Airborne Tire Wear Particles: A Critical Reanalysis of the Literature Reveals Emission Factors Lower than Expected

Abstract

Tires are a ubiquitous part of on-road transport systems serving as the critical connecting component at the interface of the motive power and road surface. While tires are essential to automobile function, the wear of tires as a source of particulate air pollution is still poorly understood. The variety of reported emissions found in the secondary literature motivated us to summarize all known mass-based tire wear emission factors for light-duty vehicles in primary research. When excluding road wear and resuspension, mean emissions of 1.1 mg/km/vehicle (median 0.2 mg/km/vehicle) were found for tire wear PM₁₀ and mean emissions of 2.7 mg/km/vehicle (median 1.1 mg/km/vehicle) when including studies with resuspended tire wear. Notably, these factors are substantially lower than broadly cited and accepted factors in the secondary literature with mean emissions of 6.5 mg/km/vehicle (median 6.1 mg/km/vehicle). As revealed by our analysis, secondary literature reports emission factors systematically higher than those of the primary sources on which they are based. This divergence is due to misunderstandings and misquotations that have been prevalent since the year 1995. Currently accepted mass-based emission factors for directly emitted airborne tire wear particles need revision, including those from the United States Environmental Protection Agency and the European Environment Agency.

Figure 1

Illustration of our definitions for tire wear particles (TWP), tire and road wear particles (TRWP), brake wear particles (BWP), and “resuspension”. These definitions for TWP, TRWP, and BWP are limited to freshly emitted particles, while any resuspended particles are classified as “resuspension” to avoid double counting.

Figure 2

Overview of all 107 citations from 14 reviews to 34 references regarding emission factors for airborne tire wear particles. Accurate and inaccurate citations are highlighted by solid green and dashed red arrows, respectively.

Figure 3

Illustration of the citations from the 14 reviews. Every pie chart represents one review. The bar chart refers to the total of all of the reviews. References that were not considered by the reviews are colored light and dark gray.

Figure 4

Emission factors for light-duty vehicles and mixed fleets with unstudded tires: (A) TWP PM₁₀, (B) TRWP PM₁₀, and (C) TWP TSP and TWP+BWP PM₁₀. Accurate and inaccurate citations to primary literature, citations to secondary literature, and estimates from primary literature are shown as squares, triangles, circles, and crosses, respectively. The year refers to the year of publication from the primary study or the cited reference.

Figure 5

Histogram of emission factors for tire wear PM₁₀ in the reviews referring to primary sources (green background) or other secondary sources (blue background). Some primary sources were found by the reviews but not quoted for unexplained reasons (gray background). Differences between reviews and the cited sources (“corrected”) are shown in red to visualize the bias introduced by misquotations.