Abstract
Abstract. Intermediate-volatility organic compounds (IVOCs) emitted from vehicles are important precursors to secondary organic aerosols (SOAs) in urban areas, yet vehicular emission of IVOCs, particularly from on-road fleets, is poorly understood. Here we initiated a field campaign to collect IVOCs with sorption tubes at both the inlet and the outlet in a busy urban tunnel (>30 000 vehicles per day) in south China for characterizing emissions of IVOCs from on-road vehicles. The average emission factor of IVOCs (EFIVOCs) was measured to be 16.77±0.89 mg km−1 (average ±95 % CI, confidence interval) for diesel and gasoline vehicles in the fleets, and based on linear regression, the average EFIVOCs was derived to be 62.79±18.37 mg km−1 for diesel vehicles and 13.95±1.13 mg km−1 for gasoline vehicles. The EFIVOCs for diesel vehicles from this study was comparable to that reported previously for non-road engines without after-treatment facilities, while the EFIVOCs for gasoline vehicles from this study was much higher than that recently tested for a China V gasoline vehicle. IVOCs from the on-road fleets did not show significant correlation with the primary organic aerosol (POA) or total non-methane hydrocarbons (NMHCs) as results from previous chassis dynamometer tests. Estimated SOA production from the vehicular IVOCs and VOCs surpassed the POA by a factor of ∼2.4, and IVOCs dominated over VOCs in estimated SOA production by a factor of ∼7, suggesting that controlling IVOCs is of greater importance to modulate traffic-related organic aerosol (OA) in urban areas. The results demonstrated that although on-road gasoline vehicles have much lower EFIVOCs, they contribute more IVOCs than on-road diesel vehicles due to its dominance in the on-road fleets. However, due to greater diesel than gasoline fuel consumption in China, emission of IVOCs from diesel engines would be much larger than that from gasoline engines, signaling the overwhelming contribution of IVOC emissions by non-road diesel engines in China.
Highlights
Intermediate-volatility organic compounds (IVOCs) refer to organics with effective saturated concentrations ranging from 103 to 106 μg m−3, roughly corresponding to the volatility range of C12–C22 normal alkanes (n-alkanes) (Donahue et al, 2006; Zhao et al, 2014). Robinson et al (2007) have demonstrated that IVOCs, as the missing secondary organic aerosol (SOA) precursors in many model studies, could efficiently narrow the gap between model-predicted and fieldobserved SOA
As liquefied petroleum gas vehicles (LPGVs) and electrical vehicles (EVs) are considered to have no IVOC emissions (Stewart et al, 2021), only gasoline vehicles (GVs) and diesel vehicles (DVs) are responsible for the inlet–outlet incremental concentrations of IVOCs
Where emission factor of IVOCs (EFIVOCs) represents the fleet-average emission factor measured during a time interval; EFDV and EFGV are the average EFIVOCs for DVs and GVs, respectively; and α is the fraction of DVs in the total IVOC-emitting DVs and GVs traveling through the tunnel
Summary
Intermediate-volatility organic compounds (IVOCs) refer to organics with effective saturated concentrations ranging from 103 to 106 μg m−3, roughly corresponding to the volatility range of C12–C22 normal alkanes (n-alkanes) (Donahue et al, 2006; Zhao et al, 2014). Robinson et al (2007) have demonstrated that IVOCs, as the missing secondary organic aerosol (SOA) precursors in many model studies, could efficiently narrow the gap between model-predicted and fieldobserved SOA. Due to a lack of direct measurements, these model simulations used the ratios of IVOCs to other species like primary organic aerosol (POA) or non-methane hydrocarbons (NMHCs) to estimate IVOC emissions. IVOCs could account for ∼ 60 % of non-methane hydrocarbons (NMHCs) from diesel vehicles and 4 %–17 % from gasoline vehicles, explaining a dominant portion of estimated SOA mass from diesel and gasoline exhaust (Zhao et al, 2015, 2016). Previous chamber simulations on SOA formation from vehicle exhaust revealed that traditional volatile organic compounds (VOCs) could not explain the formed SOA, and IVOCs instead might dominate the SOA production (Deng et al, 2020; Zhang et al, 2020). For the control of fine particle pollution in urban areas, it is necessary to compile and upgrade emission inventories for IVOCs, and more work is needed to characterize their emissions from on-road vehicles
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