Airborne particulate matter has been a highly topical subject for the past 20 years or so, although it has been a problem pollutant for very much longer. Historical records of problems with coal smoke pollution go back many centuries, but the first good quantitative records date back to the early 20th century when it was measured as “black smoke”. An episode of severe pollution by black smoke and sulfur dioxide in December 1952 is believed to have caused around 4000 premature deaths in London alone, and comparable events occurred in other parts of the world. The recognised toxicity of black smoke and sulfur dioxide led to the development of mitigation policies, which greatly improved the quality of the atmosphere to the point that it was felt that there were no significant remaining effects on public health arising from these pollutants in most developed countries. However, the application of more advanced epidemiological methods in the 1990s demonstrated that significant public health impacts of exposure to airborne particles, now measured by the mass metrics PM_2.5 and PM₁₀, existed in North America and Europe and, by implication, in other parts of the world. Whilst most aspects of local air quality continued to improve in developed countries, this was a time when less developed countries were growing their economies rapidly, with an accompanying substantial increase in the concentration of airborne particles. As a result, the World Health Organisation's Global Burden of Disease Project ranks exposure to outdoor particles as the ninth largest preventable cause of disease in the global population.

This volume of the Issues series addresses airborne particulate matter from a global perspective. The first chapter, by Marc Guevara of the Barcelona Supercomputing Centre, gives quantitative information on the sources of particle emissions in a European context. Not only are particles emitted directly into the atmosphere, but they also form within the atmosphere from the oxidation of gases and undergo chemical transformations in the atmosphere. The latter secondary fraction typically makes the dominant contribution to PM_2.5 concentrations. In the second chapter, Neil Donahue of Carnegie Mellon University and co-authors pose the question: What are the sources of particles? Their focus is on the secondary particles that are formed within the atmosphere and how the sources affect human exposure to airborne particles, both by mass and by number.

One of the key advances in the past 20 years has been the development of receptor modelling methods that take air quality data and use it to provide estimates of the quantitative contribution of different sources to measured concentrations. In the third chapter, John Watson and Judy Chow of the Desert Research Institute, Nevada, outline the principles of these methods, and in the fourth chapter, Philip Hopke of Clarkson University, New York State, provides further insights into methods and gives some case studies from North America. The following two chapters take data from two major European experimental studies to provide case studies of the application of receptor modelling methods to source apportionment. In doing so, they give powerful insights into the sources of particles in European cities. The AIRUSE LIFE+ team led by Xavier Querol of the Consejo Superior de Investigaciones Cientificas (CSIC) describes a study of cities in southern Europe. This is complemented by a study from northern Europe (the Joaquin Project) led by Edward Roekens of the Flanders Environment Agency, which has applied similar methods to five cities within northern Europe.

The subsequent two chapters provide insights into the apportionment of PM_2.5 (fine particles) and particulate matter of various size fractions in two of the world's hotspots. In Chapter 7, Mei Zheng and colleagues from Peking University consider the source apportionment of PM_2.5 within China, and in Chapter 8, Mukesh Khare and Isha Khanna of the Indian Institute of Technology, Delhi, review case studies of source apportionment from the Indian sub-continent.

No such volume would be complete without also considering the adverse health effects of airborne particles. Consequently, in Chapter 9, Frank Kelly of Kings College, London, reports on the health effects of airborne particles and how these relate to their composition, size and source.

We are delighted to have engaged such a distinguished group of authors, including several world leaders in their respective fields, to provide a truly authoritative and up-to-date volume describing many of the key aspects of airborne particulate matter. We believe that this will prove to be of widespread interest and be of great value to a range of communities, including policymakers, physical and life scientists, and students taking advanced courses in a range of environmental and health-related fields.

Ronald E. Hester

Roy M. Harrison

Xavier Querol