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
Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by photons. It is defined as the interaction of one or more photons with one target molecule. ...
of
nitrogen dioxide by photons with
wavelengths (λ) less than 420
nanometers,
which are able to reach the lowest levels of the
atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
, 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 O
2 is abundant, O (
3P) can be assumed to be in
steady state, and thus an equation linking the
concentrations of the species involved can be derived:
:
The Leighton relationship above shows how production of ozone is directly related to the solar intensity, and hence to the
zenith angle, due to the reliance on photolysis of NO
2. The yield of ozone will therefore be greatest during the day, especially at noon and during the
summer
Summer is the hottest of the four temperate seasons, occurring after spring and before autumn. At or centred on the summer solstice, the earliest sunrise and latest sunset occurs, daylight hours are longest and dark hours are shortest, wit ...
season. This relationship also demonstrates how high concentrations of both ozone and
nitric oxide
Nitric oxide (nitrogen oxide or nitrogen monoxide) is a colorless gas with the formula . It is one of the principal oxides of nitrogen. Nitric oxide is a free radical: it has an unpaired electron, which is sometimes denoted by a dot in its che ...
are unfeasible.
However, NO can react with
peroxyl radicals to produce NO
2 without loss of ozone:
: RO
2 + NO → NO
2 + 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
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Atmospheric chemistry