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photochemical smog

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  1. 1. • When fossil fuels are burned, a variety of pollutants are emitted into the earth's troposphere • Two of the pollutants that are emitted are hydrocarbons (unburned fuel) and nitric oxide (NO) • When the gas is introduced to sunlight the nitric oxide (NO) is converted to nitrogen dioxide (NO2). • NO2 is a brown gas and at sufficiently high levels can contribute to urban haze. • NO2 can absorb sunlight and break apart to produce oxygen atoms that combine with the O2 in the air to produce ozone (O3)
  2. 2. • Ozone is a powerful oxidizing agent, and a toxic gas • it is believed that the natural level of ozone in the clean troposphere is 10 to 15 parts-per-billion. • scientists have found that ozone levels in "clean air" are now approximately 30 parts- per-billion.
  3. 3. • The presence of the ozone layer in the stratosphere is vital to life at the surface since the ozone layer absorbs UV light that would otherwise reach the surface and cause damage to both animal and plant life. • the ozone layer is becoming chemically perturbed due to the presence of chlorofluorocarbons (CFCs) in the stratosphere • CFCs are used in air conditioners and as cleaning and blowing agents in the chemical industry
  4. 4. • Located over Antarctica • it appears every year in October • The amount of ozone in the stratosphere over Antarctica has decreased to half the natural level
  5. 5. • Among the chemicals that may pose a human health risk are pesticides, PCBs, polycyclic aromatic hydrocarbons (PAHs), dioxins, and volatile organic compounds (benzene, carbon tetrachloride). • Many of the more environmentally persistent compounds ( PCBs) have been measured in Arctic wildlife and, for example, in tissues of the local Innuit population.