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 Air Pollution
5. Chemistry of smog formation

Photochemical smog is one of the most important and obvious air pollutants. An important role in the air pollution chemistry, especially in the formation of ozone is played by nitrogen oxides, NOx which stands for a group of compounds such as nitric oxide (NO), dinitrogen trioxide, (N2O3),and nitrogen dioxide (NO2). These compounds, along with other hazardous gases, are emitted when coal is burned in power plants and industrial boilers for the generation of power, and from automobiles. Most of the NOx emitted from combustion is nitric oxide, formed according to the following reaction.

                                          N2 + O2 = 2NO

The high temperatures (3000o F to 4000oF) which are maintained in the combustion favor the formation of NO. However, the following reactions can also take place in the furnace, in the stack, or later, in the atmosphere:

                                                    2NO + O2 = 2NO2

                                          NO2 + NO = N2O3

                                          2NO2 = N2O4

                                          3NO2 + H2O = 2HNO3 + NO

Nitrogen dioxide (NO2) reacts with hydrocarbons which are present in the atmosphere to form aldehydes and ketones through photochemical reactions. It also can react with oxygen in the presence of sunlight to
give nitric oxide and ozone:

                                           NO2 + O2 = NO + O3

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