Air Quality Guidelines Global Update 2005 - World Health ...

11. Ozone
Paulo Hilário Nascimento Saldiva, Nino Künzli, Morton Lippmann
General description
Sources
Ozone (O3) and other photochemical oxidants are pollutants that are not directly
emitted by primary sources. Rather, they encompass a group of chemical species
formed through a series of complex reactions in the atmosphere driven by
the energy transferred to nitrogen dioxide (NO2) molecules when they absorb
light from solar radiation (represented in the equations below by hυ, where h is
Planck’s constant and υ is the frequency of light).
The precursors that contribute most to the formation of oxidant species in
polluted atmospheres are nitrogen dioxide and non-methane volatile organic
compounds (VOCs), especially unsaturated VOCs. Methane is much less reactive
than the other VOCs but is present at much higher concentrations, having
risen in concentration over the past 100 years owing to its increasing use as fuel,
and is released from rice fields and farm animals. Photochemistry involving
methane accounts for much of the rise in ozone over the oceans and remote land
areas, from about 30 μg/m 3 to about 75 μg/m 3 .
The simplified general equations that regulate atmospheric photochemistry
may be summarized as follows.
Nitrogen dioxide dissociates to form nitric oxide (NO) and atomic oxygen:
NO2 + hυ (λ ≤ 430 nm) � NO + O (1)
Atomic oxygen combines with molecular oxygen to form ozone:
O + O2 � O3
Ozone is decomposed by reacting with nitric oxide, forming nitrogen dioxide
and molecular oxygen:
NO + O3 � NO2 + O2
Thus, photochemical pollution occurs when the photostationary cycle described
in equations 1–3 is altered, either by events that consume nitric oxide or, conversely,
favour the production of nitrogen dioxide. The reaction of nitric oxide
with atmospheric peroxides (RO2) is the main cause of disturbance of the photochemical
equilibrium, as presented in reaction 4:
NO + RO2 � NO2 + RO (4)
(2)
(3)
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