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In this study, we implement a numerical model to predict secondary organic aerosol (SOA) formation from semi- and intermediate-volatility organic compounds emitted from in-use gasoline and diesel vehicles. The model is formulated based on the volatility basis set (VBS) approach, and it accounts for OH oxidation of unspeciated low-volatility organics, which are classified by their volatility. This model incorporates SOA formation data from smog chamber and emission measurements of vehicle exhaust in a Hybrid framework to calculate the contribution of both traditional and non-traditional SOA precursors to total SOA formation observed in photo-oxidation experiments. Emission and SOA formation data were acquired from a series of experiments conducted at the Center for Atmospheric Particle Studies, Carnegie Mellon University on different gasoline and diesel vehicles. Instead of assigning surrogate compounds, the source-specific mass yield for non-traditional SOA precursors are calculated directly from the experiments. The present performance of the model is in good agreement with its previous application to aircraft exhaust. Based on the model predictions, a set of average VBS mass yields for each class of vehicles is presented. The obtained yield matrix is able to reproduce the observed SOA concentrations in the smog chamber within the measurement uncertainty.

This study investigates the secondary organic aerosol (SOA) formation potential of carburetor motorcycles exhaust. This type of two-wheeler is a popular means of transport in many Asian cities. A volatility-based numerical model was employed to predict SOA formation from a fleet of motorcycles in Tehran, capital of Iran. The fleet was a combination of four-stroke, gasoline-powered motorcycles with different engine displacement volumes. Total hydrocarbon (THC) emission factors of all motorcycles were previously measured in a chassis dynamometer laboratory according to cold start Euro-3 emissions certification test procedures. Due to incomplete combustion and lack of control on exhaust emissions, unburned fuel was assumed to be a good surrogate for the exhaust of carburetor motorcycles, regarding SOA formation. 150 cc engine and 200 cc engine motorcycles had the highest SOA formation potential, under atmospheric oxidant concentration, while 125 cc engine motorcycles had the highest SOA emission factor (travel- and fuel-based). It was found out that SOA emission factor of 125 cc engine motorcycles could increase up to 20%, three to five years after production. Average SOA formation from carburetor motorcycles in the present study was 4 times higher than Euro-4 passenger cars and 20 times higher than direct emission of particles from Euro-2 motorcycles and on (according to EMEP/EEA levels for PM2.5 emission factors). Carburetor motorcycles with 180 cc engine volume in the present study, had the lowest SOA formation potential.