NO, NO 2 , and O 3 are potentially in chemical
equilibrium, a condition usually referred to as
the “photostationary state.”
Other chemical processes that can convert NO
to NO 2 involve the formation of free radicals,
which can catalyze the oxidation of emitted
hydrocarbons [20], but these reactions tend to be
comparatively slow [102].
A third-order reaction, involving the oxidation
of NO by oxygen, is of minor importance under
normal atmospheric conditions:
2NO þ O 2 ! 2NO 2
ð5Þ
However, this reaction is rapid at low temperatures and high NO concentrations, such that it is
much more rapid at the elevated levels typical of
those close to points of emission. Under wintertime pollution episode conditions, when a shallow
inversion layer can lead to a combination of high
NO x levels and stagnant air for periods of a day or
more, reaction (5) can potentially make a substantial contribution to NO2 concentrations, as during
an episode in London in December 1991 [20].
Given that model predictions show a decrease
in NO x emissions, the absence of a downward
trend in NO 2 concentrations (see earlier section
on “Historical Trends”) is not completely understood. Three contributing factors have often been
cited:
1. The likelihood that the average proportion of
NO 2 in NO x (termed f-NO 2 ) in diesel exhaust is
increasing. Analyses of NO 2 concentrations in
urban areas have indicated that a significant
proportion of NO 2 must be emitted directly
from the exhaust, and that the f-NO 2 value
has increased in recent years [104–108]. This
has two causes:
• The increasing market share for diesel vehicles in many European countries. For example, the share of first registrations of diesel
passenger cars in Finland increased from
17% in 2005 to 52% in 2008 [109]. Petrol
cars emit less NO x than diesel vehicles, and
with a smaller proportion of NO 2 .
• The increase in the use of specific aftertreatment technologies in modern diesel
vehicles which involve in situ generation
of NO 2 , such as catalytically regenerative
DPFs [106].
The changes in f-NO 2 for traffic emissions have not had a large impact on NO 2
concentrations away from the roadside,
where the dominant contribution is from
secondary NO 2 .
2. The possibility that real-world NO x emissions
from road vehicles are not decreasing as rapidly as models are predicting [110].
3. Increasing background ozone concentrations
[111]. As the O 3 concentration increases, the
amount of NO converted to NO 2 increases.
Ozone Ground-level O 3 is implicated in summer
smog, such as the one that affected large areas of
Europe in the summer of 2003, and high concentrations lead to increases in the frequency of respiratory symptoms and in deaths [112]. It is not
produced directly from emission sources but is
created by photochemical reactions such as those
described above. It can be transported over long
distances, and is therefore regarded as a regional air
pollution problem. High concentrations are typically observed downwind of large cities in the
summer when photochemical formation is
enhanced. Because road transport is a major source
of ozone precursors (e.g., NO x and hydrocarbons),
it is an important contributor to ground-level concentrations. Globally, road transport is responsible
for 5–15% of tropospheric O 3 [113, 114].
Modelling Air Pollution from Transport
Predictions from air quality models are used in a
diverse range of applications. A common one is
air quality management (AQM), which includes
the identification of high-pollution areas (“hot
spots”), the development of plans and programs
for improving air quality, the checking of progress
toward targets, and the provision of information to
the public. Applications that are closely linked to
AQM are transport planning and land-use planning. Air quality predictions are also an important
element of environmental impact assessment
(EIA).
64
Air Quality, Surface Transportation Impacts on
equilibrium, a condition usually referred to as
the “photostationary state.”
Other chemical processes that can convert NO
to NO 2 involve the formation of free radicals,
which can catalyze the oxidation of emitted
hydrocarbons [20], but these reactions tend to be
comparatively slow [102].
A third-order reaction, involving the oxidation
of NO by oxygen, is of minor importance under
normal atmospheric conditions:
2NO þ O 2 ! 2NO 2
ð5Þ
However, this reaction is rapid at low temperatures and high NO concentrations, such that it is
much more rapid at the elevated levels typical of
those close to points of emission. Under wintertime pollution episode conditions, when a shallow
inversion layer can lead to a combination of high
NO x levels and stagnant air for periods of a day or
more, reaction (5) can potentially make a substantial contribution to NO2 concentrations, as during
an episode in London in December 1991 [20].
Given that model predictions show a decrease
in NO x emissions, the absence of a downward
trend in NO 2 concentrations (see earlier section
on “Historical Trends”) is not completely understood. Three contributing factors have often been
cited:
1. The likelihood that the average proportion of
NO 2 in NO x (termed f-NO 2 ) in diesel exhaust is
increasing. Analyses of NO 2 concentrations in
urban areas have indicated that a significant
proportion of NO 2 must be emitted directly
from the exhaust, and that the f-NO 2 value
has increased in recent years [104–108]. This
has two causes:
• The increasing market share for diesel vehicles in many European countries. For example, the share of first registrations of diesel
passenger cars in Finland increased from
17% in 2005 to 52% in 2008 [109]. Petrol
cars emit less NO x than diesel vehicles, and
with a smaller proportion of NO 2 .
• The increase in the use of specific aftertreatment technologies in modern diesel
vehicles which involve in situ generation
of NO 2 , such as catalytically regenerative
DPFs [106].
The changes in f-NO 2 for traffic emissions have not had a large impact on NO 2
concentrations away from the roadside,
where the dominant contribution is from
secondary NO 2 .
2. The possibility that real-world NO x emissions
from road vehicles are not decreasing as rapidly as models are predicting [110].
3. Increasing background ozone concentrations
[111]. As the O 3 concentration increases, the
amount of NO converted to NO 2 increases.
Ozone Ground-level O 3 is implicated in summer
smog, such as the one that affected large areas of
Europe in the summer of 2003, and high concentrations lead to increases in the frequency of respiratory symptoms and in deaths [112]. It is not
produced directly from emission sources but is
created by photochemical reactions such as those
described above. It can be transported over long
distances, and is therefore regarded as a regional air
pollution problem. High concentrations are typically observed downwind of large cities in the
summer when photochemical formation is
enhanced. Because road transport is a major source
of ozone precursors (e.g., NO x and hydrocarbons),
it is an important contributor to ground-level concentrations. Globally, road transport is responsible
for 5–15% of tropospheric O 3 [113, 114].
Modelling Air Pollution from Transport
Predictions from air quality models are used in a
diverse range of applications. A common one is
air quality management (AQM), which includes
the identification of high-pollution areas (“hot
spots”), the development of plans and programs
for improving air quality, the checking of progress
toward targets, and the provision of information to
the public. Applications that are closely linked to
AQM are transport planning and land-use planning. Air quality predictions are also an important
element of environmental impact assessment
(EIA).
64
Air Quality, Surface Transportation Impacts on