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Naturally occurring ozone in the upper layer of the atmosphere forms a protective shield around the earth, filtering out harmful solar radiation. But the ozone that’s found at ground level is only partly natural, and its effects are thought to be negative. It can cause respiratory problems in people, reduce crop productivity and even damage some building materials. And it’s a major component of smog, the chemical soup that forms a brownish-yellow haze over many urban areas.

Ground-level ozone is produced by a series of reactions involving other chemicals, mostly volatile organic compounds and nitrogen oxides (NOx). Especially high levels of ozone develop in the summer, when the sunlight is intense and the air is hot and stagnant.

Most volatile organic compounds in the Tennessee Valley’s air come from natural sources, and the remaining ones are generated by motor vehicles and some industries (though not power plants).

In the TVA region, transportation sources such as on-road and off-road vehicles, boats and airplanes account for about two-thirds of total  NOx emissions. All point sources in the region such as power plants, refineries and mills account for about one-fourth of total NOx emissions, but TVA point sources only account for about one-tenth of the total NOx emissions in the region. The remaining amount comes from area sources such as decaying vegetation, industrial processes and nitrogen fertilizers.

What TVA Is Doing About It

TVA is reducing NOx emissions by closing older generating units, and by installing pollution-control equipment on units where the greatest environmental and public-health benefits can be realized.

TVA operates selective catalytic reduction systems (SCRs) on 17 coal-fired units, operates low-NOx burners or low-NOx combustion systems on 20 units, optimized combustion on all 24 units, and operates NOx control equipment year-round when units are operating (except during start-up, shutdown, and maintenance periods). TVA also operates SCR’s on its Natural Gas Fired Combined Cycle Plants.

The SCR technology controls NOx emissions by transforming them into harmless nitrogen and water vapor. TVA has reduced its overall NOx emissions by 97 percent since 1995. Reductions of this magnitude help reduce ozone levels.