consideration of sweeping or washing for reducing roadside PM 10 concentrations [150, 151], and
if sweeping or washing can remove particles that
could evolve into PM 10 , then sweeping may have
a delayed beneficial effect on air quality [144]. It
should be technically undemanding, and relatively low cost. Clearly, both sweeping and washing programs would have to be conducted during
periods when traffic flows are low, so that any
disruption to the traffic is minimized.
Dust Suppressants Dust suppressants (or binders)
appear to be an obvious choice for the reduction of
resuspension due to road vehicles. Suppressants
are chemicals applied to surfaces to maintain the
moisture levels or to actually chemically bind the
surface material to reduce fugitive dust emissions.
The main suppressants in use are water (the oldest,
and undoubtedly the cheapest) and solutions of
salts such as calcium chloride (CaCl 2 ), magnesium chloride (MgCl 2 ) and calcium magnesium
acetate (CMA). Little quantitative information
exists on their effects on airborne particulate matter, although with the need to control PM 10 the
interest in suppressants is growing. There is reasonably convincing evidence from recent pilot
studies in Scandinavia that the use of dust suppressants could make a significant difference to
roadside PM concentrations on timescales of
hours to days [144, 145, 152]. However, more
information is needed on potential adverse effects
of using dust suppressants on paved roads.
A notable concern is road safety, as road surface
friction is reduced when large amounts of suppressants are used.
Traffic Management Numerous forms of traffic
management offer the possibility of reducing
emissions from traffic. Local restrictions to the
access and/or circulation of traffic have been
introduced in many cities to reduce pollutant concentrations. Such restrictions can take a range of
forms, including tolls, congestion charges, alternate number plate schemes, and weight restrictions. Cities that have implemented these types
of schemes include London, Stockholm, Athens,
Budapest, and Prague. Low emission zones
(LEZs) are one of the more effective types of
restriction. Entry to a LEZ is usually conditional
on a vehicle meeting specified standards. These
standards can be set in various different ways,
such as emission legislation, the use of specific
exhaust aftertreatment (e.g., DPF), or vehicle age.
The potential beneficial effects of such measures can be illustrated by reference to a trial road
charging scheme introduced in Stockholm City
Centre in 2006. It was estimated that the scheme
resulted in a 15% reduction in total road use
within the charging zone. Emissions of NO x and
PM 10 from road traffic in the zone fell by 8.5%
and 13%, respectively. Air quality dispersion
modelling was applied to assess the effects of the
emission reductions on ambient concentrations. It
was found that the annual average NO x concentrations would decrease by up to 12% along the
most densely trafficked streets and PM 10 concentrations would decrease by up to 7% [153].
Other types of traffic management are designed
to impose low-emission driving styles, including
variable speed limits and synchronized traffic signals (so-called green waves).
Mitigation Measures
The possibilities for mitigation are less diverse
than those for prevention. Mitigation measures
typically involve the use of physical barriers, vegetation, or some form of air treatment.
Physical Barriers On open roads the emissions
from traffic are subject to the normal processes of
atmospheric dilution and dispersion. Barriers are
common features alongside roads with high levels
of traffic, particularly those which run through
populated areas. These features are usually introduced to attenuate noise, but can also block pollutant dispersion, increase turbulence and initial
mixing, or modify deposition. Any air flow perpendicular to the barrier can be deflected upward
by the structure, and this can increase the apparent
release height of the pollutant and increase vertical mixing due to the flow separation at the top of
the barrier. A recirculation cavity forms in the lee
of the structure, and this can also reduce pollutant
concentrations relative to an open area with no
barrier [155]. However, plume reattachment – and
74
Air Quality, Surface Transportation Impacts on
if sweeping or washing can remove particles that
could evolve into PM 10 , then sweeping may have
a delayed beneficial effect on air quality [144]. It
should be technically undemanding, and relatively low cost. Clearly, both sweeping and washing programs would have to be conducted during
periods when traffic flows are low, so that any
disruption to the traffic is minimized.
Dust Suppressants Dust suppressants (or binders)
appear to be an obvious choice for the reduction of
resuspension due to road vehicles. Suppressants
are chemicals applied to surfaces to maintain the
moisture levels or to actually chemically bind the
surface material to reduce fugitive dust emissions.
The main suppressants in use are water (the oldest,
and undoubtedly the cheapest) and solutions of
salts such as calcium chloride (CaCl 2 ), magnesium chloride (MgCl 2 ) and calcium magnesium
acetate (CMA). Little quantitative information
exists on their effects on airborne particulate matter, although with the need to control PM 10 the
interest in suppressants is growing. There is reasonably convincing evidence from recent pilot
studies in Scandinavia that the use of dust suppressants could make a significant difference to
roadside PM concentrations on timescales of
hours to days [144, 145, 152]. However, more
information is needed on potential adverse effects
of using dust suppressants on paved roads.
A notable concern is road safety, as road surface
friction is reduced when large amounts of suppressants are used.
Traffic Management Numerous forms of traffic
management offer the possibility of reducing
emissions from traffic. Local restrictions to the
access and/or circulation of traffic have been
introduced in many cities to reduce pollutant concentrations. Such restrictions can take a range of
forms, including tolls, congestion charges, alternate number plate schemes, and weight restrictions. Cities that have implemented these types
of schemes include London, Stockholm, Athens,
Budapest, and Prague. Low emission zones
(LEZs) are one of the more effective types of
restriction. Entry to a LEZ is usually conditional
on a vehicle meeting specified standards. These
standards can be set in various different ways,
such as emission legislation, the use of specific
exhaust aftertreatment (e.g., DPF), or vehicle age.
The potential beneficial effects of such measures can be illustrated by reference to a trial road
charging scheme introduced in Stockholm City
Centre in 2006. It was estimated that the scheme
resulted in a 15% reduction in total road use
within the charging zone. Emissions of NO x and
PM 10 from road traffic in the zone fell by 8.5%
and 13%, respectively. Air quality dispersion
modelling was applied to assess the effects of the
emission reductions on ambient concentrations. It
was found that the annual average NO x concentrations would decrease by up to 12% along the
most densely trafficked streets and PM 10 concentrations would decrease by up to 7% [153].
Other types of traffic management are designed
to impose low-emission driving styles, including
variable speed limits and synchronized traffic signals (so-called green waves).
Mitigation Measures
The possibilities for mitigation are less diverse
than those for prevention. Mitigation measures
typically involve the use of physical barriers, vegetation, or some form of air treatment.
Physical Barriers On open roads the emissions
from traffic are subject to the normal processes of
atmospheric dilution and dispersion. Barriers are
common features alongside roads with high levels
of traffic, particularly those which run through
populated areas. These features are usually introduced to attenuate noise, but can also block pollutant dispersion, increase turbulence and initial
mixing, or modify deposition. Any air flow perpendicular to the barrier can be deflected upward
by the structure, and this can increase the apparent
release height of the pollutant and increase vertical mixing due to the flow separation at the top of
the barrier. A recirculation cavity forms in the lee
of the structure, and this can also reduce pollutant
concentrations relative to an open area with no
barrier [155]. However, plume reattachment – and
74
Air Quality, Surface Transportation Impacts on