This section amplifies on those principles and applies them to the design, manufacture and use of fuels and engines. But we do see, hovering over our cities a brown haze we call photochemical smog. We can measure the gaseous impurities - we find ozone there. It turns out this photochemical smog is a result of the production of nitric oxide in the confined and extraordinary conditions of the automobile cylinder or in power plants at a temperature above 2000 degrees Celsius. In the atmosphere, NO molecules react with oxygen rapidly to form NO 2.
Ozone is harmful in the lower atmosphere.
More complex reactions of ozone cause photochemical smog. The brown gas, nitrogen dioxide is split back to NO and an oxygen atom and the oxygen atom reacts with an oxygen molecule to form ozone. This means there is not net increase or decrease in their concentrations - they are produced in one step and disappear in another. This sequence is called a chain reaction and it results in the conversion of oxygen to ozone. The process is not spontaneous.