Evaporative emissions of vehicles are an essential source of atmospheric volatile organic compounds (VOCs), contributing to ozone contamination, especially in urban areas. Due to the outdated standards under which in-use vehicles were constructed and the ageing of control devices, high-mileage vehicles tend to produce an enormous amount of evaporative emissions. In this study, evaporative emissions from two high-mileage light-duty gasoline vehicles were quantified using VT-SHED, and their ozone-forming potential (OFP) values were calculated based on the identified VOC species. The results show that VOCs with high boiling points are released at low rates when the temperature inside the VT-SHED ranges from 20 to 28 °C. The release rates of all VOC species increase when the VT-SHED temperature is 28-35 °C. Diurnal loss dominates evaporative emissions from high-mileage gasoline vehicles, with the levels of VOCs quantified within this stage being 3-fold higher than those during the hot-soak stage. Only during the hot-soak stage, C11-C16 n-alkanes occupy an overall increased portion in the identified VOC inventory. OFP values of the two high-mileage vehicles exceeded 600.0 mgO3/day during the 48-h diurnal-loss tests. The specific reactivity (SR) values of the diurnal-loss VOCs are deemed more relevant to fuel compositions because the two vehicles have the same fuel yield and close SR values of approximately 4.3 mgO3/mgVOCs, despite different certification standards, potentially allowing for the use of unified SR values to ease the estimation of the ozone contamination of evaporative emissions from in-use fleets.
Keywords: Diurnal loss; Hot-soak loss; OFP; Temperature; VOCs.
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