Other Answers (4)

• Photochemical smog is produced by the action of sunlight on various chemicals in the air.
• Smog, or ground-level ozone, is produced by volatile organic compounds (VOC) and nitrogen oxides (NOx) reacting in the presence of sunlight.
  
  VOC emissions come from paints, gas vapor from gas stations, etc. NOx emissions come from high temperature combustion, like: power boilers, car engines, cement kilns, etc.
• Kusheng had most of it. I might just add that a lack of movement of the air contributes to the accumulation of smog. Dust is also a large contributor. As the pavement heats up in the sun it starts a convection cycle that can cause a %26quot;dust plume%26quot;. This can even happen during a thermal inversion (when warm air traps the cooler air beneath it).
• Combustion=oxygen+fuel+spark