MOBILE6.2 Modeling of Exhaust Air Toxic Emission Factors: Trend and Sensitivity Analysis

Abstract

The newly released MOBILE6.2 has incorporated both air toxic and particulate matter emission factor modeling functions. A series of test runs were performed to gain a better understanding of the air toxic modeling function and overall model behavior. These test runs and scenarios evaluated the changes in emission factors of all six built-in air toxic compounds as affected by vehicle activities, fuel physical properties, fuel chemical compositions, oxygenated fuel additives, and environmental conditions. Results obtained indicate that exhaust emission factors for acrolein, acetaldehyde, benzene, 1,3-butadiene, formaldehyde, and methyl tertiary butyl ether are inversely proportional to freeway and arterial vehicle speeds. This phenomenon follows the trend of total organic gas emission factors. Effects from roadway facility differences indicated that the higher the percentage of vehicle miles traveled on a freeway, the lower the air toxic emission factors on a per vehicle mile traveled basis. Exhaust air toxic emission factors increase when fuel Reid vapor pressure value and sulfur content increase. Diesel sulfur content has no effect on the six toxic compound emission factors. Effects from fuel chemical compositions on all emission factors varied. However, chemical compositions do have significant effects on all air toxic compound emission factors. On the same note, both minimum and maximum temperatures affect all air toxic emissions significantly. The time series evaluation indicates that all six tested air toxic compound emissions decrease linearly from the year 2002 to 2020.