The crossword solution for the term 'SMOG INGREDIENT' is OZONE, which is a five-letter word. OZONE is a naturally occurring gas in the Earth's atmosphere that is composed of three oxygen atoms. It is found in two layers of the atmosphere, the stratosphere and the troposphere. In the stratosphere, OZONE acts as a shield from the sun's ultraviolet radiation, while in the troposphere it is a major component of smog.