The overall U.S. ozone emissions in parts per million with a standard of 0.12 ppm. Occurs naturally in the stratosphere and provides a protective layer shielding the Earth from harmful ultraviolet radiation.
Environmental Protection Agency: Ozone - http://www.epa.gov/air/ozonepollution/index.html
Ozone (O3) is a gas composed of three oxygen atoms. It is not usually emitted directly into the air, but at ground-level is created by a chemical reaction between oxides of nitrogen (NOx) and volatile organic compounds (VOC) in the presence of sunlight. Ozone has the same chemical structure whether it occurs miles above the earth or at ground-level and can be "good" or "bad," depending on its location in the atmosphere
EPA: Ozone - http://www.epa.gov/air/eac/
The Environmental Protection Agency is working with communities to get clean air as soon as possible by entering into Early Action Compacts to reduce ground-level ozone pollution. Communities with Early Action Compacts will start reducing air pollution one to at least two years earlier than required by the Clean Air Act.