Angeles, for example, is situated in a valley with
stagnant conditions during temperature inversions. The stagnant conditions lead to very low
wind speeds, and this results in little air exchange
between the valley and the surrounding areas. The
hot and sunny climate and high emissions from
traffic, industry, and other sources make the valley
act like a large pollutant reaction chamber, and
this is ultimately leading to high concentrations of
photochemical products like ozone, nitrogen
dioxide, and peroxyacetyl nitrate (PAN) with the
resultant degradation of air quality.
Wind speed through a city plays an important
role. For example, nitrogen oxide (NO x ) levels in
the street Via Senato in Milan, Italy, compared
with the street Jagtvej in Copenhagen, Denmark,
show similar concentrations at the two sites
despite the much higher traffic load in the street
of Copenhagen [23]. This is directly the result of
generally higher wind speeds in Copenhagen
compared with Milan. High wind speeds and neutral conditions prevail in Copenhagen, whereas
low wind speeds and stable or near stable conditions are frequent in Milan. Copenhagen has a
cold coastal climate, whereas Milan has a warm
subtropical climate, and the local wind conditions
are furthermore affected by the location inside the
Po Valley. Air quality planners must take their
own geography into consideration.
Impact of Topography
Most cities have characteristic wind systems as a
result of local topography. An example is the
rising air over a warm mountain side during daytime often leading to local formation of clouds and
the subsequent release of precipitation.
During the night the system reverses, and the
cooling of the air in the mountain valley leads to
stable conditions which may lead to local air pollution problems. The impact of katabatic winds is
another example affecting cities along the Norwegian coast. Katabatic winds are formed when cold
air masses move downslope (katabatic is Greek
for moving downhill) and meeting the colder
snow and glacier covered areas, which then cool
the air mass further as it continues flowing
downhill.
Katabatic winds may lead, for example, to high
levels of local dust with resulting air quality
issues, though their occurrence is highly location
specific. The warm and dry foehn wind formed on
the backside of a mountain chain, for example, on
the north side of the European Alps, is another
example. As the wind flows over the mountain,
the air mass is cooled and releases moisture. The
air is subsequently warmed as the air mass flows
downhill. This system may then form an inversion
with the effect of reducing the dispersion of local
air pollutants.
Impact of Meteorology
Ambient temperature in the urban atmosphere of
larger cities is usually a couple of degrees Celsius
higher than that found in the surrounding rural
areas due to the so-called urban heat island effect
[24]. The phenomenon is due to the much smaller
reflection or albedo of the city as compared with
its surrounding area and therefore absorbs more
energy, to release it later in the form of heat.
There is in addition a high consumption of
energy inside the city, as a result of domestic
heating and intense road traffic, which further
contributes to the higher releases of heat than
rural surroundings. Buildings act as heat reservoirs, so the city has a less pronounced diurnal
temperature variation compared with the rural
area. Lastly, buildings and other urban constructions form a physical barrier for the wind flow.
This shielding leads to less cooling of the surfaces
inside the city.
In calm weather, an urban circulation cell,
so-called heat island circulation, may be formed
by warm air rising from the city. As it moves away
from the city, the heated air may sink and then, in
another circulation, be returned to the city at a low
altitude. A similar phenomenon is known in
coastal regions, where a sea breeze may be formed
as a result of the temperature difference between
the sea and land surfaces.
A study in London showed that due to the heat
island circulation over the city, the wind speed is
never below about 1 m s
À1 [25]. Therefore, the
heat island effect is very important during low
wind speed conditions in urban areas where it
may dominate the dispersion and thereby be the
202
Urban Air Quality: Sources and Concentrations
stagnant conditions during temperature inversions. The stagnant conditions lead to very low
wind speeds, and this results in little air exchange
between the valley and the surrounding areas. The
hot and sunny climate and high emissions from
traffic, industry, and other sources make the valley
act like a large pollutant reaction chamber, and
this is ultimately leading to high concentrations of
photochemical products like ozone, nitrogen
dioxide, and peroxyacetyl nitrate (PAN) with the
resultant degradation of air quality.
Wind speed through a city plays an important
role. For example, nitrogen oxide (NO x ) levels in
the street Via Senato in Milan, Italy, compared
with the street Jagtvej in Copenhagen, Denmark,
show similar concentrations at the two sites
despite the much higher traffic load in the street
of Copenhagen [23]. This is directly the result of
generally higher wind speeds in Copenhagen
compared with Milan. High wind speeds and neutral conditions prevail in Copenhagen, whereas
low wind speeds and stable or near stable conditions are frequent in Milan. Copenhagen has a
cold coastal climate, whereas Milan has a warm
subtropical climate, and the local wind conditions
are furthermore affected by the location inside the
Po Valley. Air quality planners must take their
own geography into consideration.
Impact of Topography
Most cities have characteristic wind systems as a
result of local topography. An example is the
rising air over a warm mountain side during daytime often leading to local formation of clouds and
the subsequent release of precipitation.
During the night the system reverses, and the
cooling of the air in the mountain valley leads to
stable conditions which may lead to local air pollution problems. The impact of katabatic winds is
another example affecting cities along the Norwegian coast. Katabatic winds are formed when cold
air masses move downslope (katabatic is Greek
for moving downhill) and meeting the colder
snow and glacier covered areas, which then cool
the air mass further as it continues flowing
downhill.
Katabatic winds may lead, for example, to high
levels of local dust with resulting air quality
issues, though their occurrence is highly location
specific. The warm and dry foehn wind formed on
the backside of a mountain chain, for example, on
the north side of the European Alps, is another
example. As the wind flows over the mountain,
the air mass is cooled and releases moisture. The
air is subsequently warmed as the air mass flows
downhill. This system may then form an inversion
with the effect of reducing the dispersion of local
air pollutants.
Impact of Meteorology
Ambient temperature in the urban atmosphere of
larger cities is usually a couple of degrees Celsius
higher than that found in the surrounding rural
areas due to the so-called urban heat island effect
[24]. The phenomenon is due to the much smaller
reflection or albedo of the city as compared with
its surrounding area and therefore absorbs more
energy, to release it later in the form of heat.
There is in addition a high consumption of
energy inside the city, as a result of domestic
heating and intense road traffic, which further
contributes to the higher releases of heat than
rural surroundings. Buildings act as heat reservoirs, so the city has a less pronounced diurnal
temperature variation compared with the rural
area. Lastly, buildings and other urban constructions form a physical barrier for the wind flow.
This shielding leads to less cooling of the surfaces
inside the city.
In calm weather, an urban circulation cell,
so-called heat island circulation, may be formed
by warm air rising from the city. As it moves away
from the city, the heated air may sink and then, in
another circulation, be returned to the city at a low
altitude. A similar phenomenon is known in
coastal regions, where a sea breeze may be formed
as a result of the temperature difference between
the sea and land surfaces.
A study in London showed that due to the heat
island circulation over the city, the wind speed is
never below about 1 m s
À1 [25]. Therefore, the
heat island effect is very important during low
wind speed conditions in urban areas where it
may dominate the dispersion and thereby be the
202
Urban Air Quality: Sources and Concentrations