Photochemical smog forms through a series of chemical reactions among compounds in the atmosphere. When nitric oxide (NO), a component of the exhaust from cars and power plants, enters the atmosphere, it reacts with oxygen to produce nitrogen dioxide (NO2). Sunlight can break nitrogen dioxide down. This process initiates other chemical reactions that lead to the formation of low-level ozone. Although ozone (O3) that is high in the stratosphere filters out harmful UV radiation, ozone's presence at the ground level poses a health risk. Also, Volatile Organic Compounds (VOCs), molecules that enter the atmosphere from substances such as gasoline, cleaning solvents, and trees, play a crucial role in forming photochemical smog.