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Ground Level Ozone Chart.Ozone is an odorless, colorless gas made up of three oxygen molecules (O3) and is a natural part of the environment. It occurs both in the Earth’s upper atmosphere, or stratosphere, and at ground level in the lower atmosphere, or troposphere.
Approximately 90 percent of atmospheric ozone is located in the stratosphere, which begins about six to ten miles above the Earth’s surface and extends upwards to about 31 miles. Ozone in the stratosphere, known as the “ozone layer,” occurs naturally and forms a protective barrier that prevents ultraviolet sunlight (UV-B) radiation from the sun from reaching the Earth’s surface and harming plant and animal life. This is commonly referred to as “good ozone.”

The remaining 10 percent of atmospheric ozone is located in the troposphere, which extends from the surface of the Earth to the stratosphere. Ozone in the troposphere is not emitted directly into the air as a gas, but is formed through the photochemical reaction of nitrogen oxides (NOx) and volatile organic compounds (VOCs). Nitrogen oxides and volatile organic compounds break apart in the presence of intense sunlight and recombine into new structures, creating ozone. NOx and VOCs are known as ozone precursor gases, since their presence, concentration, and chemical reactivity both precede and lead to the production of “bad ozone."

Nitrogen oxides and volatile organic compounds are emitted by cars and trucks, industrial facilities, refineries, power plants, household products and cleaning supplies, and paints and solvents. The long-range transport of ozone and precursor emissions from local, regional, and international sources can also impact air quality. Wildfires and urban emissions from as far away as Asia contribute to elevated summertime ozone concentrations. Reducing emissions from these sources is essential to reducing the formation of ground level ozone.