increased pollutant concentrations – can take
place further downwind [155].
Inside a road tunnel the dilution and dispersion
of pollution is restricted, and unless there is the
injection of fresh air, the concentration of pollutants will increase from the entrance to the exit, and
the emissions will be concentrated at the portals or
ventilation points. Therefore, the road traffic
source of pollution is effectively removed along
the length of the tunnel and the exterior air quality
ought to improve. However, pollutant concentrations can be particularly high in the vicinity of the
exit portals. The air quality at the portals or ventilation points will depend upon the level of traffic
and whether or not the polluted air is treated in
some way before release. When considering the
potential for tunnels for improving ambient concentrations, it should be borne in mind that vehicle
occupants and tunnel workers may be exposed to
increased concentrations, which could be detrimental in public health terms.
Vegetation The planting of vegetation can have a
positive impact on air quality in the vicinity of
roads [156–158]. A number of mechanisms are
involved, but the main effects are dry deposition at
the leaf surface and the modification of dispersion.
The height, width, type, and porosity (leaf density) of the vegetation appear to be important, and
greater deposition can occur on coniferous plants
than on broadleaf plants due to the larger leaf
surface area per unit ground area [159]. However,
the results of trials have been mixed, and there are
likely to be several practical difficulties associated
with the use of vegetative barriers in built-up
areas, including available space, soiling by traffic,
and the general hostility of the roadside environment to plant life. In addition, analyses carried out
for a busy arterial road show that very large vegetation areas (in excess of 10,000 m
2 ) would be
needed to compensate for the local emissions
from vehicles [160].
Air Treatment Various air treatment approaches
have been devised with the aim of reducing air
pollution. In recent years considerable efforts
have been made to develop construction materials
and coatings, which have the potential for
reducing air pollution. These materials are often
reliant upon photocatalysis, most commonly
involving the compound titanium dioxide
(TiO 2 ). Titanium dioxide acts as a catalyst to
oxidize NO and NO 2 into nitrate (NO 3
À ) in the
presence of oxygen and ultraviolet radiation. The
nitrate is subsequently washed away by rainfall.
The effectiveness of TiO 2 coatings has been successfully demonstrated in laboratory trials [161],
but field trials have been less convincing. This is
partly because it is difficult to distinguish the
impact of chemical conversion on the catalyst
surface from fluctuations in concentrations due
to, for example, meteorology.
Several air treatment technologies have been
developed for use in tunnels. For example, a number of denitrification systems are available; most
work by either a chemical absorption or catalytic
process. Tests in Norwegian and Japanese tunnels
have indicated that removal rates for NO 2 of up to
90% are possible [162, 163]. The application of
denitrification systems at open roadside locations
does not appear to have been reported, and
appears to be impractical at present. Electrostatic
precipitators (ESPs) have been demonstrated as
being effective in removing particles from tunnel
air with varying efficiency. Some electrostatic
systems have also been developed for exterior
environments and tunnels [164], but further investigation into their effectiveness is required.
Future Directions
Outlook
The future global outlook for transport-related
pollution currently looks rather bleak. In developing countries, the rise in car ownership and travel
is predicted to be even more dramatic than it has
been in industrialized nations. For example, the
rate of car ownership in Brazil is predicted to
overtake that in the country that has historically
been the world leader – the United States – before
the middle of the century. Car ownership in China
has doubled in the last 5 years, and it already has
the third highest car sales in the world [126].
However, at the time of writing there appear to
be some signs of change, but probably not as a
Air Quality, Surface Transportation Impacts on
75
place further downwind [155].
Inside a road tunnel the dilution and dispersion
of pollution is restricted, and unless there is the
injection of fresh air, the concentration of pollutants will increase from the entrance to the exit, and
the emissions will be concentrated at the portals or
ventilation points. Therefore, the road traffic
source of pollution is effectively removed along
the length of the tunnel and the exterior air quality
ought to improve. However, pollutant concentrations can be particularly high in the vicinity of the
exit portals. The air quality at the portals or ventilation points will depend upon the level of traffic
and whether or not the polluted air is treated in
some way before release. When considering the
potential for tunnels for improving ambient concentrations, it should be borne in mind that vehicle
occupants and tunnel workers may be exposed to
increased concentrations, which could be detrimental in public health terms.
Vegetation The planting of vegetation can have a
positive impact on air quality in the vicinity of
roads [156–158]. A number of mechanisms are
involved, but the main effects are dry deposition at
the leaf surface and the modification of dispersion.
The height, width, type, and porosity (leaf density) of the vegetation appear to be important, and
greater deposition can occur on coniferous plants
than on broadleaf plants due to the larger leaf
surface area per unit ground area [159]. However,
the results of trials have been mixed, and there are
likely to be several practical difficulties associated
with the use of vegetative barriers in built-up
areas, including available space, soiling by traffic,
and the general hostility of the roadside environment to plant life. In addition, analyses carried out
for a busy arterial road show that very large vegetation areas (in excess of 10,000 m
2 ) would be
needed to compensate for the local emissions
from vehicles [160].
Air Treatment Various air treatment approaches
have been devised with the aim of reducing air
pollution. In recent years considerable efforts
have been made to develop construction materials
and coatings, which have the potential for
reducing air pollution. These materials are often
reliant upon photocatalysis, most commonly
involving the compound titanium dioxide
(TiO 2 ). Titanium dioxide acts as a catalyst to
oxidize NO and NO 2 into nitrate (NO 3
À ) in the
presence of oxygen and ultraviolet radiation. The
nitrate is subsequently washed away by rainfall.
The effectiveness of TiO 2 coatings has been successfully demonstrated in laboratory trials [161],
but field trials have been less convincing. This is
partly because it is difficult to distinguish the
impact of chemical conversion on the catalyst
surface from fluctuations in concentrations due
to, for example, meteorology.
Several air treatment technologies have been
developed for use in tunnels. For example, a number of denitrification systems are available; most
work by either a chemical absorption or catalytic
process. Tests in Norwegian and Japanese tunnels
have indicated that removal rates for NO 2 of up to
90% are possible [162, 163]. The application of
denitrification systems at open roadside locations
does not appear to have been reported, and
appears to be impractical at present. Electrostatic
precipitators (ESPs) have been demonstrated as
being effective in removing particles from tunnel
air with varying efficiency. Some electrostatic
systems have also been developed for exterior
environments and tunnels [164], but further investigation into their effectiveness is required.
Future Directions
Outlook
The future global outlook for transport-related
pollution currently looks rather bleak. In developing countries, the rise in car ownership and travel
is predicted to be even more dramatic than it has
been in industrialized nations. For example, the
rate of car ownership in Brazil is predicted to
overtake that in the country that has historically
been the world leader – the United States – before
the middle of the century. Car ownership in China
has doubled in the last 5 years, and it already has
the third highest car sales in the world [126].
However, at the time of writing there appear to
be some signs of change, but probably not as a
Air Quality, Surface Transportation Impacts on
75