Photochemical reactions are driven by light or near-visible electromagnetic radiation. In general, incoming units of energy, known as photons, excite effected molecules, raising their energy to a point where they can undergo reactions that would normally be exceedingly difficult. The process is distinguished from thermal reactions, which take place with molecules in their normal energy states. Under sunlit conditions, photochemical processes can generate small amounts of extremely reactive molecules which initiate important chemical reaction sequences.

To initiate a photochemical reaction, two requirements need to be met. First, the photon must have enough energy to initiate the photochemical reaction in the molecule. Second, the compound must be colored, in order to be able to react with visible or near-visible photon radiation.

In environmental chemistry, photochemical reactions are of considerable importance to the trace chemistry of the atmosphere. The Los Angeles photochemical smog is an example of a...