Page 42 - Curriculum Visions Dynamic Book

Smog: oxidation of exhaust fumes
Oxidation of exhaust gases is one of the main sources of pollution in the world. For example, a brown haze often hangs over some of the world’s major cities, especially those, like Los Angeles, Mexico City and Bangkok, that have heavy traffic, bright sunshine and calm air. This haze is called smog (or more properly, photochemical smog).
Smog is a result of chemical reactions that take place in the air, using the energy of sunlight. Smog begins when gases are created in the cylinders
of vehicle engines. Here oxygen and nitrogen gas combine as the fuel burns to form nitric oxide (NO, a colourless gas). Together with a cocktail of other gases, nitric oxide is pumped out of the tailpipes of vehicles.
When the gas reaches the air, more oxygen
is available from the atmosphere. The nitric oxide combines with oxygen to produce nitrogen dioxide (NO2, a brown gas). This is the gas that will eventually contribute to acid rain in wet environments. However, in dry sunny places the intense sunlight causes the nitrogen dioxide to decompose back into nitric oxide and releases oxygen atoms (O).
The formation of nitrogen dioxide is in part responsible for the brown colour of the air. But
the released oxygen atoms are very reactive and quickly combine with oxygen (O2) molecules to form ozone (O3).
Ozone (see page 12) is highly toxic and even at tiny concentrations causes irritation to the eyes, especially in congested city streets on hot sunny days.
Demonstration of the smog effect
If a gas jar of nitric oxide is put together with a gas jar of air, and the separating cover slip pulled away, the nitric oxide will mix with the air and oxidise to brown nitrogen dioxide.
Because gas molecules are in constant motion, the contents of both gas jars become uniformly brown.
 Photochemical smog over Los Angeles, California, USA.
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