In atmospheric chemistry, a null cycle is a catalytic cycle that simply interconverts chemical species without leading to net production or removal of any component.
One of the most important null cycles takes place in the stratosphere, with the photolysis of ozone by ultraviolet photons with wavelengths less than 330 nanometers.
Overall, the reaction converts UV photon energy into heat thereby warming the stratosphere.
Net: hv → H This cycle links ozone to NOx in the troposphere during daytime.
In equilibrium, described by the Leighton relationship, solar radiation and the NO2:NO ratio determine ozone abundance, maximizing around noon time.