Alamo Area Metropolitan Planning Organization
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Air Quality Standards

The Clean Air Act requires that every five years, the Environmental Protection Agency (EPA) set National Ambient Air Quality Standards (NAAQS) for six criteria pollutants (link to including particulate matter (PM), ground-level ozone (O3), carbon monoxide (CO), sulfur oxides (SO2), nitrogen oxides (NO2), and lead (Pb). Along with setting the pollutants, EPA evaluates whether or not the thresholds set for the pollutants are adequately protective of human health and the environment, based on the most recent scientific evidence. Locally, the Texas Commission on Environmental Quality (TCEQ) is responsible for monitoring air quality and reporting those findings to the EPA.

Six Pollutants Evaluated under the Clean Air Act

Of the six criteria pollutants, the one that poses the greatest challenge to our region is ground level ozone. When ozone, a naturally occurring gas, resides in the upper atmosphere, it serves as a shield from the harmful ultraviolet radiation emitted by the sun. Unlike the “good ozone” in the upper atmosphere, ozone in the lower atmosphere is a pollutant and a primary component of smog, which is why scientists and experts frequently say, “ozone good up high, bad nearby.” Emissions from industrial facilities and electric utilities, motor vehicle exhaust, gasoline vapors and chemical solvents are some of the major sources of the pollutants that when under certain meteorological conditions, can mix to form ground level ozone.

The Clean Air Act requires that every five years, the standards for the six criteria pollutants be re-evaluated to determine whether or not they are adequately protecting human health and the environment, based on the most recent scientific evidence. During the most recent ozone review cycle, the EPA determined that the old 2008 standard was not adequate and on Oct. 1, 2015, the standard became more stringent. Compliance with the standard is based on the annual fourth highest daily ozone concentration, which is then averaged over a period of three years for a rolling three year average. Under the old standard, it was 75 ppb and it is currently 70 ppb.

Ozone Season

Ozone season is a period of time in which ground-level ozone, the primary air pollution in the Alamo Area, reaches its highest concentration levels in our air. In our region, ozone season stretches from the month of April through October.
Ground-level ozone reaches its highest concentrations during these months because it forms when nitrogen oxides mix with volatile organic compounds in intense sunlight, and sunlight is strongest from April through October. It is during this time that we are most likely to have exceedances of the National Ambient Air Quality Standard for ground-level ozone.

Conformity with Air Quality Standards

Transportation conformity is a test in which transportation activities in an MPO’s long and short-range plans must be shown to not cause new air quality violations, worsen existing violations, or delay timely attainment of the NAAQS. Failure to do so within the required time frame of 12 months or the failure to meet emissions budgets or to pass one of the conformity tests results in a conformity lapse and a designation of non-attainment.
Given the more stringent standard, the Alamo Area may be declared a non-attainment region by October 2017. To learn more about what that means for our region, click on the non-attainment tab above.

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