Abstract
Abstract. In China, a rapid increase in passenger vehicles has led to the growing concern of vehicle exhaust as an important source of anthropogenic secondary organic aerosol (SOA) in megacities hard hit by haze. In this study, the SOA formation of emissions from two idling light-duty gasoline vehicles (LDGVs) (Euro 1 and Euro 4) operated in China was investigated in a 30 m3 smog chamber. Five photo-oxidation experiments were carried out at 25 °C with relative humidity at around 50 %. After aging at an OH exposure of 5 × 106 molecules cm−3 h, the formed SOA was 12–259 times as high as primary organic aerosol (POA). The SOA production factors (PF) were 0.001–0.044 g kg−1 fuel, comparable with those from the previous studies at comparable OH exposure. This quite lower OH exposure than that in typical atmospheric conditions might however lead to the underestimation of the SOA formation potential from LDGVs. Effective SOA yields in this study were well fit by a one-product gas-particle partitioning model but quite lower than those of a previous study investigating SOA formation from three idling passenger vehicles (Euro 2–4). Traditional single-ring aromatic precursors and naphthalene could explain 51–90 % of the formed SOA. Unspeciated species such as branched and cyclic alkanes might be the possible precursors for the unexplained SOA. A high-resolution time-of-flight aerosol mass spectrometer was used to characterize the chemical composition of SOA. The relationship between f43 (ratio of m/z 43, mostly C2H3O+, to the total signal in mass spectrum) and f44 (mostly CO2+) of the gasoline vehicle exhaust SOA is similar to the ambient semi-volatile oxygenated organic aerosol (SV-OOA). We plot the O : C and H : C molar ratios of SOA in a Van Krevelen diagram. The slopes of ΔH : C / ΔO : C ranged from −0.59 to −0.36, suggesting that the oxidation chemistry in these experiments was a combination of carboxylic acid and alcohol/peroxide formation.
Highlights
The formation mechanisms, magnitude and chemical composition of airborne fine-particulate matter (PM2.5) are important to evaluate its effects on human health and climate (Hallquist et al, 2009)
The mass fraction of aromatic hydrocarbons for the Euro 4 vehicle was comparable with 32.2 % for idling private cars in Hong Kong (Guo et al, 2011) and 38.3 % for Euro 3 lightduty gasoline vehicles operated through ECE cycles with an average speed around 18.7 km h−1 (Wang et al, 2013)
Both Schauer et al (2002) and Gentner et al (2013) observed that aromatic hydrocarbons contributed around 27 % of the total volatile organic compounds (VOCs) for gasoline-powered automobiles driven through the cold-start Federal Test Procedure urban driving cycle and onroad gasoline vehicles in the Caldecott tunnel, similar to that of the Euro 1 vehicle
Summary
The formation mechanisms, magnitude and chemical composition of airborne fine-particulate matter (PM2.5) are important to evaluate its effects on human health and climate (Hallquist et al, 2009). Nordin et al (2013) investigated SOA formation from idling gasoline exhausts from three passenger vehicles (Euro 2–4), finding that C6–C9 light aromatics contributed up to 60 % of the formed SOA. To exclude the influence of a small sample size, Gordon et al (2014) studied aging of emissions from 15 light-duty gasoline vehicles with model years ranging from 1987 to 2011, concluding that traditional precursors could fully explain the SOA from older vehicles and unspeciated organics were responsible for the majority of the SOA from the newer vehicles. It is urgent to investigate the SOA formation from vehicle exhaust in China to help make suitable policies to mitigate air pollution and to provide valuable parameters to chemical transport models. The magnitude and composition of the SOA formed from gasoline vehicle exhaust and whether traditional SOA precursors can explain the formed SOA was evaluated and discussed in this paper
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