A study by a team of researchers in the UK has estimated the contributions of brake dust, tire dust, and resuspended dust to non-exhaust traffic particles as 55.3 ± 7.0%, 10.7 ± 2.3%, and 38.1 ± 9.7%, respectively, at their sampling site at Marleybone Road, London.
Increasingly stringent environmental regulations have resulted in substantial reductions in the exhaust emissions from road traffic, the team notes in a paper describing their work in the ACS journal Environmental Science & Technology. However, these reductions in combustion emissions have not been accompanied by similar reductions in nonexhaust emissions—i.e. the abrasive emissions from brake, road, and tire wear, and the resuspension of materials from the highway surface. As a result, these non-exhaust emissions make up a similar proportion of the airborne particulate matter (PM) resulting from vehicle use as exhaust emissions.
A separate study has predicted that in central Europe, the contribution of nonexhaust PM to total traffic emissions will increase to 80−90% by the end of this decade. ALthough the aerodynamic diameters of these non-exhaust particulate emissions tend to be larger than those of exhaust emissions, they still can enter the respiratory system and possibly lead to adverse health impacts. Sources and types of non-exhaust atmospheric particulate matter include:
Iron, copper, antimony, and barium from brake operation.
Road surfaces are generally composed of either concrete or aggregate with a bituminous binder and abrasion of such a surface is likely to result in particulate matter of mineral origin.
Tire wear is likely to result in predominantly carbonaceous particles, although small quantities of metals, in particular zinc which is used as a vulcanization activator, may be present.
Material resuspended from the road surface may include all types of vehicle abrasion debris, in addition to material from non-road sources which has been deposited on the road surface. This may include mineral dust from the local environment, typically including silicon, aluminum, calcium, and iron—particularly in arid locations. Within the United Kingdom, winter maintenance procedures involve spreading deicing salt on roadways, but the practices that are common in some northern countries of sanding roadways and fitting studded tires are not adopted.
The new study used a novel approach combining the use of size distribution information and tracer elements. The researchers collected size-segregated particulate matter, in 10 size fractions from <0.2 μm aerodynamic diameter to greater than 21 μm aerodynamic diameter at curbside and background sites during month-long campaigns conducted in 2007, 2009, 2010, and 2011.
Among their findings were that:
Iron, copper, antimony, and barium had curbside-to-background ratios of 4.1−4.4.
Total mass, aluminum, calcium, silicon, and zinc had curbside to background ratios of 1.3−1.6.
Vanadium concentrations were slightly lower at the curbside site than at the background sites (ratio 0.9).
The curbside to background ratios of nickel, titanium, and chromium are similar to those of mass.
|Mean concentrations and curbside-to-background ratios for the main analytes. Credit: ACS, Harrison et al. Click to enlarge.|
The combination of particle size distributions and chemical tracers, even though these are often contributed by more than one source, does appear to allow a realistic reconstruction of the mass of coarse mode particles contributed by non-exhaust traffic sources. Quantitative estimates have been arrived at for the contribution of brake dust, tire dust, and resuspended particles but it should be recognized that not only are these site specific, they are also subject to considerable uncertainties which have yet to be quantified. It would be expected that in southern Europe and other hotter, drier parts of the world, the contribution of resuspended road dust would be substantially greater. On the other hand, in Scandinavia, the use of studded tires and road sanding is common practice in the winter months leading once again to a quite different pattern in contributions to non-exhaust traffic-related particulate matter in the atmosphere.—Harrison et al.
Roy M. Harrison, Alan M. Jones, Johanna Gietl, Jianxin Yin, and David C. Green (2012) Estimation of the Contributions of Brake Dust, Tire Wear, and Resuspension to Nonexhaust Traffic Particles Derived from Atmospheric Measurements. Environmental Science & Technology doi: 10.1021/es300894r