Site specific analyses of ambient NO2 and PM10 concentrations

3 Site Specific NO₂ Projections: Method

The methods used to calculate site specific projections of future annual mean NO₂ concentrations have been described in the AQS (DETR et al, 2000) and in some detail by Stedman et al (1998a) and Stedman (1999). Projections were based on measurements carried out at sites within the national automatic monitoring networks (see /netcen/airqual for details of the site locations and an archive of monitoring results). The following steps were required to project these measured concentrations forwards:

The measured concentration was divided into component parts. A map of rural concentrations was subtracted from the measured concentration to determine the local source contribution to annual mean NO_x concentration. Emission inventory maps (Goodwin et al, 1999) for 1997 were used to split the local source contribution into four emissions sectors: road traffic, domestic and services, industry and other (a total of 25 1 km squares, centred on the monitoring site local were examined). For roadside monitoring sites there was assumed to be an additional contribution from emissions on the road adjacent to the monitoring site. Annual mean background concentrations for roadside sites were derived from mapping studies similar to those described in the AQS.
Each component was then projected forwards from the measurement year to 2010 according to the projected change in emissions from each sector and added together to give an estimate of annual mean NO_x for 2010. Rural concentrations were projected on the basis of changes in UK total NO_x emissions.
Non-linear relationships between annual mean NO₂ and NO_x concentrations were then used to calculate estimates of NO₂ concentration.

Estimates of road traffic emissions were taken from Table 3. Estimates of UK total non-traffic emissions of NO_x for 1998 and earlier years are available from the NAEI. Emissions from non-traffic sources for years between 1998 and 2010 have been derived from DTI (2000) and estimated within the NAEI. These estimates therefore incorporate an assumed growth in economic activity of about 2.5% per year and the continuation of current trends towards greater use of natural gas and cleaner technologies DTI(2000). The change in non-traffic emission of NO_x is expected to be small relative to the changes in emissions from road traffic. We have estimated that non-traffic emissions in 2010 will be approximately 95% of those in 1998. These projections illustrate our best estimate of the likely result of current national and international policies.

The following non-linear relationships between annual mean concentrations were used. These have been derived from monitoring data from 1990 to 1999 inclusive and the graphs showing these relationships are reproduced in Appendix 1:

· Central London Background

NO₂ (ppb) = 1.750.NO_x (ppb)^0.7

· Elsewhere Background

NO₂ (ppb) = 2.375.NO_x (ppb)^0.6

· Roadside

NO₂ (ppb) = 1.8767.NO_x (ppb)^0.6

Figures 1 to 3 show illustrative examples of site specific projections of NO_x and NO₂ concentrations for years between 1990 and 2010. Projected concentrations have been calculated from measured concentrations in 1996, 1997, 1998 and 1999. There is good agreement between the projections for years earlier than 1996 and the measured concentrations. This gives us confidence that the split into different sectors and the emissions estimates are reasonable. The inter-year changes in measured concentrations are also put into the context of changes in emissions, enabling the identification of years with unusually efficient or poor dispersion of primary pollutants. High concentrations due to poor dispersion conditions are evident at West London in 1997 and particularly in 1991, when there was a major episode of elevated conditions. The projections also clearly illustrate the impact of emissions reductions on ambient NO_x concentrations and the correspondingly smaller changes in annual mean NO₂.

The projections of traffic emissions used to calculate the illustrative results shown in shown in Figures 1 to 3 are NAEI estimates (Murrells, 2000), which are based 1997 National Road Traffic Forecasts for Great Britain (DETR 1997). These projections pre-date the area type specific emissions projections for England listed in Table 3 and do not reflect any of the measures specified in the 10 Year Plan. The advantage of these NAEI projections is that they are available for all years from 1990 to 2010 and facilitate the comparison with measurement data (the 10 Year Plan emissions estimates are currently only available for 1996, 2000 and 2010). This enables the profile of projected concentrations changes to be compared with past trends an assessment of concentrations in years such as 2005 for which AQS objectives have been set. The NAEI projections for 2010 are similar to those for the baseline scenario within the 10 Year Plan. However the assumptions underlying the two are not the same. For example the NAEI estimates assumed that the previous policy of the fuel duty escalator would continue until 2002. The 10 year Plan baseline only includes the impacts of the fuel duty escalator to 1999, after which the policy was changed. In addition, the NAEI estimates also do not take into account the effect on road traffic volumes of the policies in the Government’s Integrated Transport White Paper (DETR, 1998). More details on the Plan baseline can be found in the background document (DETR 2000b).

Chapter 2 Chapter 4

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