Reactive Oxidative Species and Speciated Particulate Light-Duty Engine Emissions from Diesel and Biodiesel Fuel Blends

It is well established that particulate matter (PM) continues to be a major air pollutant challenge for human health globally, and vehicle exhaust PM emissions have been linked to many adverse health effects. However, the relative toxicity of biodiesel emissions compared to petroleum diesel remains unclear. Given the legislated mandates to increase biodiesel fuel use in response to energy security and climate concerns, in this study we examined the relationships between biodiesel fuel blend, exhaust particle oxidative potential (OP), and PM composition. Mechanistically, there is a growing consensus that the formation of reactive oxygen species (ROS) due to PM exposure leads to subsequent oxidative stress and inflammation at the cellular level. Here, dithiothreitol (DTT) assays were performed on impinger samples of PM obtained from light-duty diesel engine transient cycle emission tests with two biodiesel feedstocks, soybean (SOY) and waste vegetable oil (WVO), blended with ultralow sulfur petrodiesel at five different volume percentages of biodiesel, Bxx (B0, B10, B20, B50, and B100). The DTT activity per mass of PM sampled generally decreased as the percent biodiesel increased in the fuel, for both feedstocks. Mean DTT PM activity (±1 std dev) for SOY decreased from 20.9 ± 4.2 to 13.6 ± 3.8 nmol/min/mgPM for B0 and B100, respectively, and from 22.6 ± 4.5 to 8.5 ± 2.8 nmol/min/mgPM for the WVO feedstock. Results indicate biodiesel blend PM may be less toxic per unit PM mass emitted by 50–80%, depending on feedstock. By comparison of feedstocks, statistically significantly lower OP for WVO, only for blends B50 and B100, suggests different combustion products between feedstocks only for the more highly oxygenated biodiesel blends used in this study. The organic composition of WVO exhaust particles measured by GC-MS showed positive correlations between DTT PM activity and particle-phase polycyclic aromatic hydrocarbons (PAHs), n-alkanes, aromatic aldehydes, aromatic ketones, and quinones, but not aliphatic aldehydes. The results of this study point to the importance of aromatic polar organic combustion products to the redox cycling potential of PM derived from biodiesel fuel combustion. Of the redox-active metals (Fe, Cu, and Zn), only Zn showed positive correlation with OP. The decreasing trend in WVO OP points to recent improvements in waste oil biodiesel fuel production technology that may have beneficial effects on exhaust emissions toxicity. Here, WVO feedstock preprocessing steps to remove free fatty acids and the relatively high (2000 ppm) fuel antioxidant concentration may partially explain the decreasing trend of OP with increasing Bxx. Future biodiesel emissions studies should combine PM toxicity assays with detailed fuel, lubrication oil, and exhaust particle composition to better elucidate compositional factors contributing to toxicity and identify alternative biodiesel fuel blend compositions that minimize biological response from exposure to exhaust PM. This may be possible using fuel additives beyond antioxidants. Future studies should also quantify the sensitivity of biologic responses to blends commonly used in real-world engines (B0 to B20) given the variability observed in this study at low blend ratios.