Describe the process by which ozone can be formed in large cities.

Ozone can be formed when a mixture of O2 and NO2 is exposed to bright light. Such mixtures occur in the polluted air of large cities. The concentration of NO2 in air is usually very low, because N2 and O2 do not react at normal temperatures. However, in the hot, reacting gases inside the cylinders of internal combustion engines, nitrogen and oxygen can react.