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

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AIR QUALITY GUIDELINES
Atmospheric peroxides are formed by the oxidation of VOCs as represented in
the equations below, which describe oxidation of an alkene:
RCHCHR + O � RCH2 + RCO (5)
(generation of free radicals)
RCH2 + O2 � RCH2O2 (6)
(generation of peroxides)
RCH2O+ O2� RCHO + HO2
(generation of aldehydes)
RCH2O2 + NO2 � RCH2O2NO2 (8)
(generation of organic nitrates)
Reaction 8 may be seen as a way of stabilizing nitrogen dioxide and transporting
it over long distances (mainly in the form of peroxyacetyl nitrate), since this reaction
can be reversed far away from the original source of nitrogen dioxide.
There are several classes of VOCs (hydrocarbons and related compounds) in
the atmosphere, mainly in emissions from large urban centres and industrial areas.
These increase the complexity of photochemical reactions, mainly in areas
characterized by high solar radiation. Thus reactions 1–8 are depicted only to
describe some of the pathways responsible for the production of compounds in
the atmosphere that lead to the production of ozone, the most representative and
toxic pollutant of the class of ambient oxidants. For those interested in more information
about atmospheric photochemistry, there are introductory textbooks
well suited to health professionals (1,2).
The ambient concentration of ozone depends on several factors: sunshine intensity,
atmospheric convection, the height of the thermal inversion layer, concentrations
of nitrogen oxides and VOCs, and the ratio of VOCs to nitrogen
oxides. The VOC : nitrogen oxides ratio most favourable to ozone formation lies
in the range 4 : 1 to 10 : 1.
Occurrence in the air
Ozone exhibits a considerable spatial variation since, once formed, it travels with
the prevailing wind (3), tending to reach higher concentrations in suburbs and
remote downwind locations or at higher altitudes (4). This suggests that it is
likely that the highest concentrations of ozone occur in areas that lack adequate
instrumental analysis of this pollutant. This situation is particularly important
in the vicinity of large urban conglomerates in developing countries, where people
living on the outskirts of megacities are exposed to oxidants that, owing to
the absence of proper monitoring, are at levels that exceed air quality guidelines.
(7)