What are the sources and conditions responsible for exceedences of the 24 h PM10 limit value (50 μg m−3) at a heavily trafficked London site?

The European Union has set limit values for PM10 to be met in 2005. At Marylebone Road, London, where the traffic is heavy, the daily limit value of 50 μg m−3 is exceeded more than 35 times a year. A total of 185 days with daily PM10 concentrations exceeding the limit value of 50 μg m−3 measured between January 2002 and December 2004 (data capture of 89.5%) are discussed in this paper. These exceedences were more frequent in early spring and in autumn. Concentrations have been disaggregated into regional, urban (background) and local (street) contributions. Most of the episodes of gravimetric PM10 above the limit value were associated with a high regional background and very often the regional contribution dominated the PM10 mass. The secondary aerosol (especially the particulate nitrate) made a major contribution to the PM10 load. These situations were frequently observed when air masses came from the European mainland (showing that both emissions from the UK and other EU countries contributed to the exceedences), and less frequently with maritime air masses that have stagnated over the UK (showing that emissions from the UK alone less frequently contributed to the high regional background). However, the higher frequency of episodes breaching the limit value at the roadside site than at the rural site and the higher frequency of PM10 concentrations above the limit value on weekdays show that the high regional contributions are additional to local and urban emissions. Local emissions mainly due to traffic were the second important contributor to the exceedences, while the contribution of the urban background of London was less important than the local emissions and the regional background. Applying the pragmatic mass closure model of Harrison et al. [2003. A pragmatic mass closure model for airborne particulate matter at urban background and roadside sites. Atmospheric Environment 37, 4927–4933], revealed that the regional aerosol is comprised very largely of ammonium nitrate and sulphate and secondary organic aerosol. Findings suggest that international abatement of secondary aerosol precursors may be the most effective measure to fulfil the requirements of the European Directive 1999/30/CE by lowering the regional background.