10 Conclusions
Projections of PM₁₀ concentrations for future years have been calculated using the receptor modelling methods developed by the Airborne Particles Expert Group. Projections have been calculated for a business as usual scenario and indicate that significant exceedances of the National Air Quality Strategy objective for PM₁₀ are likely in 2005. Projected 99th percentiles are well in excess of 50 mgm-3 for urban background monitoring site locations in major cities and concentrations are likely to be in excess of the objective in many smaller urban areas, particularly for years with elevated secondary particle concentrations such as 1996.

The picture is similar for the EU 'Daughter Directive' indicative Stage 2 limit values for 2009 with exceedances in many areas likely with current national and international policy measures. Projected concentrations in 2004 are expected to be lower than the EU 'Daughter Directive' Stage 1 limit values at most urban background locations, with the possible exception of central London.

Concentrations at the roadside are expected to be higher than at nearby background locations. Concentrations higher than the EU Stage 1 limit value are therefore expected at the roadside of heavily trafficked roads in urban areas in 2004. Concentrations at sites with significant industrial source contributions to measured ambient PM₁₀ concentrations are also expected to be at risk of exceeding this limit value.

The receptor modelling results indicate that the projected PM₁₀ concentrations in 2005 are relatively insensitive to changes in primary combustion particle emissions due to the important contributions of secondary and 'other' particle sources to ambient concentrations. It is likely that the receptor modelling may overestimate the contribution from 'other' particle sources on some days because this concentration has been derived from the residual of the regression analysis. An examination of co-located PM₁₀ and PM2.5 measurements does, however, indicate that the 99th percentile of daily mean coarse particle concentrations (PM₁₀ - PM2.5) are typically in the range 20 - 30 mgm-3.

11 Acknowledgement
This work was funded by the UK Department of the Environment, Transport and the Regions as part of their Air Quality Research Programme.