the highest air pollution exposure [4, 5] and associated negative health impact therefore take place.
With projections for the next 50 years indicating that the worldwide urban population will
increase by two thirds [2], urban air quality will
continue to deteriorate as air pollution continues
to increase in major cities, especially those in
developing countries (such as in Brazil, Russia,
India, Indonesia, and China).
Urban Air Pollution Concentrations in General
Urban air pollution arises from the competition
between emission processes which increase pollutant concentrations and dispersion, advection,
and deposition processes that reduce and remove
them. Differences in pollution levels reflect differences in emission densities and emission patterns. They also reflect processes of pollutant
dispersion and removal. Pollution is dispersed by
the flow of wind. It is removed generally via
chemical conversion to other compounds and/or
by being deposited either through wet or dry
deposition processes.
The dispersion and removal processes are
governed by the local meteorological conditions,
which also vary heavily with the physical location
of the city. Therefore, air pollution concentrations
in an urban environment are naturally the result of
local emissions as well as contributions from pollution transport from more remote sources
(Fig. 1).
Also of importance to the local contribution to
urban air pollution are demographic/geographic
factors such as the size of the city domain and
the density of pollutant emissions inside the
domain [5].
The temporal pattern in urban air pollution
levels varies as a function of the local releases.
However, it is the variations in the meteorological
parameters that govern the dispersion and the
pollutant transport into and out of the city from
surrounding rural areas.
The temporal variations in emissions and meteorology are not the only contributors; the emission
release height also plays an important role with
respect to how quickly the air pollution is dispersed and deposited, with air quality generally
improving with areas with very tall and
technologically up-to-date smoke stacks. This is
because air pollution emitted from a high release
height will in many cases be transported out of the
urban area before being dispersed down to ground
level, although this is of course depending on the
size of the urban domain. This is the principle that
the “solution to pollution is dilution” when often
the pollution and subsequent air quality issues are
just moved from one jurisdiction to another.
Urban industries, power plants, and other
sources for which the releases come from tall chimneys therefore only rarely contribute to the local
ground level air pollutant concentrations inside the
urban area, and instead they are usually contributing primarily to the regional air pollution.
Pollutant emissions related to vehicular transport, local domestic heating, and smaller industries have low release heights [less than 10 m
above ground level (a.g.l.)]. Consequently, the
releases are not diluted as efficiently before
reaching ground level as in the case for emissions
from tall (more than 20 m a.g.l.) release heights.
The contribution from “low” sources, both
point and mobile, therefore often dominates the
pollutant concentrations at ground levels inside
the urban area.
A steady growth in vehicular transport and
centralization of domestic heating have made
road traffic the most important source of urban
air pollution in many countries including most of
the industrialized nations [7].
There are usually significant differences between
developed and developing countries when comparing emissions from different industrial sectors. With
respect to the local contribution from the various
sectors, a comparison of two so-called megacities
(Beijing and Paris) showed that aerosol particles and
volatile organic compounds (VOCs) have a complex and multi-combustion source in Beijing,
whereas a single traffic pollution source completely
dominates the urban atmospheric environment in
Paris [8].
Both municipalities recognize the above and
are taking steps to improve air quality.
Although ambient air quality is that which is
often the subject of research and the type of pollution which is heard about (and the subject of this
entry), indoor air quality cannot be forgotten.
198
Urban Air Quality: Sources and Concentrations
With projections for the next 50 years indicating that the worldwide urban population will
increase by two thirds [2], urban air quality will
continue to deteriorate as air pollution continues
to increase in major cities, especially those in
developing countries (such as in Brazil, Russia,
India, Indonesia, and China).
Urban Air Pollution Concentrations in General
Urban air pollution arises from the competition
between emission processes which increase pollutant concentrations and dispersion, advection,
and deposition processes that reduce and remove
them. Differences in pollution levels reflect differences in emission densities and emission patterns. They also reflect processes of pollutant
dispersion and removal. Pollution is dispersed by
the flow of wind. It is removed generally via
chemical conversion to other compounds and/or
by being deposited either through wet or dry
deposition processes.
The dispersion and removal processes are
governed by the local meteorological conditions,
which also vary heavily with the physical location
of the city. Therefore, air pollution concentrations
in an urban environment are naturally the result of
local emissions as well as contributions from pollution transport from more remote sources
(Fig. 1).
Also of importance to the local contribution to
urban air pollution are demographic/geographic
factors such as the size of the city domain and
the density of pollutant emissions inside the
domain [5].
The temporal pattern in urban air pollution
levels varies as a function of the local releases.
However, it is the variations in the meteorological
parameters that govern the dispersion and the
pollutant transport into and out of the city from
surrounding rural areas.
The temporal variations in emissions and meteorology are not the only contributors; the emission
release height also plays an important role with
respect to how quickly the air pollution is dispersed and deposited, with air quality generally
improving with areas with very tall and
technologically up-to-date smoke stacks. This is
because air pollution emitted from a high release
height will in many cases be transported out of the
urban area before being dispersed down to ground
level, although this is of course depending on the
size of the urban domain. This is the principle that
the “solution to pollution is dilution” when often
the pollution and subsequent air quality issues are
just moved from one jurisdiction to another.
Urban industries, power plants, and other
sources for which the releases come from tall chimneys therefore only rarely contribute to the local
ground level air pollutant concentrations inside the
urban area, and instead they are usually contributing primarily to the regional air pollution.
Pollutant emissions related to vehicular transport, local domestic heating, and smaller industries have low release heights [less than 10 m
above ground level (a.g.l.)]. Consequently, the
releases are not diluted as efficiently before
reaching ground level as in the case for emissions
from tall (more than 20 m a.g.l.) release heights.
The contribution from “low” sources, both
point and mobile, therefore often dominates the
pollutant concentrations at ground levels inside
the urban area.
A steady growth in vehicular transport and
centralization of domestic heating have made
road traffic the most important source of urban
air pollution in many countries including most of
the industrialized nations [7].
There are usually significant differences between
developed and developing countries when comparing emissions from different industrial sectors. With
respect to the local contribution from the various
sectors, a comparison of two so-called megacities
(Beijing and Paris) showed that aerosol particles and
volatile organic compounds (VOCs) have a complex and multi-combustion source in Beijing,
whereas a single traffic pollution source completely
dominates the urban atmospheric environment in
Paris [8].
Both municipalities recognize the above and
are taking steps to improve air quality.
Although ambient air quality is that which is
often the subject of research and the type of pollution which is heard about (and the subject of this
entry), indoor air quality cannot be forgotten.
198
Urban Air Quality: Sources and Concentrations