Air quality impacts were estimated through sophisticated computer modeling based on predictions of the amount and nature of traffic under worst-case scenarios. The emissions models are based on extensive emissions testing that the U.S. Environmental Protection Agency (EPA) has conducted on thousands of vehicles representative of the ages and models of the vehicle fleet on the roads today.
The carbon monoxide and particulate matter (PM10) analyses demonstrated that the freeway will not contribute to any new localized violations, increase the frequency or severity of any existing violation, or delay timely attainment of the National Ambient Air Quality Standards or any required interim emissions reductions or other milestones.
Since ozone is a regional pollutant, there is no requirement to analyze potential impacts and no possibility of localized violations of ozone to occur at the project level. The Maricopa Association of Governments is responsible for developing plans to reduce emissions of ozone precursors in the Maricopa area. The South Mountain Freeway is included in the Regional Transportation Plan that has been determined by the U.S. Department of Transportation to conform to the State Implementation Plan on February 12, 2014.
The emission modeling developed for the freeway showed that for the mobile source air toxics study area, there would be little difference in total annual emissions of mobile source air toxics emissions (less than a 1 percent difference) in 2025 and 2035. In addition, beginning in 2017, the new EPA Tier 3 vehicle and fuel standards will reduce tailpipe and evaporative emissions and reduce mobile source air toxics.
The Record of Decision discusses the air quality analyses in greater detail.