Particle size and composition distribution analysis of automotive brake abrasion dusts for the evaluation of antimony sources of airborne particulate matter

Abstract

Abrasion dusts from three types of commercially available non-steel brake pads were generated by a brake dynamometer at disk temperatures of 200, 300 and 400 °C. The number concentration of the abrasion dusts and their aerodynamic diameters (D_p) were measured by using an aerodynamic particle sizer (APS) spectrometer with high temporal and size resolution. Simultaneously, the abrasion dusts were also collected based on their size by using an Andersen low-volume sampler, and the concentrations of metallic elements (K, Ti, Fe, Cu, Zn, Sb and Ba) in the size-classified dusts were measured by ICP-AES and ICP-MS. The number distributions of the brake abrasion dusts had a peak at D_p values of 1 and 2 μm; this peak shifted to the coarse side with an increase in the disk temperature. The mass distributions calculated from the number distributions have peaks between D_p values of 3 and 6 μm. The shapes of the elemental mass distributions (Ti, Fe, Cu, Zn, Sb and Ba) in size-classified dusts were very similar to the total mass distributions of the brake abrasion dusts. These experimental results indicated that the properties of brake abrasion dusts were consistent with the characteristics of Sb-enriched fine airborne particulate matter. Based on these findings and statistical data, the estimation of Sb emission as airborne particulate matter from friction brakes was also discussed.

Introduction

The atmosphere contains airborne particulate matter (APM) of different sizes. Since fine particles with diameters of less than 2.5 μm (PM2.5) enter the lungs during respiration and remain in their peripheries, they may cause an adverse effect on human health (Calcabrini et al., 2004; Oberdoerster, 2005). According to an epidemiological study, there is a significant relationship between the concentration of PM2.5 and human mortality (Borja-Aburto et al., 1998). It was reported that a 10 μg m⁻³ increase in PM2.5 would be associated with a 1.4% increase in mortality (Borja-Aburto et al., 1998). With regard to the respective chemical species, toxic substances with carcinogenicity and chronic toxicity, such as polycyclic aromatic hydrocarbons and toxic metallic elements, are found in PM2.5 collected in an urban district (Gao et al., 2002; Lim et al., 2005). According to previous studies, there appears to be a significant relationship between these toxic substances and chronic diseases such as cardiopulmonary function failure, genotoxic hazard, and so on (Lippmann and Ito, 2000; Magari et al., 2002).

The results of the long-term monitoring of APM in Tokyo (Furuta et al., 2005) indicate that fine particles with a diameter of less than 2 μm are extremely enriched with antimony (Sb) when compared with crustal materials. Sb-enriched APM have also been observed in several countries, e.g. USA (Gao et al., 2002), Germany (Weckwerth, 2001) and Argentina (Gomez et al., 2005). Furthermore, Krachler et al. (2005) reported the increasing of atmospheric Sb contamination in the northern hemisphere from the observation of snow and ice core samples. In our previous study, particle shape could be morphologically classified into three types (spherical, cotton-like and edgy-shaped; typical images were shown in the paper (Furuta et al., 2005)), and the results of the composition analysis by SEM-EDX for single particle have revealed that a vast majority of edgy-shaped fine particles contain high concentrations of Sb. This observation suggests that such particles are produced by the mechanical abrasion of Sb-enriched materials. Some non-steel (non-asbestos organic) brake pads used in the disk brake system of an automobile contain Sb₂S₃ as a solid lubricant that reduces the wear of friction materials under high load conditions (Jang and Kim, 2000). According to the morphological observation by using SEM, brake abrasion dusts were mainly edgy-shaped as we expected (Furuta et al., 2005). Hence, the brake abrasion dust generated by the friction brake of automobiles might be one of the possible sources of Sb-enriched APM (Weckwerth, 2001; Sternbeck et al., 2002; Pakkanen et al., 2003; Adachi and Tainosho, 2004; Lough et al., 2005; Furuta et al., 2005). In addition, Uexküll et al. (2005) have suggested that some amount of Sb₂S₃ could be oxidized to Sb₂O₃, one of the possible carcinogenic compounds, by frictional heating during braking (IARC, 1989). Thus, we should pay more attention to automotive brake abrasion dusts from the viewpoint of toxicology. However, few studies have investigated the physical or chemical properties of the brake abrasion dusts such as the particle size distributions and size-classified metallic element compositions. Garg et al. (2000) used a micro-orifice uniform deposit impactor (MOUDI) to demonstrate that some particle mass distributions of the brake abrasion dusts generated by a brake dynamometer installed in a closed chamber show a bimodal (aerodynamic diameter range <0.1 and >10 μm) profile. Sanders et al. (2003) have also used a MOUDI and reported that the particle mass distribution of the brake abrasion dusts generated by a brake dynamometer installed in an open system covered with a hood exhibits a peak at approximately 6 μm, which was considerably different from the results of Garg et al. (2000). However, no reports are available for the distributions of elemental concentrations (including Sb) in size-classified abrasion dusts. In order to demonstrate that brake abrasion dusts are one of the important Sb sources in fine APM, it is necessary to determine the particle size distribution of abrasion dusts and the elemental distribution in size-classified dusts emitted during the actual braking process.

In this study, we measured the aerodynamic particle size distributions of the brake abrasion dusts produced by a brake dynamometer; additionally, we analyzed the concentrations of metallic elements present in the size-classified abrasion dusts that were produced in the dynamometer abrasion tests. Moreover, the estimation of Sb emission from the automotive brake abrasion dusts is also discussed.