing a modern scooter (Euro 5) and the rest on-road vehicles. Euro 6 gasoline on-road cars exhibited higher SOA_FP values than Euro 6 diesel vehicles. These results highlight the disproportionate contribution of scooters and off-road vehicles to urban SOA, underlining the need for targeted emission control strategies.