inversions and low wind speeds associated with
stable high-pressure systems can restrict dispersion and lead to high pollutant concentrations.
Dispersion is also influenced by the local topography, and is reduced by the presence of local
obstacles such as buildings. The effect is most
pronounced in the case of so-called street canyons. Moreover, vehicle-induced turbulence
needs to be taken into account.
Transformation
Some of the transformation processes that influence the concentrations of PM, NO 2 and ozone are
summarized below. Meteorological factors,
including solar radiation, precipitation, and
humidity, also play important roles in these
processes.
Particulate Matter The emission of primary
particles from combustion sources was described
earlier. Nucleation mode particles have a relatively short existence in the atmosphere since
they readily transform into larger particles and
deposit quite efficiently to surfaces because of
their highly diffusive nature. Accumulation
mode particles are too large to be subject to
rapid diffusion and too small to settle from the
air rapidly under gravity. Their further growth is
inhibited because they do not coagulate quickly
and there are diffusion barriers to their growth by
condensation. Particles in the accumulation
mode can therefore have a long atmospheric lifetime (typically 7–30 days), although incorporation into rain can shorten it significantly. Within
the coarse particle size range, gravitational settling velocities become appreciable and therefore
atmospheric lifetimes are much shorter than for
accumulation mode particles. Particles larger
than ~100 mm in diameter rapidly settle out of
the air and are of minor health significance
because, although they can be inhaled, they do
not generally penetrate beyond the nose and
mouth [99].
A substantial fraction of the fine PM mass,
especially at background locations, is secondary
in nature [99–101]. Secondary particles are
formed by atmospheric reactions involving both
inorganic and organic gaseous precursors,
several of which are emitted by transport. For
example, particles can be produced by the oxidation of precursor gases such as SO 2 and NO x to
acids followed by neutralization with ammonia
(NH 3 ), or by the partial oxidation of organic
compounds. The formation of secondary particles is relatively slow. Their persistence in the
atmosphere is therefore prolonged, and they are
distributed more evenly throughout the air than
primary particles. They can also travel large distances, resulting in the transport across national
boundaries.
Nitrogen Dioxide The ambient concentration of
NO 2 is dictated by various complex processes and
factors. Under the majority of atmospheric conditions the following reaction is the dominant pathway by which NO is converted to NO 2 :
NO þ O 3 ! NO 2 þ O 2
ð2Þ
At polluted locations comparatively close to
sources of NO x (such as road vehicles), NO is in
large excess and the reaction proceeds quickly
(seconds), with the availability of O 3 limiting the
quantity of NO 2 that can be produced. Primary
NO 2 emissions are therefore important at such
locations. At unpolluted locations, when O 3 is
present in excess, the timescale for conversion of
NO to NO 2 is longer (minutes). When neither NO
nor O 3 is in large excess, the reaction progressively depletes both reagents, and can become
very slow [20].
Under normal ambient daytime conditions, the
reverse process also occurs – the destruction of
NO 2 by photolysis to form NO and ozone, as
shown in reactions (3) and (4):
NO 2 þ sunlight ! NO þ O
ð3Þ
O þ O 2 þM
ð
Þ ! O 3 þM
ð
Þ
ð4Þ
where M is a third body, most commonly
nitrogen.
The mean daylight lifetime of NO 2 with
respect to photolysis is around 3 min. In winter,
it is typically a factor of 2 or 3 longer. Under
conditions when photolysis is sufficiently rapid,
Air Quality, Surface Transportation Impacts on
63
stable high-pressure systems can restrict dispersion and lead to high pollutant concentrations.
Dispersion is also influenced by the local topography, and is reduced by the presence of local
obstacles such as buildings. The effect is most
pronounced in the case of so-called street canyons. Moreover, vehicle-induced turbulence
needs to be taken into account.
Transformation
Some of the transformation processes that influence the concentrations of PM, NO 2 and ozone are
summarized below. Meteorological factors,
including solar radiation, precipitation, and
humidity, also play important roles in these
processes.
Particulate Matter The emission of primary
particles from combustion sources was described
earlier. Nucleation mode particles have a relatively short existence in the atmosphere since
they readily transform into larger particles and
deposit quite efficiently to surfaces because of
their highly diffusive nature. Accumulation
mode particles are too large to be subject to
rapid diffusion and too small to settle from the
air rapidly under gravity. Their further growth is
inhibited because they do not coagulate quickly
and there are diffusion barriers to their growth by
condensation. Particles in the accumulation
mode can therefore have a long atmospheric lifetime (typically 7–30 days), although incorporation into rain can shorten it significantly. Within
the coarse particle size range, gravitational settling velocities become appreciable and therefore
atmospheric lifetimes are much shorter than for
accumulation mode particles. Particles larger
than ~100 mm in diameter rapidly settle out of
the air and are of minor health significance
because, although they can be inhaled, they do
not generally penetrate beyond the nose and
mouth [99].
A substantial fraction of the fine PM mass,
especially at background locations, is secondary
in nature [99–101]. Secondary particles are
formed by atmospheric reactions involving both
inorganic and organic gaseous precursors,
several of which are emitted by transport. For
example, particles can be produced by the oxidation of precursor gases such as SO 2 and NO x to
acids followed by neutralization with ammonia
(NH 3 ), or by the partial oxidation of organic
compounds. The formation of secondary particles is relatively slow. Their persistence in the
atmosphere is therefore prolonged, and they are
distributed more evenly throughout the air than
primary particles. They can also travel large distances, resulting in the transport across national
boundaries.
Nitrogen Dioxide The ambient concentration of
NO 2 is dictated by various complex processes and
factors. Under the majority of atmospheric conditions the following reaction is the dominant pathway by which NO is converted to NO 2 :
NO þ O 3 ! NO 2 þ O 2
ð2Þ
At polluted locations comparatively close to
sources of NO x (such as road vehicles), NO is in
large excess and the reaction proceeds quickly
(seconds), with the availability of O 3 limiting the
quantity of NO 2 that can be produced. Primary
NO 2 emissions are therefore important at such
locations. At unpolluted locations, when O 3 is
present in excess, the timescale for conversion of
NO to NO 2 is longer (minutes). When neither NO
nor O 3 is in large excess, the reaction progressively depletes both reagents, and can become
very slow [20].
Under normal ambient daytime conditions, the
reverse process also occurs – the destruction of
NO 2 by photolysis to form NO and ozone, as
shown in reactions (3) and (4):
NO 2 þ sunlight ! NO þ O
ð3Þ
O þ O 2 þM
ð
Þ ! O 3 þM
ð
Þ
ð4Þ
where M is a third body, most commonly
nitrogen.
The mean daylight lifetime of NO 2 with
respect to photolysis is around 3 min. In winter,
it is typically a factor of 2 or 3 longer. Under
conditions when photolysis is sufficiently rapid,
Air Quality, Surface Transportation Impacts on
63