Better-quasi-ordering

In atmospheric chemistry, the Leighton relationship is an equation that determines the concentration of tropospheric ozone in areas polluted by the presence of nitrogen oxides. Ozone in the troposphere is primarily produced through the photolysis of nitrogen dioxide at wavelengths (λ) less than 430 nm, which are able to reach the lowest levels of the atmosphere, through the following mechanism:

NO₂ + hν (λ < 240 nm) → NO + O (³P) (J₁)

O (³P) + O₂ + M → O₃ + M (k₂)

NO + O₃ → NO₂ + O₂ (k₃)

Since O (³P) is very reactive it can be assumed to be in steady state, and thus an equation linking the concentrations of the species involved can be derived:

$[O_{3}]=J_{1}[NO_{2}]/k_{3}[NO]$

The Leighton relationship above shows how production of ozone is directly related to the solar intensity and hence to the zenith angle. The yield of this molecule will therefore be a maximum during the day, especially at noon and in the summer season; it also demonstrates how high concentrations of both ozone and nitric oxide are unfeasible. However, NO can react with peroxyl radicals to give back NO₂ without loss of ozone:

RO₂ + NO → NO₂ + RO

providing another pathway to allow the buildup of O₃.

This relationship is named after Philip Leighton, who wrote a significant book in 1961 describing air pollution, as recognition of his contributions in the understanding of tropospheric chemistry.