Regulation of Fuels and Fuel Additives: Renewable Fuel Standard ...

change gasoline sulfur levels, so emissions of sulfur dioxide and any resultant ambient
concentrations of sulfate PM are not expected to change. Increased ethanol use is
expected to increase NOx emissions, as described above. Thus, the possibility exists that
ambient nitrate PM levels could increase. Increased ethanol is generally expected to
increase VOC emissions, which could also impact the formation of secondary organic
PM. However, some VOC emissions, namely exhaust VOC emissions, are expected to
decrease, while non-exhaust VOC emissions are expected to increase and the impact on
PM is a function of the type of VOC emissions.
The formation of secondary organic PM is very complex, due in part to the wide
variety of VOCs emitted into the atmosphere. Whether or not a specific gaseous VOC
reacts to form PM in the atmosphere depends on the types of reactions that VOC
undergoes, which in turn can depend on other pollutants present, such as ozone, NOx and
other reactive compounds. The relative mass of secondary PM formed per mass of
gaseous VOC emitted can also depend on the concentration of the gaseous VOC and the
organic PM in the atmosphere. Most of the secondary organic PM exists in a continually
changing equilibrium between the gaseous and PM phases. Both the rates of these
reactions and the gaseous-PM equilibria depend on temperature, so seasonal differences
can be expected.
Recent smog chamber studies have indicated that gaseous aromatic VOCs can
form secondary PM under certain conditions. These compounds comprise a greater
fraction of exhaust VOC emissions than non-exhaust VOC emissions, as non-exhaust
VOC emissions are dominated by VOCs with relatively high vapor pressures. Aromatic
VOCs tend to have lower vapor pressures. As increased ethanol use is expected to reduce
exhaust VOC emissions, emissions of aromatic VOCs should also decrease. In addition,
refiners are expected to reduce the aromatic content of gasoline by 5 volume percentage
points as ethanol is blended into gasoline. Emissions of aromatic VOCs should decrease
with lower concentrations of aromatics in gasoline. Thus, emissions of gaseous aromatic
VOCs could decrease for both reasons.
Overall, we expect that the decrease in secondary organic PM is likely to exceed
the increase in secondary nitrate PM. In 1999, NOx emissions from gasoline-fueled
vehicles and equipment comprised about 20% of national NOx emissions from all
sources. In contrast, gasoline-fueled vehicles and equipment comprised over 60% of all
national gaseous aromatic VOC emissions. The percentage increase in national NOx
emissions due to increased ethanol use should be smaller than the percentage decrease in
national emissions of gaseous aromatics. Finally, in most urban areas, ambient levels of
secondary organic PM exceed those of secondary nitrate PM. Thus, directionally, we
expect a net reduction in ambient PM levels due to increased ethanol use. However, we
are unable to quantify this reduction at this time.
EPA currently utilizes the CMAQ model to predict ambient levels of PM as a
function of gaseous and PM emissions. This model includes mechanisms to predict the
formation of nitrate PM from NOx emissions. However, it does not currently include any
mechanisms addressing the formation of secondary organic PM. EPA is currently
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