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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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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.
