High ground-level ozone concentrations in Central Texas are the result of a complex interaction of emissions and meteorology.

• Ground-level ozone is not emitted directly, but rather, is formed through chemical reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs)
• There are both anthropogenic (man-made) and biogenic (naturally occurring) sources of NOx and VOC emissions
• In some areas such as Central Texas, high ground-level ozone concentrations are more influenced by NOx emissions, and in other areas, they are more influenced by VOC emissions
• Ground-level ozone concentrations are influenced by long-range pollution transport:
  • Ozone levels in Central Texas would be about 27 ppb in 2017 without any biogenic or anthropogenic emissions from the U.S.
  • Biogenic emissions contribute about 4-5 ppb
  • Anthropogenic emissions in the U.S. outside of Texas contribute about 10-11 ppb
  • Anthropogenic Texas sources, including the Austin-Round Rock metro area, contributes about 20-22 ppb
  • On high ozone days, the "background" ozone level before emissions from the Austin-Round Rock metro area are added is about 59-61 ppb
  • CAPCOG estimates that anthropogenic emissions from the Austin-Round Rock metro area contributed 8 ppb to the region's 2013-2015 design value of 68 ppb

NOx v. VOC Emissions

NOx emissions are generated primarily from anthropogenic combustion sources, while VOC emissions are generated primarily from vegetation.

• NOx emissions contribute about 30-40 times more to high ozone levels in the Austin-Round Rock metro area than VOC emissions
• Peak ozone levels in Travis County are 40-140 times more responsive to reductions in local anthropogenic NOx emissions than they are to reductions in local anthropogenic VOC emissions
• It would take a reduction of about 10-13 tons per day of local NOx emissions (a 11-14% reduction) to achieve a 1 ppb reduction in peak ozone levels
• Eliminating all local anthropogenic VOC emissions would only achieve about a 0.3 ppb reduction in peak ozone levels

NOx Emissions by Source

As the figure below shows, about 1/2 of the typical ozone season weekday NOx emissions in the region are from on-road (cars, trucks, buses, etc.) mobile sources and non-road (construction equipment, agricultural equipment, etc.) mobile sources, with the rest coming from large point sources and smaller stationary "area" sources.