The chemical reactions that occur in the atmosphere are normally driven by the electromagnetic radiation from the sun. The reactions are called photochemical reactions. They are driven by the absorption of a photon of light not by intermolecular collisions as in normal reactions. The amount of energy in a photon depends on the frequency of the radiation as shown by the equation below:-
Energy=Plank's constant x frequency
E=hu
Most reactions are driven by the absorption of ultra-violet radiation which has a higher frequency than visible light. Therefore U.V light has more energy than visible light so is more likely to break chemical bonds and start reactions. Photochemical reactions generally produce three main types of particle, all of which are more reactive than molecules:-