and has been estimated to be approximately 2% of
the gross domestic product (GDP) in developed
countries, whereas it is even 5% of the GDP in
developing countries (www.unep.org/urban_envi
ronment/issues/-urban_air.asp). There may also
be significant associated losses in productivity [5].
Origins of Air Quality: Sources and Formation
of Urban Air Pollution
Urban ambient air pollution concentrations arise
from the continuous competition between the emission processes that increase pollutant concentrations
and the dispersion, advection, and deposition processes that reduce and remove them. Pollutant differences between different urban areas are naturally
reflecting differences in emission densities and pattern but also in the dispersion and removal processes. The local meteorological conditions are
governing the impact on pollution levels of these
dispersion and removal processes, and these meteorological conditions vary heavily with the physical
location of the city. Air pollution concentrations in
an urban environment are naturally the result of the
contribution from local emissions, but also contributions arising from pollution from more remote
sources may play a very important role (Fig. 1).
The size of the city domain together with the density
of pollutant emissions governs the local contribution
to urban air pollution [6]. Naturally, the temporal
pattern in urban air pollution levels is a function of
the temporal pattern in the local releases. However,
the extent of the urban area and the spatial distribution in the local emission density also play central
roles for the local contribution to air pollution levels
in the urban environments. The local contribution to
pollution levels also varies heavily from one pollutant to another. In addition, the meteorological conditions greatly affect the actual pollution levels in a
given situation as they govern the dispersion conditions and the pollutant transport in and out of the city
area. In this context, the presence of the building
obstacles plays a crucial role in causing generally
high pollutant levels in the urban environment. This
is especially true inside the “street canyons” formed
with buildings on both sides of the street and where
the canyon vortex flow governs the pollution
distribution.
Air Quality and Health Effects
Air quality and health effects have been shown to
be linked. Particulate matter (PM) is generally
believed to be the most hazardous of ambient
pollutants. Reducing ambient air concentrations
of PM10 from 70 to 20 mg m
À3 would lower the
number of air quality-related deaths by approximately 15% [1].
Greater than half of the world’s population
reside in cities, and the number is expected to
grow [2]. Cities and especially their urban centers
are where air quality is generally the poorest, and
City increment
Street increment
Distance
Concentration
Regional
Urban Air Quality: Sources and Concentrations,
Fig. 1 A schematic illustration of the air pollutant contribution from the regional transport, the city area, and the
street. The relative magnitude of the various contributions
depends on the considered pollutant and the actual dispersion conditions (governed by the meteorology)
Urban Air Quality: Sources and Concentrations
197
the gross domestic product (GDP) in developed
countries, whereas it is even 5% of the GDP in
developing countries (www.unep.org/urban_envi
ronment/issues/-urban_air.asp). There may also
be significant associated losses in productivity [5].
Origins of Air Quality: Sources and Formation
of Urban Air Pollution
Urban ambient air pollution concentrations arise
from the continuous competition between the emission processes that increase pollutant concentrations
and the dispersion, advection, and deposition processes that reduce and remove them. Pollutant differences between different urban areas are naturally
reflecting differences in emission densities and pattern but also in the dispersion and removal processes. The local meteorological conditions are
governing the impact on pollution levels of these
dispersion and removal processes, and these meteorological conditions vary heavily with the physical
location of the city. Air pollution concentrations in
an urban environment are naturally the result of the
contribution from local emissions, but also contributions arising from pollution from more remote
sources may play a very important role (Fig. 1).
The size of the city domain together with the density
of pollutant emissions governs the local contribution
to urban air pollution [6]. Naturally, the temporal
pattern in urban air pollution levels is a function of
the temporal pattern in the local releases. However,
the extent of the urban area and the spatial distribution in the local emission density also play central
roles for the local contribution to air pollution levels
in the urban environments. The local contribution to
pollution levels also varies heavily from one pollutant to another. In addition, the meteorological conditions greatly affect the actual pollution levels in a
given situation as they govern the dispersion conditions and the pollutant transport in and out of the city
area. In this context, the presence of the building
obstacles plays a crucial role in causing generally
high pollutant levels in the urban environment. This
is especially true inside the “street canyons” formed
with buildings on both sides of the street and where
the canyon vortex flow governs the pollution
distribution.
Air Quality and Health Effects
Air quality and health effects have been shown to
be linked. Particulate matter (PM) is generally
believed to be the most hazardous of ambient
pollutants. Reducing ambient air concentrations
of PM10 from 70 to 20 mg m
À3 would lower the
number of air quality-related deaths by approximately 15% [1].
Greater than half of the world’s population
reside in cities, and the number is expected to
grow [2]. Cities and especially their urban centers
are where air quality is generally the poorest, and
City increment
Street increment
Distance
Concentration
Regional
Urban Air Quality: Sources and Concentrations,
Fig. 1 A schematic illustration of the air pollutant contribution from the regional transport, the city area, and the
street. The relative magnitude of the various contributions
depends on the considered pollutant and the actual dispersion conditions (governed by the meteorology)
Urban Air Quality: Sources and Concentrations
197