Ozone pollution at the earth's surface is formed within the atmosphere by the interaction of sunlight with chemical precursor compounds (or starting ingredients): the nitrogen oxides (NOx) and volatile organic compounds (VOCs). In the United States, the efforts of the Environmental Protection combustion burning, or rapid oxidation, accompanied by release of energy in the form of heat and light
Agency (EPA) to reduce ozone pollution are therefore focused on reducing the emissions of the precursor compounds. VOCs, a primary focus of many regulations, arise from the combustion of fossil fuel and from natural sources (emissions from forests). Increasingly, attention is turning to reducing the emissions of NOx compounds, which also arise from the combustion of fossil fuels. The use of cleaner fuels and more efficient vehicles has caused a reduction in the emission of ozone precursors in urban areas. This has led to a steady decline in the number and severity of episodes and violations of the one-hour ozone standard established by the U.S. Environmental Protection Agency (EPA) (which is 120 parts per billion or ppb, meaning that out of a billion air molecules, 120 are ozone). In 1999 there were thirty-two areas of the country that were in violation of the ozone standard, down from 101 just nine years earlier. Despite these improvements, ground-level ozone continues to be one of the most difficult pollutants to manage. An additional, more stringent ozone standard proposed by the EPA to protect public health, eighty ppb averaged over eight hours, was cleared in early 2001 for implementation in the United States. For comparison, Canada's standard is sixty-five ppb averaged over eight hours. see also Air Pollution; Asthma; CFCs (Chlorofluorocarbons); Electric Power; Halon; Montreal Protocol; NOx (Nitrogen Oxides); Smog; Vehicular Pollution; Ultraviolet Radiation; VOCs (Volatile Organic Compounds).
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