What is Ozone
Ozone and numerous reactive organic compounds are formed as a result of chemical reaction between nitrogen oxides and volatile organic carbons. Because sunlight provides the energy necessary to propel the reactions, these products are also known as photochemical oxidants. In preindustrial times, ozone concentration ranged from 10 to 15 parts per billion (ppb), well below harmful levels. Summer concentrations of ozone is unpolluted air in North America now range from 20 to 50 ppb. Polluted air may contain ozone concentration of 150ppb or more, a level considered quite unhealthy if encountered for long periods. EPA data indicate that ambient ozone level, monitored across the United States at some 1,100 sites, declined by 20% between 1980 and 1992, leveled off during 1990s, and then declined only a few percent in the 2000s. Ozone air quality standards are the leaders in non-attainment across the country, and more than 132 million people live in countries that still do not meet ozone standards.
Nitrogen dioxide absorbs light energy and splits to form nitric oxide and atomic oxygen, which rapidly combines with oxygen gas to form ozone. If other factors are not involved, ozone and nitric oxide then react to form nitrogen dioxide and oxygen gas. A steady state concentration of ozone results, and there is no appreciable accumulation of the gas. When VOCs are present, however, the nitric oxide reacts with them instead of with the ozone, causing several serious problems. First, the reaction between nitric oxide and the VOCs leads to highly reactive and damaging compounds known as peroxyacetyl nitrates, or PANs. Second, numerous aldehyde and ketone compounds are produced as the VOCs are oxidized by atomic oxygen, and these compounds are also noxious. Finally, with nitric oxide tied up in this way, the ozone tends to accumulate. Because of the complex air chemistry involved, ozone concentrations may peak 30 to 100 miles (50 to 160 km) downwind of urban centers where the primary pollutants are generated. As a result, ozone concentrations above the air quality standards are sometimes found in rural and wilderness areas.
The stratospheric ozone layer protects Earth from harmful ultraviolet radiation. In the upper atmosphere, 15-50 km above the Earth, oxygen and ozone absorb much of the incoming short wave radiation. These are ultraviolet (UV), X – and gamma – rays which are the main rays harmful to living organisms, damaging their genetic material. High in the atmosphere oxygen molecule are dissociated into oxygen atoms which combine with oxygen molecule to ozone. The reaction is reversible by sunlight. Ozone exists at an equilibrium level in the ‘ozone layer’ at a concentration of 1ppm.