Influence of Oxidized Biodiesel Blends on Regulated and Unregulated

May 24, 2010 - Laboratory of Fuels Technology and Lubricants, School of Chemical Engineering, National Technical University of Athens, 9 Iroon Polytec...
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Environ. Sci. Technol. 2010, 44, 5306–5312

Influence of Oxidized Biodiesel Blends on Regulated and Unregulated Emissions from a Diesel Passenger Car G E O R G I O S K A R A V A L A K I S , * ,† EVANGELOS BAKEAS,‡ AND STAMOS STOURNAS† Laboratory of Fuels Technology and Lubricants, School of Chemical Engineering, National Technical University of Athens, 9 Iroon Polytechniou Str. Zografou Campus, 157 80, Athens, Greece, and Laboratory of Analytical Chemistry, Chemistry Department, National and Kapodistrian University of Athens, Panepistimioupolis, 15771, Athens, Greece

Received March 15, 2010. Revised manuscript received May 6, 2010. Accepted May 13, 2010.

This paper investigates the effects of biodiesel blends on regulated and unregulated emissions from a Euro 4 diesel passenger car, fitted with a diesel oxidation catalyst and a diesel particlefilter(DPF).Emissionandfuelconsumptionmeasurements were conducted for the New European Driving Cycle (NEDC) and the Artemis driving cycles. Criteria pollutants, along with carbonyl, polycyclic aromatic hydrocarbon (PAH) and nitrate PAH and oxygenate PAH emissions, were measured and recorded. A soy-based biodiesel and an oxidized biodiesel, obtained from used frying oils, were blended with an ultra low sulfur diesel at proportions of 20, 30, and 50% by volume. The results showed that the DPF had the ability to significantly reduce particulate matter (PM) emissions over all driving conditions. Carbon monoxide (CO) and hydrocarbon (HC) emissions were also reduced with biodiesel; however, a notable increase in nitrogen oxide (NOx) emissions was observed with biodiesel blends. Carbon dioxide (CO2) emissions and fuel consumption followed similar patterns and increased with biodiesel. The influence of fuel type and properties was particularly noticeable on the unregulated pollutants. The use of the oxidized biodiesel blends led to significant increases in carbonyl emissions, especially in compounds which are associated with potential health risks such as formaldehyde, acetaldehyde, and acrolein. Sharp increases in most PAH compounds and especially those which are known for their toxic and carcinogenic potency were observed with the oxidized blends. The presence of polymerization products and cyclic acids were the main factors that influenced the PAH emissions profile.

Introduction Biodiesel is chemically synthesized via the transesterification from vegetable oils, used frying oils, and animal fats. Biodiesel properties are similar to those of diesel fuel; it has low sulfur content, is free of aromatic compounds, is nontoxic and readily biodegradable, and possesses a higher cetane number, * Corresponding author phone: +30 210 7723213; fax: +30 210 7723163; e-mail: [email protected]. † National Technical University of Athens. ‡ National and Kapodistrian University of Athens. 5306

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higher flash point, and better lubricity performance. Despite its many advantages, biodiesel has poor oxidation stability with respect to petroleum diesel. Fuel stability may be affected by the type of feedstock, the presence of naturally occurring antioxidants, and the storage conditions (1). Oxidation stability is of great importance in the context of possible problems with engine parts, as well as the impact of emissions. The main oxidation products are peroxides and hydroperoxides. During further degradation, these products form shorter-chain compounds such as low molecular weight acids, aldehydes, ketones, and alcohols. Further reactions of the unstable hydroperoxide species with another fatty acid chain may form high molecular weight materials, such as dimer or trimer acids which may lead to filter blocking, injector failures, and deposit formation (2, 3). The use of biodiesel for reducing the environmental impact of diesel emissions has been widely investigated. In fact, many studies have centered their research on the criteria pollutants, such as carbon monoxide (CO), unburned hydrocarbons (HC), nitrogen oxides (NOx), and particulate matter (PM). Many authors have shown that the addition of biodiesel can reduce most of the aforementioned pollutants, with the exemption of NOx (4–6). It has been demonstrated that the positive or adverse effect of biodiesel fuels on PM emissions varied significantly among vehicles, engine technology, and test cycle (7). A number of authors showed that PM emissions for the New European Driving Cycle (NEDC) may be adversely affected by biodiesel, a phenomenon which is mainly attributed to certain physicochemical parameters of biodiesel and to the cold-start conditions (8, 9). In the literature, there are two diverse interpretations concerning NOx emissions of biodiesel. Some studies reported higher NOx production (10, 11), whereas others have shown lower NOx emissions in respect to diesel fuel (12, 13). The increase in NOx emissions is still not very well explained, but several parameters, including fuel type, fuel quality, fuel spray characteristics, operating conditions, and engine technology are implicated (14–16). Limited information is available regarding the effects of biodiesel on emissions of carbonyl compounds and polycyclic aromatic hydrocarbons (PAHs). Concerning carbonyl emissions, there are some divergences when considering the results obtained with diesel fuel and biodiesel blends. Many studies showed a clear increasing trend of these emissions when biodiesel was used (17, 18); however, some studies reveal decreases or even insignificant differences (19, 20). It should be noted that carbonyl compounds from vehicular exhaust are of great importance since some species are toxic, mutagenic, and even carcinogenic to the human body. They also play a critical role to the tropospheric chemistry, as they are important precursors to free radicals (HOx), ozone, and peroxyacylnitrates (21). PAH emissions are released during incomplete combustion of fossil fuels, are widely distributed in the atmosphere, and are one of the first pollutants to have been identified as suspected carcinogens (22). Nitrated and oxygenated PAHs, which have elicited the most concern due to their mutagenic and carcinogenic properties, are also products of fuel combustion (23, 24). The information given in literature about the effect of biodiesel on PAH emissions is limited and often contradictory. The majority of authors have observed some decrease in PAH emissions with biodiesel, although a noticeable dependence on engine operation conditions is usually acknowledged (25–27). On the other hand, some authors found some increase with biodiesel, which may be attributed to the test cycle and the fuel chemical structure (19, 28). 10.1021/es100831j

 2010 American Chemical Society

Published on Web 05/24/2010

All of the above show that biodiesel impact on engine emissions could be quite significant. Few studies are available on modern passenger cars, employed common-rail engine systems, and after-treatment technologies, and even fewer studies report results, which are not necessarily representative of actual driving conditions, making it difficult to assess the fuel impact on diesel car fleet emissions. This work has three main objectives: (i) to evaluate the impact of fuel source material on the formation of exhaust emissions from a modern passenger car representative of the current European fleet, (ii) to investigate the effect of the use of oxidized fuel on the formation of carbonyl and PAH emissions, and (ii) to study the influence of biodiesel concentration and driving cycle on the exhaust emissions.

Experimental Section Test Fuels and Vehicle. A total of seven fuels were evaluated in this study. An ultra low sulfur diesel meeting the current European Union (EU) fuel quality requirements for diesel vehicles (EN 590:2008) was used as reference fuel and to create blends with two types of methyl esters. Soy-based methyl ester (SME) and used frying oil methyl ester (UFOME) were blended with the reference diesel at proportions of 20, 30, and 50% v/v. The neat biodiesels were examined according to the automotive fatty acid methyl ester (FAME) standard EN 14214 (Table S1-S2 in the Supporting Information). It should be mentioned, that UFOME was an oxidized fuel, since it was naturally aged during long-term storage. The main quality properties of the diesel fuel and its blends with both biodiesels are given in the Supporting Information section (Table S2-S3). A 2007 model year Subaru Forester 2.0D XS (SUV type), equipped with a common-rail direct injection diesel engine and meeting Euro 4 emission standards, was used in this study. Emissions in this vehicle were controlled by a diesel oxidation catalyst (DOC) and a silicon carbide (SiC) diesel particulate filter (DPF). All emission tests were performed with the vehicle in its original configuration. The technical specifications of the vehicle are listed in Table S3-S4 in the Supporting Information. Driving Cycles and Measurement Protocol. In order to investigate the impact of biodiesel on the exhaust emissions and fuel consumption, the vehicle was driven on a chassis dynamometer over the certification NEDC and the nonlegislated Artemis driving cycles. The Artemis cycles are distinguished into an urban (Urban), a rural (Road), and a motorway (Motorway) part, each representative of the corresponding driving condition. The speed vs time profiles of the applied driving cycles can be found elsewhere (9). The daily measurement protocol started with the NEDC, which is a cold-start driving cycle. This comprises two parts: an urban part (UDC), where the engine starts from room temperature, and an extra-urban part (EUDC), which aims at testing the car at higher than urban speeds. The NEDC was then followed by the three Artemis cycles. This protocol was repeated twice per fuel blend, while two sets with the reference fuel were conducted at the beginning and end of the campaign. Prior to each measurement, the vehicle was conditioned for about 350-400 km before testing whenever a fuel change was required. Exhaust Sampling and Emission Analyzers. Emission measurements were conducted following the European regulations (Directive 70/220/EEC and amendments). Gaseous and PM mass were sampled according to the constant volume sampling (CVS) technique, with a dilution tunnel. A schematic of the sampling system and detailed information regarding the emission analyzers are given in ref 27. Carbonyls and PAHs Analysis. A detailed description regarding the sampling and analysis of carbonyls and PAHs is given in the Supporting Information section.

FIGURE 1. (a-b) CO2 emissions and fuel consumption for all fuel/cycle combinations.

Results and Discussion Fuel Consumption and CO2 Emissions. Figure 1a-b shows the experimental results obtained for the CO2 emissions (a) and fuel consumption (b). Regarding CO2, some increases were observed with biodiesel over all driving conditions. Higher increases were found over the NEDC, which may be attributed to the cold-start UDC phase of the cycle. This phenomenon indicates a possible drop in engine efficiency when biodiesel under low-speed load conditions is used. The biodiesel impact on CO2 emissions ranged on average from 1 to 6% over the NEDC and the Artemis cycles. Fuel consumption, expressed in L/100 km, presented similar patterns with CO2 emissions. An increase in fuel consumption proportional to the difference in energy content of the fuels was observed for tested fuels. This increase was in the order of 1 to 6% on average over all cycles for the biodiesel blends. It should be stressed that the oxidized blends produced slightly higher consumption when compared to the SME blends. This observation was probably based upon the fact that the oxidized biodiesel contained a higher amount of oxygen (due to the formation of oxidation products) and, hence, presented lower energy content than the SME. VOL. 44, NO. 13, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. (a-d) NOx (a), PM (b), HC (c), and CO (d) emission measurement results for the tested fuels over the NEDC and the Artemis driving cycles. Regulated Emissions. In order to facilitate result presentation, the regulated emissions of NOx, PM, HC, and CO are presented in Figure 2a-d. Regarding NOx emissions, an increasing trend was observed when increasing biodiesel concentration over all driving cycles, with the exception of SME-50 blends which resulted in some reductions over the NEDC and Artemis Urban for no obvious reason, since the results were repetitive. When biodiesel is used, NOx emission levels over the NEDC were found to be above the Euro 4 specification limits (0.25 g/km). The increase was in the order of 9 and 8% on average for SME and UFOME blends, respectively. Biodiesel application during the Artemis driving cycles did not change NOx profile; however, the emission levels were higher than those of the NEDC. The highest NOx emissions were observed for the Artemis Urban cycle. Those were 3.1-3.7 higher than equivalent emissions observed for NEDC. The aforementioned observations support the hypothesis that the engine control is optimized to use regular automotive diesel, and its calibration was based upon the operating points of the NEDC. The lower NOx emission levels for the NEDC, as opposed to Artemis cycles, were as expected. NEDC is characterized by low vehicle speed and low engine load and, thus, low exhaust gas temperature. As a higher load induces a higher combustion temperature and NOx emissions increase with temperature, NOx emissions are higher at higher loads. Additionally, the presence of oxygen also enchases combustion temperature resulting in higher NOx formation and, thus, higher exhaust temperature. The impact of biodiesel source material and type on NOx emissions may be potentially explained by several mechanisms. It has been reported that the increased number of double bonds and, thus, the lower cetane number, along with the oxygen content in the ester molecule, may affect NOx formation (5, 14, 29). Under the present test conditions, both biodiesels were mainly composed of unsaturated fatty 5308

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esters and, therefore, an increase in NOx may result from the role of the double bonds in the combustion chemistry. PM emission results are shown in Figure 2b. Large reductions in PM were achieved with biodiesel blends over all driving cycles. These reductions can be explained by the increase in oxygen content in the fuel which contributes to more complete fuel oxidation even in locally rich zones and the lack of sulfur and aromatics since these compounds increase the soot nucleation rate (30). Another major contributing factor was the exhaust configuration of the vehicle (DOC + DPF), which led in full oxidation of PM inside the catalyst. In general, PM emissions were found well below the Euro 4 limit (0.025 g/km) over all driving cycles but surprisingly above the future Euro 5 limit (0.005 g/km). The highest reductions were observed over the NEDC and ranged from -7 to -23% and -9 to -24% for the SME and UFOME blends, respectively. Smaller reductions were achieved over the Artemis cycles, which were in the order of -15, -4, and -3% on average for all biodiesel blends over Urban, Road, and Motorway, respectively. However, the emission levels obtained during Road and Motorway operation were lower than those of the NEDC. These results suggest that the combined effect of the cold-start UDC phase and the lower volatility of biodiesel adversely influenced the formation of PM emissions during the NEDC. This could be a consequence of the poorer combustion, of lower fuel vaporization at low temperature, which was the case with the biodiesel blends, and also of the reduced efficiency of the catalytic converter, which was below the light-off temperature in the starting phase of the cycle (8, 9, 29, 31). A trend toward higher HC emission levels was observed with biodiesel over the legislated NEDC (Figure 2c). This phenomenon may be due to the lower volatility of the biodiesel blends and the cold-start effect during the UDC operation. A different behavior was observed for both

biodiesel blends over the Artemis cycles. In general, emission levels seem to be reduced as the mean driving cycle power increases. Similar to HC, CO emissions decreased as the content of biodiesel increased over all driving conditions (Figure 2d). Higher CO emission levels were observed over the NEDC which can be attributed to the poor catalyst efficiency; while during Artemis operation, these emissions were noticeably lower. Again, it should be noted that the increase of the mean load and speed of the cycle, along with the oxygen availability of biodiesel, positively affected CO emissions (32). In addition, during Artemis operation, the catalytic activity is higher due to the increased exhaust temperature and, thus, leads to lower HC and CO emissions. Carbonyl Emissions. Thirteen carbonyl compounds (aldehydes and ketones) were identified in the exhaust gases, and the concentrations measured over all driving cycles are listed in Tables S4-S7 and S5-S8 in the Supporting Information. Consistent with other studies (33–35), low molecular weight compounds such as formaldehyde, acetaldehyde, acrolein, and propionaldehyde were found to be the most abundant carbonyls emitted. However, heavier compounds such as crotonaldehyde, methacrolein, and butyraldehyde were present in the exhaust in relatively high concentrations. The addition of biodiesel, independent of its origin, provided significant increases in emissions of carbonyl compounds over all cycles. A clear trend of increase in carbonyl emissions was observed with the increase in biodiesel content. On the other hand, aromatic aldehydes produced discordant results. Benzaldehyde was reduced with biodiesel as expected, since its formation depends mainly from the aromatic content of the fuel (34). However, p-tolualdehyde and hexanaldehyde increased with biodiesel, an observation which indicates that the formation of these compounds may be influenced by other parameters than the aromatic content. Under the present test conditions, the level of formaldehyde emissions significantly increased over reference levels for both types of biodiesel blends. The highest increases over the NEDC were 109 and 54% for UFOME-50 and SME-50, respectively. During Artemis operation, the blends of UFOME resulted in average increases of 38, 42, and 38% over Urban, Road, and Motorway, respectively. The same trend was found for the SME blends where the average increases were 15, 23, and 27% over Artemis Urban, Road, and Motorway, respectively. Acetaldehyde emissions followed a similar pattern with formaldehyde and increased with biodiesel fraction in the fuel. The average increases for the UFOME blends were in the order of 26, 22, 43, and 50%, while for the SME blends the increases were 12, 12, 30, and 35% over NEDC, Urban, Road, and Motorway, respectively. The higher level of these compounds with biodiesel may be attributed to the presence of short-chain esters that favor formation of the shortest chain aldehydes (namely formaldehyde and acetaldehyde) during combustion, since formaldehyde in vehicle exhaust is mainly produced from the incomplete combustion of saturated aliphatic hydrocarbons (36). This was probably the case of used frying oil methyl ester, which was initially composed of unsaturated methyl esters, where during the repeated deep frying process the formation of short-chain components was favored from the breakdown of unsaturated fatty acids (20). Another major contributing factor for the higher carbonyl emission levels was the presence of oxygen in the ester molecule (4, 18). A strong increase was observed in acrolein emissions for all fuel/cycle combinations. A clear trend toward higher emissions with the application of UFOME blends was observed as regards the SME blends. The average increases for the UFOME blends were in the order of 19, 17, 35, and 40%, while for the SME blends these increases were 17, 13, 29, and 32% over the NEDC, Urban, Road, and Motorway, respectively. Acrolein emissions are mainly originated by the

FIGURE 3. Total combinations.

carbonyl

emissions

for

all

fuel/cycle

oxidation of glycerol, glycerides, and fatty acid residues present in the biodiesel (36). This result is in agreement with the higher amount of glycerol and glycerides content found in UFOME as compared to SME, which met the strict specifications. Figure 3 shows the total carbonyl emissions for all fuel/ cycle combinations. Results indicate that the presence of the UFOME blends led to significantly higher emissions than the corresponding SME blends and diesel fuel. This phenomenon may be explained by the fact that the parent oil already had an amount of carbonyls and carboxylic acids, which were formed during the thermal stressing of the oil and finally remained in the methyl ester during transesterification. In line with the above, a considerable amount of aldehydes and ketones, especially those of high molecular weight, would be present in the fuel due to its oxidized nature (37, 38). As the oxidation and auto-oxidation mechanisms proceed, the hydroperoxides which are formed during the primary oxidation stage decompose to ultimately form aldehydes (secondary oxidation products). A more in depth analysis of the individual carbonyl compounds shows that the use of UFOME blends led to higher emission levels of heavier compounds than the blends of SME. Therefore, it is reasonable to assume that the application of UFOME would be more prone to higher formation of carbonyl emissions when compared to SME. Concerning the influence of the driving cycle on carbonyl compounds, notable differentiations were observed between the employed cycles. During operation over the NEDC, higher emissions levels were obtained when compared to the Artemis cycles. This phenomenon can be ascribed to the cold-start effect of the UDC phase and the partial deactivation of the oxidation catalyst. The exhaust concentration of all carbonyls were lower over the transient Artemis cycles, which was due to the increased exhaust temperatures and, thus, the higher performance of the oxidation catalyst. On the other hand, the increased combustion efficiency, due to the higher average speed and load of such driving modes, possibly led to reduced carbonyl emissions as these compounds are mainly products of incomplete combustion (39). PAH, Nitro-PAH, and Oxy-PAH Emissions. Although the DPF are very effective in capturing PM and leaving engine exhaust gases nearly clean of PM, there is no clear evidence of their effect on PAH emissions. A total number of 12 PAHs, 4 nitro-PAHs, and 6 oxy-PAHs were identified and quantified in the vehicle’s exhaust. Detailed information on these VOL. 44, NO. 13, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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emissions is shown in Tables S6-S9 and S7-S11 in the Supporting Information. It is evident from the results that the application of UFOME blends led to higher PAH emissions compared to those of diesel fuel. On the contrary, the blends of SME produced lower PAH emissions for all fuel/cycle combinations. It should be noted that the use of DPF positively influences the PAH profile in the exhaust, since it is assumed that these species were either absorbed in the DPF or probably converted to CO2. Another critical observation is that the concentrations of both lower and higher molecular weight PAHs were found in similar levels. These findings are not consistent with the results reported in previous studies, which showed that PAH levels are higher in the absence of DPF while light PAHs are usually the predominant compounds in diesel exhaust (26, 40). Heavier PAH compounds such as those of indeno[1,2,3c,d]pyrene and benzo[g,h,i]perylene were only detected for UFOME blends during all driving conditions. For the SME blends, these species remained almost undetectable. However, the concentrations of these microcontaminants were lower than those of light PAHs. It is possible that PAHs with higher ring number are retained more efficiently in DPFs (41). The increased emissions of these species might be due to pyrosynthesis of lower molecular weight aromatic compounds to larger PAHs and to the contribution of the lubricant oil (42). Low molecular-weight PAHs (containing 3-5 aromatic rings), such as those of phenanthrene, anthracene, fluoranthene, and pyrene, were found in higher concentrations with the use of UFOME blends in respect to diesel fuel and SME blends. The higher levels of light PAHs suggest that these compounds were pyrolyzed from incomplete combustion of the fuel (22, 43, 44). With increasing market share of DPF vehicles, it is expected that PAH profiles in urban air will change. However, under the present test conditions, PAH compounds which are known for their toxic, mutagenic, and carcinogenic properties, such as chrysene, benzo[a]anthracene, and benzo[a]pyrene were significantly increased with the application of UFOME blends. This phenomenon may be attributed to fuel composition and the de novo formation of these species in the DPF. Of the nitro-PAH compounds analyzed, only 1-nitropyrene and 6-nitro-benzo[a]pyrene were found in quantifiable levels in the particle phase. 6-Nitro-benzo[a]pyrene emissions were reduced for all fuel/cycle combinations when compared to diesel fuel. On the other hand, emission levels of 1-nitro-pyrene showed discordant results. The use of UFOME blends led to strong increases as compared to diesel fuel, while the use of SME blends provided some reductions with the exemption of a marginal increase over Artemis Road. It is possible that DPFs can support nitration chemical reactions, maybe due to the high NOx emissions constantly passing through the filter. In any case, further research is required in order to strengthen this hypothesis. Oxygenated PAH levels were found in significant lower amounts than their parent PAHs. It should be mentioned that four of the six oxy-PAH compounds in the test matrix were found in quantifiable levels. The prominent oxy-PAHs emitted were anthraquinone, benzanthrone, benz[a]anthracene-7,12-dione, and 9-fluorenone. The relatively high levels of anthraquinone, benzanthrone, and benz[a]anthracene-7,12-dione can be attributed to the fact that these compounds are the most stable fragments of oxidized PAHs (45). The general picture showed that the addition of biodiesel had a negative effect on the emissions of these compounds. This was particularly noticeable with the use of UFOME blends, which led in sharp increases over all driving conditions. Despite the fact that the SME blends presented lower oxy-PAH emissions compared to diesel fuel, their emission levels increased with increasing biodiesel content. In general, the higher levels of oxy-PAH emissions may be related to the 5310

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FIGURE 4. (a-c). Total PAH, nitro-PAH, and oxy-PAH emissions for all fuel/cycle combinations. oxygenated group in the methyl ester. These observations generate scepticism on the true impact of biodiesel on oxy-

toxicity of the tested fuels over the several driving cycles. As it can be observed, there is a clear reduction in the overall toxicity of SME blends compared to diesel fuel. On the other hand, TEF was significantly increased for the UFOME blends over the legislated NEDC and Artemis Urban, which mean that these emissions have higher health risks than conventional fuel emissions. This phenomenon was attributed to the increased levels of the highly carcinogenic compounds of benzo[a]anthracene and benzo[a]pyrene during these driving modes. This result suggests that the application of an oxidized biodiesel obtained from used frying oils eliminates the benefits of the biofuel with respect to the soybased biodiesel. It was also evident from the results that the overall PAH toxicity was influenced by the driving cycle. The lower toxicity for all fuel blends during Road and Motorway driving implies that the emitted PAH compounds were of low molecular weight, which have lower toxicities compared to heavier species. On the contrary, the formation of large PAHs with higher toxicities was favored by low-load conditions and the cold-start effect.

FIGURE 5. Toxicity equivalent factors (TEFs) for diesel fuel and its blends with biodiesel. PAH emissions because of the specific toxicity of quinoid and other oxygenated compounds (46). Figure 4a-c shows the total PAH, nitro-PAH, and oxyPAH emissions, which are the sum of the concentrations of the 12 PAH, 4 nitro-PAH, and 6 oxy-PAH components, respectively. Total PAH emissions confirm the negative and beneficial performance of UFOME and SME blends, respectively. It is probable that fuel composition and quality make a major contribution to the PAHs found in the exhaust of the vehicle. As mentioned previously, the combined effect of biodiesel source material and its oxidized nature led to such increases in PAH emissions. It is reasonable to assume that the biodiesel obtained from used frying oils would contain dimers, trimers, polymerization products, and cyclic acids originated from breakdown between two fatty acid chains in the same triacylglyceride molecule (via, e.g., Diels-Alder reaction). These diesters can be formed during thermal stressing, and they would be present in the final fuel and, thus, in the exhaust (45, 47). The case of SME presents some differences when compared to UFOME. The PAH emission reduction may be attributed to the presence of excess oxygen in biodiesel and the absence of aromatic and polyaromatic compounds in the fuel (25). A clear correlation was observed between PAH emissions and the applied driving conditions. The exhaust concentrations of all PAHs were quite low over Artemis Road and Motorway cycles when compared to NEDC and Artemis Urban, i.e., simulating driving at low average speeds in city conditions. The observed reductions in PAH emissions can be attributed to the higher average speed and engine load during these driving conditions, which increases exhaust temperatures and thus results in better oxidation of these compounds both inside the vehicle catalyst and the sampling system (39, 48). The higher PAH emissions obtained over the NEDC may be due to the cold-start effect and the reduced catalyst efficiency, along with the lower fuel vaporization at low-temperature, which was the case with the biodiesel blends (45). Toxicity. The health risk associated with inhalatory exposure to PAHs is commonly assessed on the basis of benzo[a]pyrene, which is known for its high carcinogenic and toxic properties (47). The carcinogenic effect of each individual PAH can be determined by means of a conversion factor (toxic equivalent factor, TEF). TEFs for individual PAHs were used to estimate human health risk associated with inhalatory exposure to PAHs (44). Figure 5 shows the general

Acknowledgments This paper is dedicated to the memory of Professor Stamos Stournas.

Supporting Information Available Including the main fuel quality properties, description of sampling, and analysis of carbonyls and PAHs, as well as the individual carbonyl and PAH compounds emitted for all fuel/ cycle combinations.This material is available free of charge via the Internet at http://pubs.acs.org.

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