Photochemical Smog
Ozone is a by product of fossil fuel combustion. When pollutants such as
hydrocarbons and nitrogen oxides react with sunlight, to produce ozone following
a series of free radical reactions. Photochemical smog is a complex pollutant
generated when hydrocarbons and oxides of nitrogen react in strong sunlight,
forming a series of secondary pollutants.
The major components of photochemical smog are:
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Ozone
Peroxyacyl nitrates
Aldehydes
Alkyl Nitrates
Air borne particles
The formation of a photochemical smog is typically initiated by photolysis of
nitrogen dioxide, which leads to the production of ozone and in turn the
reformation of NO2. This is one of the most important photochemical reactions in
the lower atmosphere: Return to Top
Low Level Ozone Synthesis:
NO2 + hv
NO + O (1)
When nitrogen dioxide is excited by light nitric oxide and atomic oxygen are
formed. The rate of this equation is k1[NO2]. k1 is the rate coefficient for this
reaction and is first order. The rate of the reaction is not only proportional to the
concentration of nitrogen dioxide. It also depends on the intensity of light. At
night k1 approaches 0 and reaches its maximum during hours of high sunlight
intensity.
The first reaction of the atomic oxygen tends to be with the more abundant
diatomic oxygen (O2)
O2 + O + M
O3 + M (2)
M is an unreactive 3rd molecule which is needed to absorb excess energy from the
reaction. The rate coefficient of this reaction is k2. There is a third and final
reaction which completes this this reaction:
O3 + NO
NO2 + O2 (3)
This reaction has the rate coefficient k3.
Each of the 3 reactions is very fast, and a photostationary state of equilibrium is
established, this governs the ratio of NO2/NO in air, which in turn can be shown to
be proportional to the concentration of ozone.
[NO2]/[NO] = k3[O3]/k1 (4)
So as k1approaches 0 (night time conditions. The presence of excess ozone
increases and the lowest [NO2]/[NO] ratios are during hours of extreme day light.
The reactions (1) and (2) produce ozone and mono-atomic oxygen, which go on to
react with hydrocarbons released from fossil fuels. Alkenes are particularly
susceptible to attack from these oxygen allotropes. A typical reaction is shown
below:
CH3CH=CHCH3+O3
CH3CHO2+CH3CHO(5)
The reaction chain is then propagated:
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C2H5+O2
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C2H5O2+NO
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C2H5O+O2
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HO2+NO
C2H5O2(6)
C2H5O+NO2(7)
CH3CHO+HO2(8)
NO2CHO2+OH(9)
Reactions (7) and (9)convert NO to NO2 without the use of ozone, keeping NO
concentrations reasonably low. Reaction 3 is relatively unimportant meaning that
high levels of ozone are able to build up. Photochemical smog's are a particular
problem in built up cities where there are high numbers of vehicles and industries
relying on combustion of fossil fuels. Eventually winds disperse the smog.