Introduction: Focus on Airborne Particulate Matter from the Transport Sector

This issue of Aerosol Science and Technology includes two peer-reviewed papers presented at the conference Transport and Air Pollution (TAP) 2009 held at Toulouse, France, between June 2–4, 2009. This conference was the seventeenth of a series of conferences organized since 1986 with the aim of assessing the scientific knowledge on air pollution due to emissions from transportation systems. Similarly to the other TAP conferences, discussions covered a wide spectrum of topics, from the characterization of atmospheric emissions from mobile engines, and their impact on air quality and on human health to the efficiency and acceptability of possible transport policies and mitigation measures to reduce both climate- and health-relevant emissions. However, the distinctive feature of this seventeenth edition was the special emphasis on airborne particulate matter that is now well recognized to be deleterious to human health.

It is currently acknowledged that the transport sector is a major source of airborne particulate matter. In urban areas of developed and developing countries, particle mass (PM), and particle number (PN) concentrations are strongly related to road transport. Both PM and PN concentrations have not decreased over the last decade despite the imposition of policies to reduce airborne particulate matter and its precursors (Harrison et al. 2008; Pérez et al. 2010/this issue). This statement of fact clearly shows that the knowledge of sources of airborne particulate matter is not as good as it was previously thought. In particular, it becomes obvious that the contributions of the transport sector to atmospheric particulate concentrations are not fully defined. First, on-road exhaust particulate emissions are likely not very well established because of unrealistic driving cycles (Joumard et al. 2000) and/or dilution conditions (Shi and Harrison 1999; Lipsky and Robinson 2006) used to produce vehicle emissions factors from chassis dynamometer experiments. Second, airborne particulate matter from the on-road transport sector also includes mechanically produced particles (from the abrasion of brakes, tires, and road surfaces, and emissions from the resuspension of road dusts) and secondary particles (formed in the atmosphere from gaseous and semi-volatile vehicular emissions). Little is known on these non-exhaust emission sources and secondary sources, as well as their respective contribution to PM concentrations in urban and regional atmospheres. However the available researches suggest possible substantial contributions. On the one hand, some research studies on non-exhaust emissions have shown that particulate emissions from resuspension may locally dominate total vehicular emissions (e.g., Abu-Allaban et al. 2003). In such areas the use of aftertreatment technologies, such as the diesel particle filter (DPF), or the introduction of electric vehicles would not dramatically change the total PM vehicular emissions. On the other hand, the very first research studies on the aging of vehicular exhaust emissions show significant photochemical formations of particulate organic compounds during the first hours after emissions (Chirico et al. 2009). Finally, data on particulate emissions from non-road transports (air, rail, and maritime) and from off-road transports (construction engines and engines used in agriculture and forestry) are scarce; new emission inventories suggest that non- and off-road engines may largely contribute to total particulate emissions (Helms and Lambrecht 2009). Similarly to the on-road engines, non- and off-road engine emissions include exhaust and non-exhaust emissions. The latter emissions may largely influence the atmosphere of subway and railway stations (Mazoué 2009; Blomqvist 2009).

In urban areas nanoparticles, currently widely measured as particle number concentrations, are in general much closer related to road traffic than coarser particles. Newer diesel vehicles emit lower particle mass but higher particle number concentrations than older diesel vehicles. Aftertreatment technologies such as the diesel particulate filter (DPF) effectively remove the solid components of particulate matter. However, nucleation happens during the dilution and cooling of the exhaust in the road atmosphere since the DPF do not remove the volatile material that passes it in the gas phase. Without DPF, the volatile material condenses on the solid particles formed in the combustion chamber; while, in case of solid particle removal with a DPF, the volatile material nucleates to form new particles. The nucleation process is enhanced by oxidation of SO₂ by catalytic devices (catalysts used for the DPF regeneration) or by the use of fuel additives (metals used as catalytic assistance for the regeneration of the DPF). Also, hydrocarbons from lube oils and unburned fuels are responsible for the formation of nuclei mode particles in the exhaust of diesel vehicles. Nanoparticles would be more deleterious to human health than coarser particles.

Virtually all of the above selected topics provided discussion materials for the 17th Transport and Air Pollution conference through about 30 presentations and two keynote speeches dedicated to airborne particulate matter. The conference proceedings papers and keynote speeches are available on the conference website (www.ettap09.inrets.fr) and on request ([email protected]).

Cheung et al. (2010/this issue) presents an extensive work on the chemical and toxicological properties of particulate matter from passenger vehicles sampled from chassis dynamometer experiments. Particulate emissions from different vehicle motorization, using various options of fuels and/or after-treatment technologies, have been compared to assess their oxidative potential in relation to a number of inorganic or organic species. Outcomes are given on the role of aftertreatment technologies and fuels on PM emissions, PM-induced toxic activities, and their relationships. In particular, a number of tracers of lube oil were associated to PM-induced toxic activities of vehicular exhaust emissions. Even though it was not the first aim of this paper, insights are also given for source apportionment studies for which current information is lacking.

The influence of on-road vehicle emissions on urban background particle levels is the purpose of the other paper published in this issue (Pérez et al. 2010/this issue). Most particle metrics measured at a background site of Barcelona, Spain, were strongly related to road traffic activities. The finer the particles, the closer the relationship. The examination of the chemical composition revealed the contribution of both exhaust and non-exhaust emissions to the different PM_× fractions. The very large dataset available has allowed the examination of trends. PM₁ concentrations have significantly increased from 1999 to 2006 and this was clearly attributed to increases of both road traffic activities and of the diesel fleet.