atmospheric chemistry Atmospheric chemistry is a branch of atmospheric science in which the chemistry of the Earth's atmosphere and that of other planets is studied. It is a multidisciplinary approach of research and draws on environmental chemistry, physics, meteorol ...

, the Leighton relationship is an equation that determines the concentration of

tropospheric ozone Ground-level ozone (O3), also known as surface-level ozone and tropospheric ozone, is a trace gas in the troposphere (the lowest level of the Earth's atmosphere), with an average concentration of 20–30 parts per billion by volume (ppbv), with c ...

in areas

polluted Pollution is the introduction of contaminants into the natural environment that cause adverse change. Pollution can take the form of any substance (solid, liquid, or gas) or energy (such as radioactivity, heat, sound, or light). Pollutants, the ...

by the presence of

nitrogen oxides Nitrogen oxide may refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds: Charge-neutral *Nitric oxide (NO), nitrogen(II) oxide, or nitrogen monoxide *Nitrogen dioxide (), nitrogen(IV) oxide * Nitrogen trioxide (), or ...

Ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...

in the

troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...

is primarily produced through the photolysis of

nitrogen dioxide Nitrogen dioxide is a chemical compound with the formula . It is one of several nitrogen oxides. is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year for use primarily in the productio ...

by photons with

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, t ...

s (λ) less than 420

nanometers 330px, Different lengths as in respect to the molecular scale. The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm) or nanometer (American and British English spelling differences#-re ...

, which are able to reach the lowest levels of the atmosphere, through the following mechanism: This series of reactions creates a null cycle, in which there is no net production or loss of any species involved. Since O (³P) is very reactive and O₂ is abundant, O (³P) can be assumed to be in

steady state In systems theory, a system or a process is in a steady state if the variables (called state variables) which define the behavior of the system or the process are unchanging in time. In continuous time, this means that for those properties ''p' ...

, and thus an equation linking the

concentration In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', ''molar concentration'', '' number concentration'', ...

s of the species involved can be derived: :

\frac

The Leighton relationship above shows how production of ozone is directly related to the solar intensity, and hence to the

zenith angle The zenith (, ) is an imaginary point directly "above" a particular location, on the celestial sphere. "Above" means in the vertical direction ( plumb line) opposite to the gravity direction at that location ( nadir). The zenith is the "highe ...

, due to the reliance on photolysis of NO₂. The yield of ozone will therefore be greatest during the day, especially at noon and during the summer season. This relationship also demonstrates how high concentrations of both ozone and nitric oxide are unfeasible. However, NO can react with peroxyl radicals to produce NO₂ without loss of ozone: : RO₂ + NO → NO₂ + RO thus providing another pathway to allow for the buildup of ozone by breaking the above null cycle. This relationship is named after Philip Leighton, author of the groundbreaking 1961 book ''Photochemistry of Air Pollution'', as recognition of his contributions in the understanding of tropospheric chemistry. Computer models of atmospheric chemistry utilize the Leighton relationship to minimize complexity by deducing the concentration of one of ozone, nitrogen dioxide, and nitric oxide when the concentrations of the other two are known.

References

{{Reflist Atmospheric chemistry