Tell me a bit about the science behind smog

During the third stroke, gasoline is combusted by a spark ignited in the internal combustion engine of the car. The gasoline, being an explosive, explodes and transfers the energy as torque to provide motion to the car.

The part that is of interest during the smog test is the combustion of the gasoline and the byproducts that are released from the exhaust.

Gasoline is a cocktail of hydrocarbons and upon combustion, in presence of air in a high pressure environment releases partially combusted hydrocarbons, Nitrogen Oxides (called NOx), Carbon Monoxide, Carbon Dioxide and Water.

There are several technical reasons for releasing hydrocarbons into the air and one of the factors is the Air/Fuel ratio. The higher the air/fuel ratio in the combustion chamber, the lower the release of Hydrocarbons (HC) and CO but to the most part has an inverse effect on NOx emissions. Additionally the reduction of HC and CO with increase of Air/Fuel ratio is not linear and larger increase of this ration causes only a marginal difference in the reduction of HC and CO emissions. The ideal air/fuel ratio of emissions to get the best of performance with least unwanted emissions is referred to as the Stoichiometric ratio. At Air/Fuel values above the Stoichiometric ratio, your vehicle is referred to as running lean in fuel and below this level, rich in fuel.

In most modern automobiles a catalytic converter destroys a large portion of HC, NOx and CO.
CO reacts with NOx to release Nitrogen and carbon dioxide.

Left over CO reacts with oxygen to release carbon dioxide. HC reacts oxygen to form carbon dioxide and water. This reduces the amount of pollutants released into the atmosphere.