Secondary organic aerosol formation from photochemical aging of light-duty gasoline vehicle exhausts in a smog chamber

摘要

In China, a rapid increase in passenger vehicles has led to the growingconcern of vehicle exhaust as an important source of anthropogenic secondaryorganic aerosol (SOA) in megacities hard hit by haze. In this study, the SOAformation of emissions from two idling light-duty gasoline vehicles (LDGVs)(Euro 1 and Euro 4) operated in China was investigated in a 30 m3 smogchamber. Five photo-oxidation experiments were carried out at 25 °Cwith relative humidity at around 50 %. After aging at an OH exposure of5 × 106 molecules cm?3 h, the formed SOA was 12–259times 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 theprevious studies at comparable OH exposure. This quite lower OH exposure thanthat in typical atmospheric conditions might however lead to theunderestimation of the SOA formation potential from LDGVs. Effective SOAyields in this study were well fit by a one-product gas-particle partitioningmodel but quite lower than those of a previous study investigating SOAformation from three idling passenger vehicles (Euro 2–4). Traditionalsingle-ring aromatic precursors and naphthalene could explain 51–90 % ofthe formed SOA. Unspeciated species such as branched and cyclic alkanes mightbe the possible precursors for the unexplained SOA. A high-resolutiontime-of-flight aerosol mass spectrometer was used to characterize thechemical composition of SOA. The relationship between f₄₃ (ratio ofm/z 43, mostly C₂H₃O+, to the total signal in mass spectrum)and f₄₄ (mostly CO₂+) of the gasoline vehicle exhaust SOA issimilar to the ambient semi-volatile oxygenated organic aerosol (SV-OOA). Weplot 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 acombination of carboxylic acid and alcohol/peroxide formation.