NO and NO2 are rapidly interconverted in the sunlit atmosphere on a timescale of seconds, primarily through the reaction of NO with ozone, and the photolysis of NO2; consequently, introduction of either species affects the abundance of the other; neglecting certain processes, the balance may be expressed through the Leighton ratio:
NO+O3 to NO2+O2 k1 [1]
NO2+hv to NO+O j2 [2]
[NO2]/[NO] = k1[NO][O3]/j2 [3]