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

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AIR QUALITY GUIDELINES
occurs on a larger regional scale in areas such as western Europe, where the emissions
from many cities combine together to form a highly polluted atmosphere
in which ozone forms and is transported over long distances. The presence of
peroxy radicals and their involvement in reaction 4 will also allow the creation of
high concentrations of nitrogen dioxide, which far exceed those predicted by the
photostationary state (reactions 1–3). One consequence of reaction 1 in polluted
cities is that ozone concentrations within the city itself are often lower than those
in the surrounding countryside, because fresh nitric oxide emissions from traffic
suppress high concentrations of ozone entering in air from the surrounding
countryside.
Secondary particulate matter
In some parts of the world, secondary particles can represent up to 50% of the
total concentration of particles in the air. They comprise three main components.
The first is sulfate, which arises from the atmospheric oxidation of sulfur dioxide
and leads initially to the formation of sulfur trioxide, which rapidly condenses
with water to form sulfuric acid. In regions with low ammonia emissions, sulfuric
acid comprises the major form of sulfate. In many places, however, there are
ample emissions of ammonia, which neutralizes the sulfuric acid to form solid
particles of ammonium sulfate. Nitrogen dioxide is also oxidized in the atmosphere
(typically faster than sulfur dioxide) to form nitric acid, which is present
in the air as a vapour. Nitric acid, however, tends to react either with ammonia
or with materials such as calcium carbonate or sodium chloride, leading to the
formation of solid particles of nitrate. When these are in the form of ammonium
nitrate, the formation process is appreciably reversible:
HNO3 (nitric acid) + NH3 (ammonia) ↔ NH4NO3 (ammonium nitrate) (5)
Ammonium nitrate can dissociate back to nitric acid and ammonia, a process
favoured by high temperature and low relative humidity. There can therefore be
important diurnal and seasonal fluctuations in the amounts of ammonium nitrate
in the air.
The third major form of secondary PM is secondary organic aerosol (SOA).
This comprises oxidized organic compounds formed in the atmosphere by reactions
of VOC. Biogenic VOC such as α-pinene emitted by trees are highly reactive
in this context, and in some areas provide a very significant source of SOA.
Anthropogenic VOC emissions are also capable of atmospheric oxidation, forming
species of lower volatility that condense to form SOA.
Typically, the formation of secondary aerosol is relatively slow, taking a day or
more. Consequently, the airborne concentrations of species such as sulfate tend
to be rather uniform over quite large distances. In the case of nitrates and SOA,
the formation processes are more rapid, and in the case of ammonium nitrate
may be reversible, and therefore higher spatial gradients are to be expected.