This document describes the air quality modeling performed by EPA in support of the 2017-2025 Light-Duty Vehicle Greenhouse Gas Final Rule (hereafter referred to as LD GHG). A national scale air quality modeling analysis was performed to estimate the impact of the vehicle standards on future year: annual and 24-hour PM2.5 concentrations, daily maximum 8-hour ozone concentrations, annual nitrogen and sulfur deposition levels, and select annual and seasonal air toxic concentrations (formaldehyde, acetaldehyde, benzene, 1,3-butadiene and acrolein) as well as visibility impairment. To model the air quality benefits of this rule we used the Community Multiscale Air Quality (CMAQ) model. 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. Emissions and air quality modeling decisions are made early in the analytical process to allow for sufficient time required to conduct emissions and air quality modeling. For this reason, it is important to note that the inventories used in the air quality modeling and the benefits modeling, which are presented in Section 6.2 and 6.3, respectively of the RIA, are slightly different than the final vehicle standard inventories presented in Chapter 4 of the RIA. However, the air quality inventories and the final rule inventories are generally consistent, so the air quality modeling adequately reflects the effects of the rule.