Weitkamp Emily A, Lambe Andrew T, Donahue Neil M, Robinson Allen L
Center for Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA.
Environ Sci Technol. 2008 Nov 1;42(21):7950-6. doi: 10.1021/es800745x.
Triterpanoid hopanes and steranes are petroleum biomarkers used to apportion fine particulate matter to motor vehicle emissions. To investigate the chemical stability of these compounds, aerosolized motor oil was exposed to the hydroxyl radical (OH) in a smog chamber and the reaction rate constants of hopanes, steranes, and n-alkanes were measured. The experiments were conducted across a range of atmospheric conditions including low and high relative humidity (RH) and with mixtures of lubricating oil and secondary organic aerosol. Hopanes and steranes were found to react at atmospherically significant rates across the entire range of experimental conditions; they are estimated to have lifetimes on the order of several days at average summertime OH levels. The one experimental parameter that strongly influenced the effective rate constants was RH; oxidization of hopanes and steranes was about a factor of 4 slower at 75% RH than at 10% RH. Chemical mass balance (CMB) analysis was performed to illustrate the effects of oxidation on source apportionment estimates. As the extent of oxidation increases, traditional CMB analysis increasingly underestimates the contribution of gasoline vehicles butthe diesel estimates are largely unaffected. The results demonstrate that even modest levels of oxidation can alter policy-relevant conclusions about the total and relative contribution of gasoline and diesel vehicle emissions to ambient fine particle concentrations.
三萜类藿烷和甾烷是用于将细颗粒物归因于机动车排放的石油生物标志物。为了研究这些化合物的化学稳定性，将雾化的机油在烟雾箱中暴露于羟基自由基（OH），并测量了藿烷、甾烷和正构烷烃的反应速率常数。实验在一系列大气条件下进行，包括低和高相对湿度（RH），以及润滑油和二次有机气溶胶的混合物。发现在整个实验条件范围内，藿烷和甾烷以具有大气意义的速率发生反应；据估计，在夏季平均OH水平下，它们的寿命约为几天。对有效速率常数有强烈影响的一个实验参数是RH；在75%RH下，藿烷和甾烷的氧化速度比在10%RH下慢约4倍。进行了化学质量平衡（CMB）分析，以说明氧化对源分配估计的影响。随着氧化程度的增加，传统的CMB分析越来越低估汽油车的贡献，但柴油车的估计基本不受影响。结果表明，即使是适度的氧化水平也会改变关于汽油车和柴油车排放对环境细颗粒物浓度的总贡献和相对贡献的与政策相关的结论。
