The work in which PM2.5 and PM10 have been measured simultaneously over a period of many months clearly indicates that road traffic exhaust emissions and secondary aerosol together can account for essentially all of the atmospheric loading of PM2.5. It follows that these are the sources which should be the main target if controls on PM10 are required. Firstly, the projected reductions in particulate matter emissions assume that renewal of the vehicle fleet will continue at a rate comparable to that in the past, and that new vehicles will conform to the EC emissions limits and continue to do so even as they age. Secondly, the dataset from which current airborne concentrations are taken comes from measurements at central urban background locations. In summer, the seasonal average contribution of vehicle emissions to PM10 is considerably less than that in winter, and the significant correlation of peak hourly PM10 with peak hourly ozone clearly indicates the importance of photochemically produced secondary particles at this time of year.
Coarse particles can also make a substantial contribution in summer.
A complementary strategy would involve control of coarse particles also, but current understanding of the sources and hence implications for control is insufficient in relation to coarse particles. Detailed analyses of the air quality data clearly show the immense importance of road traffic emissions in influencing PM2.5 and PM10 concentrations when these are elevated. Currently, diesel vehicles, in particular trucks and buses, are the main contributor to PM10 emissions from this sector. In our Second Report, the results of predictive calculations of future emissions of particulate matter from road transport were presented. These indicated the importance of control of diesel vehicles in reducing emissions of particulate matter from road traffic.