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23 to be a valuable guide, it is not universal. The tests under- taken by this project have, in conjunction with observations from other aircraft emission studies, revealed some subtle but important deviations of the near-idle VOC proportional scaling behavior. Two important observations derived from these tests are discussed below. The emission of benzene in aircraft exhaust arises from several sources: unburned benzene in the fuel, dealkylation of higher molecular weight aromatics present in the fuel, and formation through radical-radical recombination reac- tions occurring within the combustor. The near-idle scaling observation permits these fuel effects to be extracted because it focuses on changes in the exhaust composition. Deducing compositional information from individual emission indices requires that one correct for the dominating influences of engine power and ambient temperature from the measurements. Figure VI-4. Correlation of 1,3-butadiene and The scaling process naturally accomplishes this because the ethane emissions. temperature and fuel flow corrections are applied equally to both compounds. The results presented in Sections III and IV demonstrated that different compounds were equally affected by fuel flow and ambient temperature. A plot of the 1,3-butadiene emission index versus ethene The relationship of benzene and formaldehyde is investigated emission index is shown in Figure VI-4. While the data are in Figure VI-3. The slopes of these plots provide information highly correlated, linear fits always suggest a negative intercept. regarding the influence of fuel structure. While the variability Note that plots versus formaldehyde show similar results. in the plots appears to be scatter, a more complete analysis of This observation results from the fact that the emission of 1,3-butadiene has a stronger dependence on engine power all of the existing data reveals that the benzene emissions are in than do other VOCs, producing a violation of the near-idle fact nonlinearly related to the aromatic fuel content. This result VOC proportional scaling rule. The emission of 1,3-butadiene is discussed further in the fuel effects section. scales more rapidly with changes in engine power, which A second example that demonstrates a deviation from the demonstrates that errors (either positive or negative) can be near-idle VOC scaling behavior is the emission of 1,3-butadiene, made by applying a single-scaling variable drawn from EPA's an important HAP. The development of the NO+ reagent Speciate database (EPA 2008). ion mode for the PTR-MS has permitted the first real-time measurement of 1,3-butadiene (Knighton et al. 2009). This technique was employed during this project. VI.3Effect of Fuel Composition on Emissions Aromatic fuel content influences benzene emissions. The lower panel of Figure VI-5 depicts the scaled benzene emis- sion index (normalized by the formaldehyde emission index) plotted versus the fuel aromatics content. In the upper panel, the shaded gray distribution of fuel aromatics was computed from the US military fuel stocks for 2007. Although the fuel stock used in commercial aviation is different from the military sources, it is likely that commercial aviation fuel typically contains 15%22% aromatics. The larger blue points were collected at the MDW 2009 test and represent some of the most precise measurements of benzene performed to date. Additional data from other testing campaigns have been included for comparison purposes. The alternative fuels exhaust measurements in orange circles and red diamonds Figure VI-3. Correlation between emission indices show that benzene emissions flatten out (i.e., do not decrease of benzene and formaldehyde. to zero for 0% aromatic content). It is plausible that at the
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24 Figure VI-5. Benzene emission index and the aromatics fuel content. The upper panel shows the distribution of aromatics in the fuel analysis for JP8 (assumed to be a good proxy for JetA). The lower panel contains the benzene/ formaldehyde emission fraction. modest combustion temperatures at near-idle, there are two aromatic content. These data imply that if benzene emissions general pathways for the formation of benzene: one from are targeted for regulation, the fuel aromatic content could assembly of small radical precursors and a second from the be reduced to cut benzene emissions. The data also suggest pyrolysis of larger aromatic compounds. The second pathway that there is a point of diminishing value from reducing fuel would have a dependence on the content of larger aromatic aromatics, with little benzene reduction below 12% aromatic precursors, while the first would be less dependent on fuel content.