This document describes the air quality modeling performed by EPA in support of the Tier 3 motor vehicle emission and fuel standards. A national scale air quality modeling analysis was performed to estimate the impact of the Tier 3 standards on future year annual and 24-hour PM2.5 concentrations, daily maximum 8-hour ozone concentrations, annual nitrogen dioxide concentrations, annual nitrogen and sulfur deposition levels, annual ethanol and select annual and seasonal air toxic concentrations (formaldehyde, acetaldehyde, benzene, 1,3-butadiene, acrolein and naphthalene) as well as visibility impairment. To model the air quality benefits of this rule we used the Community Multiscale Air Quality (CMAQ) model.1 CMAQ simulates the numerous physical and chemical processes involved in the formation, transport, and destruction of ozone, particulate matter and air toxics. In addition to the CMAQ model, the modeling platform includes the emissions, meteorology, and initial and boundary condition data which are inputs to this model.