Obscured Contribution of Oxygenated Intermediate-Volatility Organic Compounds to Secondary Organic Aerosol Formation from Gasoline Vehicle Emissions

Abstract

Secondary organic aerosol (SOA) formation from gasoline vehicles spanning a wide range in emission types was investigated using an oxidation flow reactor (OFR) by conducting chassis dynamometer tests. Aided by advanced mass spectrometric techniques, SOA precursors, including volatile organic compounds (VOCs), intermediate/semi-volatile organic compounds (I/SVOCs), were comprehensively characterized. The reconstructed SOA produced from the speciated VOCs and I/SVOCs can explain 69% of SOA measured downstream of OFR upon 0.5-3 days’ OH exposure. While VOCs can only explain 10% of total SOA production, contribution from I/SVOCs is 59%. We also found that oxygenated I/SVOCs (O–I/SVOCs, e.g., benzylic or aliphatic aldehydes and ketones), as an obscured source, accounted for 16% of total nonmethane organic gas (NMOG) emission and 20% of SOA production. More importantly, with the improvement in emission standards, the NMOG was effectively mitigated by 35% from China 4 to China 6, which is predominantly attributed to the decrease of VOCs. Real-time measurements of different NMOG components as well as SOA production further revealed that the current emission control measures, such as three-way catalytic converters (TWCs), are effective in reducing the “light” SOA precursors (i.e., single ring aromatics), but not for the I/SVOC emissions, indicating that the catalyst are selective upon reacting with different exhaust components. Our results highlight the neglected contribution from I/SVOCs, especially O-I/SVOCs to SOA formation and the urgent need in further investigation in their origins, i.e., incomplete combustion, lubricating oil, which requires improvements in real-time molecular-level characterization of I/SVOC molecules and in turn will benefit the future design of control measures.