Sources and properties of non-exhaust particulate matter from road traffic: A review
Introduction
Airborne particulate matter covers a wide particle size range, from diameters of a few nanometres (nm) to around 100 micrometres (µm). For the purposes of monitoring and regulation, there are two commonly used particle metrics — PM10 and PM2.5. PM10 can be defined as particles having an aerodynamic diameter of less than 10 µm; PM2.5 is similarly defined, referring to particles smaller than 2.5 µm. Another term used for PM2.5 is the fine fraction. The coarse fraction of PM10 comprises particles with diameters in the size range between 2.5 µm and 10 µm (AQEG, 2005). Unless explicitly stated, these definitions of fine and coarse particle fractions will be adhered to in the following review.
In both developed and less developed countries, emissions from road traffic comprise a substantial proportion of primary particulate matter within urban areas and an even larger proportion at roadside (Charron et al., 2007). Abatement policies to reduce emissions from individual vehicles have to date focused almost exclusively on reducing the exhaust emissions of particulate matter from road vehicles and considerable success has been achieved in reducing particle emissions from diesels. Various non-technological approaches have also been implemented in an attempt to reduce emissions through a reduction in traffic volume, such as the introduction of congestion charge schemes, park-and-ride systems, and car sharing initiatives. The reductions achieved in exhaust emissions have highlighted the fact that particulate matter from non-exhaust sources is also a significant contributor to airborne concentrations. Non-exhaust particles typically arise from abrasive sources which include brake wear, tyre wear and abrasion of the road surface. Brake and tyre wear are an important source of trace metals in the urban environment, and at locations influenced by traffic can be more important than industrial emissions. Additionally, particle resuspension from the road surface can be very significant especially in dryer climates (Abu-Allaban et al., 2003).
Using data from a number of European cities, Querol et al. (2004) showed that exhaust and non-exhaust sources contributed approximately equal amounts to total traffic-related emissions of particulate matter. In Berlin, Lenschow et al. (2001) similarly demonstrated that around half of the observed elevation of PM10 at roadside above the urban background could be attributed to resuspension of road dust particles. Using coarse particles as a crude measure of non-exhaust particles, Harrison et al. (2001) found that non-exhaust sources accounted for an almost equal concentration to that from engine exhaust on a heavily-trafficked London road. In some northern European countries where winter road sanding and use of studded tyres is commonplace, a more extreme situation can arise in which the non-exhaust fraction of PM10 can account for up to 90% of airborne particulate matter (Omstedt et al., 2005, Forsberg et al., 2005).
Quantification of the mass concentration of non-exhaust particles and attribution to specific sources is currently difficult. The available methods depend upon the use of chemical tracers, or possibly of particle size distributions as indicative of specific sources. This review is concerned primarily with an examination of the chemical and physical properties of non-exhaust particles from road traffic and their potential use as source tracers. In the last section, a review of some of the more important studies of trace metals in the roadside atmosphere is included with a view to illustrating the ways in which trace element profiles have been used to elucidate particle sources. The focus of the review is upon physico-chemical characteristics of the particulate matter rather than the quantification of emissions from the various sources. Data sources and methods relevant to the latter have been discussed in an earlier paper (Thorpe et al., 2007).
Section snippets
Brake wear emissions
Frictional contact between brake system components during forced deceleration is an important source of particulate matter emissions from motor vehicles. The action of braking results in wear of the brake lining materials and the brake disc/drum. The rate of wear is determined largely by the composition of the linings used and the mode of driving to which the brakes are subjected. This in turn will influence the physical and chemical characteristics of the emitted particles.
In spite of the
Tyre wear emissions
Tyre wear is an important contributor of PM10 emissions to the atmosphere, with annual losses of rubber from tyres in the UK estimated to be 53 × 106 kg (Environment Agency, 1998). Tyre wear may be the largest source of non-exhaust PM10 after resuspension (Lukewille et al., 2001), although recent calculations for a major road in the UK indicate that brake wear may represent a greater source (Thorpe et al., 2007). Frictional contact between the road surface and tyre tread results in the abrasion
Road wear
Most road surfaces in the UK comprise the same component materials; a mixture of aggregates of various grain size, bitumen, and modifiers such as fillers and adhesives. However, variations in precise chemical composition are apparent throughout the country (Nicholls, 1998). Road surfacings may be broadly categorised as concrete or asphalt. Asphalt is comprised primarily (~ 95%) of mineral aggregates, for which many different geological materials are commonly exploited (Woodside, 1998); the
Ambient air measurements of non-exhaust traffic particles
Ambient airborne particulate matter is a complex mixture of particles from a wide range of sources. Identifying the different sources and assessing the contribution of each source to the total atmospheric particulate loading is problematic. However, the chemical composition of ambient particulate matter samples can be useful in addressing these difficulties. Prior to the elimination of Pb additives from petrol, Pb was a useful tracer element for exhaust emissions. With more stringent controls
Inferences
It is apparent from the literature that Cu and Sb may be used as reliable tracers of the presence of brake wear particles in the urban environment. A number of authors have concluded a diagnostic Cu:Sb concentration ratio of around 5:1 to be reflective of brake-related particles; this ratio is markedly different from the typical crustal ratio of around 125:1 and that associated with metallurgical processes, which is typically in excess of 10:1 (Arditsoglou and Samara 2005).
Tyre wear has been
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