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Submitted to: Journal of Environmental Science and Technology
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 6/28/2017 Publication Date: 6/28/2017 Citation: Park, J.B. 2017. Reactive oxidative species formation and unregulated particulate emissions from blended diesel and biodiesel light-duty engine emissions. Journal of Environmental Science and Technology. doi: 10.1021/acs.energyfuels.7b00698. Interpretive Summary: 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. 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. OP compared between feedstocks showed statistically significant lower OP for WVO B20, B50 and B100 blends suggesting 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 polycyclic aromatic hydrocarbons (PAHs), n-alkanes, aromatic aldehydes, aromatic ketones, quinones, but not aliphatic aldehydes. The results of this study point to the importance of aromatic polar organic compounds 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. Recycling WVO to manufacture biodiesel fuel offers potentially less adverse biological effects than petrodiesel and soy biodiesel, but future biodiesel studies should combine PM toxicity assays with detailed fuel, lubrication oil and exhaust particle composition to quantitatively evaluate alternative biodiesel fuel blend compositions that minimize biological response from exhaust PM. This may be possible using fuel additives beyond antioxidants. Future studies should quantify the sensitivity of biologic response to blends commonly used in real-world engines (B0 to B20) given the relatively high variability observed in this study at low blend ratios. Technical Abstract: 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 (± one standard deviation) 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. OP compared between feedstocks showed statistically significant lower OP for WVO B20, B50 and B100 blends suggesting 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 polycyclic aromatic hydrocarbons (PAHs), n-alkanes, aromatic aldehydes, aromatic ketones, quinones, but not aliphatic aldehydes. The results of this study point to the importance of aromatic polar organic compounds 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. Recycling WVO to manufacture biodiesel fuel offers potentially less adverse biological effects than petrodiesel and soy biodiesel, but future biodiesel studies should combine PM toxicity assays with detailed fuel, lubrication oil and exhaust particle composition to quantitatively evaluate alternative biodiesel fuel blend compositions that minimize biological response from exhaust PM. This may be possible using fuel additives beyond antioxidants. Future studies should quantify the sensitivity of biologic response to blends commonly used in real-world engines (B0 to B20) given the relatively high variability observed in this study at low blend ratios. |
