Can additional local measures in urban areas
help in reducing national emissions and
transboundary air pollution?
Paper prepared for 27th meeting of the Task Force on Integrated
Assessment Modelling in Oslo, 13-15 May 2002
Prof Helen ApSimon,
Department of Environmental Science and Technology
Imperial College, London SW7 2BP
e.mail: h.apsimon@ic.ac.uk
Can additional local measures in urban areas help in reducing national emissions
and transboundary air pollution?
In applying IAM to derive cost-effective strategies to reduce long-range
transboundary air pollution, the emphasis has been on add-on technological measures,
and the objective to close the gaps between current levels of deposition or exposure
and critical loads/levels. In parallel at a more local level in urban areas, local
authorities (LAs) in cities are addressing air quality, and drawing up plans for the
attainment of air quality objectives. In such assessment the emissions are far more
dominated by traffic, where the LAs assume similar improvements from the
introduction of newer vehicles and stricter emission standards to those in national
assessments. The emphasis is concentrated on reducing peak emissions in certain
areas, and on just one or two “difficult” pollutants where it s difficult to meet targetsfor example in the case of London NO2 and PM10. This paper will discuss aspects of
the interaction between measures to attain air quality objectives and control traffic
emissions in urban areas, and the reduction of national emissions to meet emission
ceilings.
Meeting air quality objectives
Air quality objectives (see table 1) have been established in EC daughter
directives in order to protect human health, and set limits on peak and/or annual
concentrations. In effect these limit the maximum exposure to an individual inhaling
ambient air concentrations at the worst locations. These tend to be along busy roads,
especially those enclosed by tall buildings (“street canyons”) or at busy intersections,
where local sources are superimposed on the urban background, which in turn is
superimposed on longer range imported pollutants. Typically a city will have a central
more congested part, where such canyons are more common, a surrounding area with
mixed commercial and residential use, and an outer part which tends to be more
residential and less densely built up. Generally for a large city there will be a ringroad or bypass taking a lot of traffic round the outskirts and reducing traffic through
the centre. There may be other particularly busy areas too, such as, in the case of
London, a major airport on the city outskirts.
Figure 1 illustrates source apportionment for this kind of structure in the case
of annual average concentrations of PM10, taking a row of grid squares from west to
east across London. Each column gives the concentration at the side of the busiest
road or street where the top section of each column indicates the contribution from the
local street, superimposed on the respective contributions from central, inner and
outer London, and the M 25 ring road. The two bands beneath represent the imported
contribution from long-range transport, and an additional unattributed contribution
from the coarser part of the PM10 between 2.5 and 10 microns whose origin is
currently not adequately understood or represented in the emission inventory. A
similar picture would emerge for NO2, except that in this case there is no unattributed
portion of the primary NOx emission, but instead the complexity of chemical
oxidation of NOx to NO2.
The air quality objectives are set as targets to be achieved by 2005 (except for
certain major cities where this is not possible and a longer time is required for some
pollutants). A combination of monitoring and measurement is required to assess the
areas still likely to experience exceedance in the target year, taking into account
changes in the vehicle fleet and traffic volumes. This sets the starting point for
deciding what further measures are required in order to achieve the targets. These are
generally a combination of technological measures to reduce exhaust emissions,
and/or measures to reduce traffic demand.
Technological measures
The technological measures that can be applied in urban areas are largely the
same as those assumed in national projections, related to successive improvements in
emissions with newer vehicles, and stricter enforcement of inspection and
maintenance to eliminate failed catalysts or ill-tuned vehicles. Similarly there may be
technological measures that can be applied to reduce any industrial emissions, or
domestic energy requirements- for example through combined heat and power
schemes, or greater use of electricity generated outside the city.
With respect to vehicle emissions it should be noted that both national and
urban emission projections may have been over-optimistic, especially for NOx, in
view of recent revisions in emission factors for newer vehicles. In the past it has often
been anticipated that emissions from newer vehicles will reduce in proportion to the
corresponding EURO standards for the date of registration of the vehicles. However
as the newer vehicles are introduced more comprehensive measurements can be
undertaken of in service emissions, as has been done in the UK based on work by
TRL. In particular this has led to some significant revisions in NOx emission factors,
and corresponding updating of projected transport emissions in the UK National
Atmospheric Emissions Inventory, NAEI, and in the London inventory compiled by
the Greater London Authority. As an example this has increased estimated NOx
emissions in London for 1999 as a recent year by 26% from 50.3 to 63.7 kilotonnes
per year; and projected emissions for 2005 by 62% from 25.4 to 41.1 kilotonnes. This
has had a major impact on the assessment of areas of exceedance in 2005. A similar
but less marked effect also applies to PM10. Total UK urban emissions, though with a
slightly different vehicle mix, are also significantly changed. More details can be
found on the NAEI web site – www.NAEI.org.uk, covering different vehicle types
and age/category.
Fuel switching
In deriving national cost curves for use in European scale integrated
assessment modelling with RAINS and ASAM, official energy projections have been
retained with no allowance for switching of fuels. This left room for minor
adjustments such as ultra low sulphur fuels, giving reduced particle emissions.
However in practice many countries have increased their use of natural gas, making it
much easier to attain national emission ceilings. This was illustrated when IIASA
undertook a subsidiary study with RAINS, based on a scenario from the Kyoto
protocol to reduce greenhouse gas emissions.
In urban areas conversion of vehicles to LPG or LNG can also be beneficial,
reducing both NOx and PM10substantially ( e.g. for a Euro 1 vehicle NOx emissions
from LPG are about 35-55% lower than those for a similar vehicle running on diesel,
and around 90% lower with respect to PM10). Such conversion depends on fuel supply
services and infrastructure. In urban areas such conversion is especially applicable for
captive fleets operating within the city boundaries- for example taxis, municipal
services such as refuse collection and some delivery services, plus buses and public
transport. The proportion of emission covered is very variable from one city to
another, and though potentially a cost effective measure that can help towards air
quality objectives in certain areas of the city, is likely to have a limited effect on the
overall emissions of the city unless extended to a wider proportion of vehicles.
Traffic demand measures
There are also non-technical measures to reduce traffic demand, or kilometres
driven. Examples include:
Modal switch to public transport, induced by improvements of public transport
in terms of availability, time of journey, and convenience; park and ride schemes, car
sharing schemes, coordinated or shared delivery schemes
Financial measures or disincentives such as road user or congestion charging,
parking charges or parking restrictions within the city, limited access and
pedestrianised areas, and low emission zones with restrictions on vehicles eligible to
enter.
Such measures can require substantial implementation costs, but also
revenues- for example the introduction of a cordon round central London involving a
daily charge to enter, requiring pre-purchased access with automatic checks at a large
number of entry points. The definition of costs is also ambiguous, since there are both
costs of implementation and increased costs incurred for travel and transport of goods
and services. However, unlike the investment in technological change, some of the
money from non-technological measures is revenue that may be recycled into the
community, and hence the net cost overall may be lower.
The effectiveness of non-technological measures is more difficult to quantify
than for technological measures. In some cases the effect of a measure can be
interpreted in terms of an additional journey cost for journeys between different zones
of the city, and an elasticity of demand in response to increasing cost used to derive a
reduction in kilometres driven in different zones. Such elasticities are however very
variable according to circumstance and the purpose of journeys and the type of cost.
For example the Atkins study (1999) assumed an elasticity of -0.13 to increases in
journey cost for such measures as parking or congestion charging, but noted that
elasticity with respect to increases in fuel cost could be as low as 0.02 to 0.03. The
elasticity also depends on alternatives- for example if public transport is improved at
the same time as road user charges, then there is likely to be a greater willingness to
reduce kilometres driven.
The response to low emission zones in particular is difficult to estimate. These
may impose different levels of restriction, for example a modest elimination of older
pre Euro 1 vehicles, or restriction to Euro III or newer. Faced with such a restriction,
the response may be not to make the journey at all, or to switch to some other mode of
transport, or to invest in a newer vehicle that meets the requirements - thus
accelerating the introduction of cleaner vehicles; or in some cases a different route
may be taken that avoids the LEZ, potentially leading to increased emissions overall.
Usually there will be some mix of these responses, but quantifying this mix and its
effect on emissions involves significant assumptions.
The study by W S Atkins attempted to look at the magnitude of emission
reductions achievable by both technological and non-technological measures such as
those above in central, inner and outer London (subsequently used to illustrate the
applicability of integrated assessment modelling to London with the Urban Scale
Integrated Assessment Model, USIAM, presented at the TFIAM in Brussels in May
2001. It is the central areas of London where air quality standards are exceeded most,
and hence reductions here are particularly important ; whereas it is reduction in the
overall city emissions that is relevant to meeting national emission ceilings. The
Atkins study indicated emission reductions in NOx and PM10 due to different
combinations of non-technical measures ranging from modest levels of 5% or less up
to 15 to 20% in central London. However the reductions in total city emissions were
much less, in the range 1 to 10% at most. Larger emission reductions were achieved
when combined with technical measures such as fuel switching, but are dependent on
realistic assumptions about the proportion of vehicles to which this could be
applicable within the time horizon of 2005. The effectiveness of low emission zones
will be affected by the assumptions about emission factors for newer vehicles as
discussed above.
Other factors affecting emissions
It should also be noted that there are other changes in cities which may affect
emissions, some driven by safety or other environmental factors such as noise and
congestion. In some cases these may increase total emissions- for example the
introduction of by-passes, out-of-town shopping centres, or route changes such as
one-way systems, may all increase kilometres driven and hence emissions. Changes in
speed, and acceleration and deceleration also have significant effects, for example
traffic calming schemes and “sleeping policemen”. The trend towards greater
commuter distances, influenced by increased housing prices, can also be a factor in
travel demand. However in the longer term there may be increased working from
home as greater use of IT is exploited. The overall effect of such changes on a
national scale could be highly variable.
Looking at longer time scales other measures such as electric vehicles, and
eventually hydrogen as a fuel, may do a lot to improve air quality in urban areas. At a
national level however it will depend on the how such energy sources are derived.
Conclusions
There are several alternative ways to improve urban air quality, apart from the
technological measures which have been considered in relation to national emissions,
and included in national cost curves for integrated assessment with respect to
transboundary air pollution. However the emphasis in strategies to achieve air quality
objectives is focused on limited areas of high concentration where exceedance occurs,
particularly close to major roads and in city centres. As such they are likely to have
modest effects on overall national emissions, and in some cases may even lead to
increased emissions.
Nevertheless it is beneficial to consider both attainment of national ceilings
and air quality objectives together, and to recognise the importance of assumptions
about technological measures and fuel switching in achieving both.
Acknowledgments
I would like to thank Tim Murrells of AEA Technology and Lucy Sadler of the GLA
for providing information on the recent updating of emission inventories reflecting
adoption of new new Euro 1 and II emission factors from TRL , and the significant
impact this has on NOx emissions. Also the Department of Environment. Food and
Rural Affairs, DEFRA, for supporting research in the Integrated Assessment Unit at
Imperial College as the UK National Centre for Integrated Assessment Modelling.
References:
W S Atkins (1999) An evaluation of transport measures to meet NAQS objectives.
Stage 2 final report to DETR (now DEFRA).
A Mediavilla (2002) Integrated assessment at the urban scale. PhD thesis (papers in
preparation with H ApSimon)
Web site on revised emission factors: www.NAEI.org.uk
Figure 1: Source apportionment for PM10 for a row of grid cells across London
(From USIAM model using source-receptor relationships derived using ADMS- Urban.)
Urban Scale Integrated Assessment Model (USIAM) Blame Matrix for Central London Trajectory Including
Coarse and Secondary Particulate Contributions (1996), (Imperial College, a.mediavilla@ic.ac.uk)
60
Background
Secondary
Area LEZ
Area Inner
Area Outer
Area M25
Area Not in London
1996 Local Street Canyon
55
50
45
40
35
30
25
20
15
Secondary particulates
10
5
Coarser PM10 – PM2.5:  ?
1830
1828
1826
1824
1822
1820
1818
1816
1814
1812
1810
1808
1806
1804
1802
1800
1798
1796
1794
1792
1790
1788
1786
1784
1782
1780
1778
1776
1774
1772
1770
0
Table 1: AQ objectives (taken from The Air Quality Strategy for England, Scotland,
Wales and N Ireland,2001)
Pollutant
Objective
Benzene
1,3 butadiene
Carbon monoxide
Lead
Nitrogen dioxide
16.25 g/m3 (5 ppb)
running annual mean
3
2.25 g/m (1 ppb)
“
11.6 mg/m3 (10ppm)
running 8 hour mean
0.5 g/m3
annual mean
3
200 g/m (105 ppb)
1 hour mean
not to be exceeded > 18 times per year
40 g/m3 (21 ppb)
annual mean
Particles PM10
50 g/m3 not to be
24 hour mean
exceeded >35 times per year
40 g/m3
annual mean
3
350 g/m (132 ppb) 1 hour mean
not to be exceeded.>24 times per year
125 g/m3 (47 ppb)
24 hour mean
not to be exceeded.3 times per year
266 g/m3 (100 ppb)
15 minute mean
not to be exceeded >35 times per year
Sulphur dioxide
Concentration measured as
Addendum :Impact the new emissions factors make in London ( information from
the GLA)
For your information, we have included some initial calculations of the total road
transport emissions in London based on the new factors for NOX and PM10. Further
calculations will be made in due course for the other pollutants. We have used the
same flows and vehicle stock as in the existing release of the inventory – only the
emissions factors are different.
The tables below summarise the preliminary results by vehicle class. The percentages
given refer to the increase or decrease in emissions compared with the original
factors.
Table 3 NOX Emissions (t/annum)
Vehicle Class
Cars and taxis
LGVs
HGVs
Buses
Motorcycles
TOTAL
Old Factors
24781
3833
18110
3524
55
50303
1999
New Factors
27618 (+11.4 %)
5131 (+33.9 %)
26441 (+46.0 %)
4429 (+25.7 %)
55 (no change)
63675 (+26.6 %)
Old Factors
9154
1511
12301
2352
45
25363
2005
New Factors
14100 (+54.0 %)
2953 (+95.4 %)
20625 (+67.7 %)
3410 (+45.0 %)
45 (no change)
41133 (+62.2 %)
Table 4 PM10 Emissions (t/annum)
Vehicle Class
Cars and taxis
LGVs
HGVs
Buses
Motorcycles
TOTAL
1999
Old Factors
New Factors
1037
1087 (+4.2 %)
821
662 (-19.4 %)
928
1144 (+23.3 %)
168
165 (-1.8 %)
19
19 (no change)
2974
3077 (+3.4 %)
2005
Old Factors New Factors
496
608 (+22.6 %)
447
272 (-39.2 %)
383
678 (+ 77.0 %)
71
56 (-21.1 %)
16
16 (no change)
1413
1630 (+15.4 %)
For NOX, the following observations can be made:
1.
Total emissions for 1999 have increased by over 25 %.
2.
All vehicle types show an increase in NOX except motorcycles.
3.
The most significant increase is for HGV emissions.
4.
Re-calculated 2005 total emissions have increased significantly (by over
60 %).
5.
LGV emissions in 2005 are almost double previous estimates.
6.
HGV emissions are also significantly higher than previous estimates.
For PM10, the following observations can be made:
1.
There has been a small increase in total emissions for 1999.
2.
The change for different vehicle types is variable. HGVs, and to a lesser
extent cars, have increased compared with the previous factors.
Conversely, LGVs and buses have shown a decrease.
3.
For 2005, total emissions have increased by 15 %.
The variation between different vehicle types is more pronounced than for 1999.
HGVs in particular show a large increase.