In the lower atmosphere, ozone is formed by the reaction between

NO2  +  hv  →   NO+OO2+O+M→ O3+Mwhere, M represents a third body (a molecule or a particle) needed to absorb the excess energy of the reaction, without which there would just be an exchange of oxygen atoms within the oxygen molecule. Reaction with NO keeps the concentration of O3 in balance called steady state (0.025 ppm).