Within the vortex relatively clean air flows from
rooftop height and down the windward face of the
street, across the road at street level, in the reverse of
the wind direction at roof top, thus transporting the
pollutants from the road level traffic to the leeward
face of the canyon. The result is that pollution
concentrations are up to ten times higher on the
leeward side compared with the windward side of
the street. This means that air quality depending on
prevailing wind conditions may often be much
poorer on one side of the street than another.
Nitrogen Dioxide Pollution in Urban
Areas
For example, the chemistry of nitrogen oxides
[NO x : the sum of nitrogen monoxide (NO) and
nitrogen dioxide (NO 2 )] in urban streets may be
described by only two reactions: the reaction
between ozone (O 3 ) and NO forming NO2 and
the photodissociation of NO 2 [27]:
NO þ O 3 ! NO 2
(1)
NO 2 þ hv ! NO þ O
3 P
À Á
(2)
O
3 P
À Á þ O 2 ! O 3
(3)
where O(
3 P) is ground state atomic oxygen. Reaction (3) is very fast, and therefore for most practical applications, it may be disregarded. The
products of reaction (2) may thus be considered
to be NO and O 3 .
NO x is therefore mainly emitted as NO and, to
a lesser extent, NO 2 .
Long-term exposure to elevated NO 2 levels is a
health hazard and may decrease lung function and
increase the risk of respiratory symptoms such as
acute bronchitis, cough, and phlegm, particularly
in children [28], whereas NO at current ambient
air concentrations is (considered to be) harmless.
The fraction of NO x directly emitted as NO 2
used to be only about 5 to 10% in countries with a
small fraction of diesel engines. However, due to
the use of catalytic converters and an increasing
number of diesel engines with high fraction of
NO 2 in the exhaust, this value may in some
regions be now up to as much as 40% [29]. This
extremely simple chemical mechanism of two
reactions and a direct emission describes very
well the concentrations of NO 2 inside urban
streets [27]; for many cities it may also be applied
for describing NO 2 concentrations in urban background air (see [6] documenting this the case for
many Northern European cities).
The use of catalytic converters in automobiles
has in recent years led to a major reduction in NO x
concentrations in the urban streets of many industrialized countries.
For several reasons NO 2 levels have not
followed the same trends (Fig. 4). Part of the
explanation is the chemical conversion of NO to
NO 2 in the reaction with O 3 , but the other explanation is, as mentioned above, the increased fraction of NO 2 in the NO x emission from vehicles
with catalytic converters.
Despite the overall reduction in NO x emissions, this is contributing to elevated NO 2
concentrations.
The Impact of Particle Pollution
Ambient urban air contains a complex mixture of
particles from natural and anthropogenic sources
of varying sizes and chemical composition
[30–34]. The particle size is crucial for the atmospheric fate [35] and the human health impact as
both the particles’ atmospheric behavior and their
deposition in the human respiratory system is
governed in part by its size [36]. Particles in
ambient air generally appear in two rather distinct
size classes (referred to as “modes”) usually
termed fine particles (diameter: 0.l–2.5 mm) and
coarse particles (diameter: >2.5 mm). In addition,
it is common to talk about the ultrafine particles
(diameter: 0.01–0.1 mm or 10–100 nm) that usually dominate the number concentrations (Fig. 5).
Particle Mass Concentrations
Mass concentration of particles <10 mm
and <2.5 mm in aerodynamic diameter (PM 10
and PM 2.5 ) is generally used as an indicator for
suspended particulate matter (e.g., regulated in
204
Urban Air Quality: Sources and Concentrations
rooftop height and down the windward face of the
street, across the road at street level, in the reverse of
the wind direction at roof top, thus transporting the
pollutants from the road level traffic to the leeward
face of the canyon. The result is that pollution
concentrations are up to ten times higher on the
leeward side compared with the windward side of
the street. This means that air quality depending on
prevailing wind conditions may often be much
poorer on one side of the street than another.
Nitrogen Dioxide Pollution in Urban
Areas
For example, the chemistry of nitrogen oxides
[NO x : the sum of nitrogen monoxide (NO) and
nitrogen dioxide (NO 2 )] in urban streets may be
described by only two reactions: the reaction
between ozone (O 3 ) and NO forming NO2 and
the photodissociation of NO 2 [27]:
NO þ O 3 ! NO 2
(1)
NO 2 þ hv ! NO þ O
3 P
À Á
(2)
O
3 P
À Á þ O 2 ! O 3
(3)
where O(
3 P) is ground state atomic oxygen. Reaction (3) is very fast, and therefore for most practical applications, it may be disregarded. The
products of reaction (2) may thus be considered
to be NO and O 3 .
NO x is therefore mainly emitted as NO and, to
a lesser extent, NO 2 .
Long-term exposure to elevated NO 2 levels is a
health hazard and may decrease lung function and
increase the risk of respiratory symptoms such as
acute bronchitis, cough, and phlegm, particularly
in children [28], whereas NO at current ambient
air concentrations is (considered to be) harmless.
The fraction of NO x directly emitted as NO 2
used to be only about 5 to 10% in countries with a
small fraction of diesel engines. However, due to
the use of catalytic converters and an increasing
number of diesel engines with high fraction of
NO 2 in the exhaust, this value may in some
regions be now up to as much as 40% [29]. This
extremely simple chemical mechanism of two
reactions and a direct emission describes very
well the concentrations of NO 2 inside urban
streets [27]; for many cities it may also be applied
for describing NO 2 concentrations in urban background air (see [6] documenting this the case for
many Northern European cities).
The use of catalytic converters in automobiles
has in recent years led to a major reduction in NO x
concentrations in the urban streets of many industrialized countries.
For several reasons NO 2 levels have not
followed the same trends (Fig. 4). Part of the
explanation is the chemical conversion of NO to
NO 2 in the reaction with O 3 , but the other explanation is, as mentioned above, the increased fraction of NO 2 in the NO x emission from vehicles
with catalytic converters.
Despite the overall reduction in NO x emissions, this is contributing to elevated NO 2
concentrations.
The Impact of Particle Pollution
Ambient urban air contains a complex mixture of
particles from natural and anthropogenic sources
of varying sizes and chemical composition
[30–34]. The particle size is crucial for the atmospheric fate [35] and the human health impact as
both the particles’ atmospheric behavior and their
deposition in the human respiratory system is
governed in part by its size [36]. Particles in
ambient air generally appear in two rather distinct
size classes (referred to as “modes”) usually
termed fine particles (diameter: 0.l–2.5 mm) and
coarse particles (diameter: >2.5 mm). In addition,
it is common to talk about the ultrafine particles
(diameter: 0.01–0.1 mm or 10–100 nm) that usually dominate the number concentrations (Fig. 5).
Particle Mass Concentrations
Mass concentration of particles <10 mm
and <2.5 mm in aerodynamic diameter (PM 10
and PM 2.5 ) is generally used as an indicator for
suspended particulate matter (e.g., regulated in
204
Urban Air Quality: Sources and Concentrations