Ozone Modeling Predictions Sensitivity to Speciation of Exhaust VOC Emissions.

摘要

The focus of this study is on the effect of the speciation of tailpipe exhaust emissions on ambient O3. The speciation of VOCs released in the exhaust of motor vehicles reflects the chemical composition of the fuel consumed, with additional species formed and emitted as products of incomplete combustion. Photochemical air quality models represent differences in reactivity between individual VOCs to characterize their effect on O3. Condensed chemical mechanisms (e.g., Carbon Bond 5 (CB05)) use model species to represent the multitude of VOCs present in emissions. Some model species are explicit VOCs whereas others represent classes of VOCs. A “speciation profile” assigns emissions of total organic gas (TOG, which includes all organic gas compounds emitted to the atmosphere including low-reactivity compounds such as methane, ethane, acetone and others) to model species. The speciation profile influences photochemistry and consequently affects the predicted tropospheric O3 concentrations. The speciation profile depends on the emissions source category and process and may be subject to considerable uncertainty and/or variability. The objective of this study is to use a test case to investigate the sensitivity of modeled ambient summertime O3 concentrations to the variability in the speciation of on-road gasoline light-duty vehicle (gLDV) exhaust emissions in the United States (U.S.) with focus on the eastern U.S. Emissions standards for organic gases from LDVs have become increasingly stringent in the evolution from the Tier 0 to Tier 3 standards. Thus, while considerable differences exist between the composite EPA 8757 profile and the hot-start F2B3 speciation profile modeled in this project, the change in speciation has a very small effect on total O3 due to all sources because of the small fraction (<4%) of gLDV VOCs in the total U.S. anthropogenic VOC inventory in 2030 and because several parts of the U.S., especially the eastern U.S., are NOx-limited due to large biogenic emissions. Therefore, the relative change in MDA8 O3 in July 2030 on altering the speciation is also small, with a maximum reduction of 0.7 ppb for the monthly maximum of MDA8 O3 (off the coast of New Jersey/New York) and 0.3 ppb for the monthly mean of MDA8 O3 (in the Los Angeles basin). The larger differences between the F2B3 and the EPA profile than between the F2B1 and EPA profiles are attributable to the greater fraction of methane within the F2B3 profile and the reduced fraction or absence of more reactive VOCs such as xylene and ethene.