广州机动车尾气中乙醛稳定碳同位素特征和排放因子

为了获得机动车排放源中乙醛的δ13C(稳定碳同位素丰度)特征及其影响因素,进行不同负荷下的发动机台架试验,采集不同怠速下的机动车尾气样品. 利用气相色谱-燃烧-同位素比值质谱(GC-C-IRMS)分析乙醛δ13C值,并与ρ(乙醛)进行分析. 结果显示:①在发动机燃烧过程中,乙醛的生成和消除反应同时存在. 在发动机低负荷运行时,乙醛的δ13C分馏值为负(-1.4‰~-0.4‰),表明生成反应占主导；而在高负荷运行时,分馏值为正(0.5‰~1.3‰),表明消除反应占主导. ②乙醛的δ13C值与其质量浓度无明显相关性,主要受发动机燃烧温度和尾气净化装置的影响. 整车尾气中乙醛的δ13C值在-29.1‰～-24.4‰之间,平均值为-26.5‰±1.6‰. 其中,汽油车为-25.9‰~-24.4‰,平均值为-24.9‰±0.5‰；柴油车为-29.1‰~-27.0‰,平均值为-28.0‰±0.6‰. ③南方机动车尾气排放源与植物排放源中的乙醛的δ13C值范围不同,表明δ13C值可用于大气乙醛的源解析. 通过机动车尾气中c(乙醛)/c(CO₂)估算广州汽油车和轻型柴油车乙醛的排放因子,二者分别为(13±16)和(169±106)mg/L. 

Carbon Isotopic Characterization and Emission Factors of Acetaldehyde Emitted by Automobiles in Guangzhou

Acetaldehyde (CH₃CHO) is the most abundant carbonyl compound in the atmosphere, and vehicle exhaust emissions are one of the most important sources. In this study, automobile emissions were characterized using CH₃CHO carbon isotopic ratios. Vehicle exhausts were sampled under different idling states, and CH₃CHO carbon isotopic rations were measured by gas-chromatograph-combustion-isotopic ratio mass spectrometry (GC-C-IRMS). CH₃CHO directly emitted from stand-alone engines (gasoline and diesel) running at different loads were sampled and measured concurrently. The samples were measured with HPLC for analyzing CH₃CHO concentrations. The carbon isotopic ratios indicated that both production and consumption of CH₃CHO occurred in the engine. The isotopic fractionation (ε) between CH₃CHO isotopic ratio and fuel isotopic ratio was depleted (-1.4‰ to -0.4‰) when the engine was operated with lower load (i.e., lower cylinder temperature). This result showed a dominant production of CH₃CHO. In contrast, the isotopic fractionation (ε) between CH₃CHO isotopic ratio and fuel isotopic ratio was negative (0.5‰ to 1.5‰) when the engine was operated with higher load (i.e., higher cylinder temperature); this showed a dominant consumption of CH₃CHO. The δ13C data were significantly influenced by engine combustion cylinder temperature and catalytic converter, but showed no correlation with CH₃CHO concentrations. The CH₃CHO carbon isotopic ratios in vehicle exhaust varied from -29.1‰ to -24.4‰ with average of -26.5‰±1.6‰, while the δ13C of CH₃CHO from gasoline vehicles and diesel vehicles varied from -25.9‰ to -24.4‰ (average -24.9‰±0.5‰), and -29.1‰ to -27.0‰ (average -28.0‰±0.6‰), respectively. The δ13C of CH₃CHO from vehicle exhaust and biogenic emissions in southern China were different, which might provide valuable information for source identification of atmospheric CH₃CHO in southern China. Using the proportion of mole concentration CH₃CHO/CO₂, emission factors for CH₃CHO were calculated for gasoline and diesel vehicles. They were determined to be (13±16)mg/L (HCHO mass versus fuel volume) for gasoline vehicles, and (169±106) mg/L for light-duty diesel vehicles in Guang zhou.