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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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