Chemical
Composition of Gas-Phase Organic Carbon Emissions from Motor Vehicles
and Implications for Ozone Production
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Abstract
Motor vehicles are
major sources of gas-phase organic carbon, which includes volatile
organic compounds (VOCs) and other compounds with lower vapor pressures.
These emissions react in the atmosphere, leading to the formation
of ozone and secondary organic aerosol (SOA). With more chemical detail
than previous studies, we report emission factors for over 230 compounds
from gasoline and diesel vehicles via two methods. First we use speciated
measurements of exhaust emissions from on-road vehicles in summer
2010. Second, we use a fuel composition-based approach to quantify
uncombusted fuel components in exhaust using the emission factor for
total uncombusted fuel in exhaust together with detailed chemical
characterization of liquid fuel samples. There is good agreement between
the two methods except for products of incomplete combustion, which
are not present in uncombusted fuels and comprise 32 ± 2% of
gasoline exhaust and 26 ± 1% of diesel exhaust by mass. We calculate
and compare ozone production potentials of diesel exhaust, gasoline
exhaust, and nontailpipe gasoline emissions. Per mass emitted, the
gas-phase organic compounds in gasoline exhaust have the largest potential
impact on ozone production with over half of the ozone formation due
to products of incomplete combustion (e.g., alkenes and oxygenated
VOCs). When combined with data on gasoline and diesel fuel sales in
the U.S., these results indicate that gasoline sources are responsible
for 69–96% of emissions and 79–97% of the ozone formation
potential from gas-phase organic carbon emitted by motor vehicles