7 Discussion

We have used the estimates of emissions reductions for specific types of area (including London) to estimate concentrations of NO₂ and PM₁₀. We have only modelled the impact of reductions in emissions from road traffic. Taking into account the impact of increases in rail emissions would partly offset the estimated reductions in concentrations.

DETR 2000b explains that the estimates of road traffic emissions and concentrations reductions should be treated with caution. This is because DETR’s strategic road traffic modelling work has necessarily had to make broad assumptions about how the key decision-makers, particularly local authorities and the Mayor of London, will choose to spend the funds being made available by the Plan. It is unlikely to represent accurately the decisions that those bodies will make in the context of their local transport strategies and air quality management plans. In practice expenditure might be more targeted on localised problems.

The forecast average percentage reductions in annual average concentrations for the various scenarios are listed in Table 11 along with the range. The averages were calculated by calculating a 4-year average percentage reduction in annual mean concentration for each site for the weighted scenarios and then calculating an average value across all the sites for which analysis has been carried out. Similarly the ranges represent the maximum and minimum reductions in the 4-year average results across the range of monitoring sites. A similar process was followed for the un-weighted scenarios (A to E) to give a better indication of the likely range of reductions across individual sites. DETR 2000b quotes (paragraphs 74-75) the average reductions in concentrations for each scenario and the range from the un-weighted scenario analysis.

Table 11 Summary of percentage reductions in annual mean concentrations in 2010 relative to the baseline scenario

The Plan is estimated to reduce annual average NO₂ concentrations by, on average 3.1% (range 0.6–7.3%), compared to the baseline in 2010, with the biggest reductions predicted at roadside sites. The illustrative scenarios are estimated to produce reductions of, on average, 4.6 % (range 1.0-9.8%). On the basis of the assumptions underlying our air quality modelling, there would still be areas in London (centrally and near very busy roads) where the EC limit value and the Air Quality Strategy objective would not be attained, and possibly also near heavily trafficked roads in other large cities.

Projections of annual mean NO₂ concentrations have been calculated using monitoring data from 1996, 1997, 1998 and 1999. The results in terms of percentage reductions in 2010 annual mean concentrations relative to the baseline were found to be similar for all base years. The year to year variation in reductions for the plan scenario at each site (the difference between the highest and lowest projected reduction in concentrations for the different base years) was on average 0.03 % (range 0 to 0.05 %) of the 2010 baseline concentration at background sites and 0.2 % (range 0.2 to 0.3 %) at roadside monitoring sites. The fact that this variation from year to year is small reflects that while absolute concentrations measured at a monitoring site may show considerable variation due to changes in meteorology, and thus dispersion conditions, a change in emissions can be expected to have an approximately proportional effect on the measured concentration.

There are two sets of EC Directive limit values for PM₁₀ concentrations: mandatory Stage 1 limit values for 2005, and more stringent non mandatory "indicative" Stage 2 limit values for 2010. Our analysis has focussed on the Plan’s contribution to achievement of the indicative annual mean Stage 2 limit value in 2010 - the timeframe of the Plan. Analyses presented in the Air Quality Strategy showed that this indicative limit value is likely to be widely exceeded across the country in 2010, with highest levels generally occurring next to heavily trafficked roads. Although higher levels of PM₁₀ are generally recorded next to roads, transport emissions are only one contribution to a relatively high background concentration, which is made up of a wide variety of sources. Because of this, reductions in transport PM₁₀ emissions do not necessarily lead to a proportionate reduction in overall PM₁₀ concentrations. The estimated reductions in concentrations arising from the Plan and illustrative scenarios are small: on average 1.8 % (range 0.6-5.4%) compared to the baseline for the Plan scenario; and on average 2.4 % (range 1.0-6.2%) under the illustrative scenarios. The reductions will nonetheless contribute to the Government’s broader strategy of reducing PM₁₀ levels, which involves addressing non-transport sources in the UK and emissions from the rest of Europe.

Annual mean PM₁₀ concentrations show considerable year to year variability. Changes in emissions of primary PM₁₀ from traffic may not have the same percentage impact on annual mean PM₁₀ concentrations for different base years due to variations in other important contributions to ambient PM₁₀ concentrations, such as secondary and coarse particles. The percentage reduction in annual mean concentration in 2010, relative to the baseline, is generally greatest for projections based on 1997 monitoring data. Of the four years studied, 1997 was the year with the largest relative contribution to total PM₁₀ from primary particles. The year to year variation in reductions for the Plan scenario at each site was on average 0.5 % (range 0.3 to 0.9 %) of the 2010 baseline concentration at background sites and 0.5 % (range 0.2 to 0.8 %) at roadside monitoring sites.

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