This describes a condition in which there is equilibrium in a photochemical reaction. For this condition to take place the rate of photodissociation or rate of chemical reaction of one or more of the reactants has to be equal to the rate of their recombination or reformation. The reactions producing ozone in the atmosphere under certain conditions are described this way. For instance, the three important tropospheric reaction involving the formation and consumption of ozone are:

O₃ + NO -------> O₂ + NO₂ (ozone consumption by NO)

NO₂ + hv -------> O + NO (NO₂ photolysis)

O₂ + O ----M---> O₃ (ozone formation)

At night in the clean troposphere, the concentration of NO is very low and so O₃ consumption is very slow, and therefore ozone's concentration stays relatively constant overnight even though ozone is no longer being produced. Likewise in polluted, urban settings at night, as before, ozone is no longer being produced but NO's concentration at the end of the day can be quite high, and so O₃ in that setting can be completely destroyed overnight. There are, therefore, some concentrations of NO, NO₂, sunlight, and O₃ that would interact according to the equations above--and given tropospheric reaction rates and sunlight intensities--would yield a constant concentration of ozone. This would be the photostationary state concentration of ozone; however, it is important to note that this concentration--if no other reagents are considered and at normal tropospheric temperatures and sunlight intensities--is too low to account for the common ground-level ozone concentrations routinely found in polluted cites like Houston, Texas or Beijing, China. Instead, there must be other means of NO oxidation to NO₂ besides oxidation by ozone ocurring to produce the O₃ levels that are wide spread in highly polluted environments. And there are: peroxy compounds like methyl peroxy radical, hydroperoxy radical, and other volatile organic compounds are also involved in reactions that yield ozone in cities all over the planet.