Air Quality Modeling & Analysis

Primary Purpose

SCAG staff is responsible for the Transportation Air Quality Conformity Determination on the Regional Transportation Plan (RTP) and the Regional Transportation Improvement Program (RTIP). These conformity responsibilities now include the new air quality standards for fine particulate matter (PM2.5) and 8-hour Ozone.

The recent passage of Sustainable Communities and Climate Protection Act of 2008 (SB 375) also gives SCAG a new area of responsibility. The purpose of SB 375 is to implement the state’s greenhouse gas emissions (GHG) reduction goals in the sector of cars and light trucks. This mandate requires SCAG to meet its per-capita GHG emissions reduction targets at two points in the future – 2020 and 2035.

About SCAG Air Quality

Air Basin in SCAG Region

Air Districts in the SCAG Region

8-hour Ozone Federal Non-Attainment Area

Particulate Matter2.5 (PM2.5) Federal Non-Attainment Area

Particulate Matter10 (PM10) Federal Non-Attainment Area

Nitrogen Dioxide (NO2) Federal Maintenance Area

Carbon Monoxide (CO) Federal Maintenance Area



The EMFAC model is developed by the California Air Resources Board (ARB) and is used to calculate on-road motor vehicle emissions and transportation conformity process. EMFAC 2011 is the most recent version of the model. For information about the EMFAC 2011 Model please refer to the California Air Resources Board.

Analysis Framework

Regional Emission Analysis: The transportation activity data used in the EMFAC model are Vehicle Miles Traveled (VMT) by ranges in speed for light and medium-duty vehicles and by air basins for heavy-duty trucks. The light and medium-duty vehicles, by ARB’s definition of vehicle class, are passenger cars, light-duty trucks, medium-duty trucks, and motorcycles. The activity data are based on the following output from the transportation model:

  • Highway link information such as volumes, distance, and congested speed.
  • Intra-zonal trips, average travel time and distance.

Transportation Conformity Analysis: Transportation conformity is required by the Clean Air Act section 176© (42 U.S.C 7506(c)) to ensure that federal funding and approval are given to highway and transit projects that are consistent with (“conform to”) the air quality goals established by a state air quality implementation plan (SIP). Conformity to the purpose of the SIP means that transportation activities will not cause new air quality violations, worsen existing violations, or delay timely attainment of the relevant National Ambient Air Quality Standards (NAAQS). Conformity applies to non-attainment and maintenance areas for the following transportation-related criteria pollutants: ozone, PM2.5, PM10, CO, and NO2.