Characterization of non-exhaust coarse and fine particles from on-road driving and laboratory measurements

doi:10.1016/j.scitotenv.2013.04.040

Science of The Total Environment

Volumes 458–460, 1 August 2013, Pages 273-282

https://doi.org/10.1016/j.scitotenv.2013.04.040 Get rights and content

Highlights

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We investigate the characterization of non-exhaust fine and coarse particles.
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The particles were measured under on-road driving and laboratory measurements.
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The dependence of particle mass & number on vehicle speeds and braking was observed.
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Chemical analysis can identify the main source of non-exhaust particles.

Abstract

We investigated the physical and chemical properties of non-exhaust coarse and fine particles generated by on-road driving and in a laboratory setting using a mobile sampling system. The on-road driving and laboratory measurements performed under constant speed driving revealed that particles produced by tire wear had a size distribution in the range of 2–3 μm, while roadway particles (RWPs) measured behind the front tire during on-road driving largely comprised crustal materials such as road surface wear particles and road dust as well as tire wear particles (TWPs). The mode diameters of particles obtained from on-road driving under cornering conditions were similar to those obtained under constant speed conditions, but with higher concentrations of crustal elements. Under braking conditions, the particulate matter (PM) concentrations of brake wear particles (BWPs) sampled near the brake pad increased significantly and were much higher than the concentration of RWPs during deceleration, indicating that BWPs are one of the main sources of non-exhaust emissions. In addition, BWPs observed from on-road and laboratory measurements had a broader PM size range (1–10 μm) than RWPs. Size-segregated chemical analysis of PM samples indicated that the concentrations of Fe and Ca were highest in the coarse fraction emitted under constant speed and cornering conditions, while Fe, Ba, and Ti were most abundant in the fine fraction emitted during braking events.

Graphical abstract

Introduction

In recent years, growing concern about climate change and air pollution has resulted in stringent legislative actions, such as limits for particulate matter (PM). To date, substantial reductions in traffic-related pollution have focused on exhaust emissions, but recent studies have reported that non-exhaust sources such as road dust, resuspension of materials from the pavement surface, and wear of vehicular parts (tires, brakes, etc.) contribute as much as exhaust emissions to traffic-related pollution (Harrison et al., 2001, Lenschow et al., 2001, Querol et al., 2004, Dahl et al., 2006).

In Korea, more than half of the annual rainfall is concentrated in the summer (Choi et al., 2010), and relatively dry conditions persist for the rest of year. For this reason, in metropolitan areas, large amounts of resuspended road dusts, which consist mainly of crustal matter, are considered one of the largest PM₁₀ emission sources, in addition to tire wear and brake wear emissions (Jung et al., 2006). De-icing salts distributed onto roads during winter are also present in high concentrations in road dusts during winter (Lee et al., 2013). However, in the current Clean Air Policy Support System (CAPSS) suggested by the Korean Ministry of Environment (KME), the contributions of non-exhaust sources to PM sectors are not accounted for due to the lack of published studies and no accurate estimation method for measuring non-exhaust emissions from vehicles (Kim et al., 2004).

Non-exhaust particles consist mainly of PM₁₀ (PM with an aerodynamic diameter less than 10 μm) and PM_2.5 (PM with an aerodynamic diameter less than 2.5 μm) and are of special concern due to their adverse effects on human health (Dockery and Pope, 1994). In particular, fine particles (PM_2.5) may cause respiratory diseases, and their concentrations are highly correlated with human mortality (Hoek et al., 2002, Pope et al., 2002). In view of the health risks, daily PM_2.5 will be regulated to 50 μg/m³ in Korea by 2015.

Several studies have been conducted to investigate the characteristics of tire wear particles (TWPs), which are particles collected during road simulator experiments (Camatini et al., 2001, Kupiainen and Tervahattu, 2005, Dahl et al., 2006, Gustafsson et al., 2009, Sjödin et al., 2010). Camatini et al. (2001) identified rubber particles from tire wear experiments in the laboratory and found Zn using SEM-EDX (Scanning Electron Microscope-Energy Dispersive X-ray Spectroscopy) analysis. Sjödin et al. (2010) investigated particle mass size distributions of three different tires using a road simulator. They also determined the chemical composition of TWPs and reported that the coarser fractions generated by studded tires were dominated by Si, Ca, K, and Fe.

In addition to investigating tire-related particles, several authors have characterized brake wear particles (BWPs) emitted from the brake lining during vehicle deceleration in the laboratory (Garg et al., 2000, Sanders et al., 2003, Iijima et al., 2007). Garg et al. (2000) determined mass size distributions and conducted a chemical analysis of BWPs in a closed chamber. They reported that the mode diameters of BWPs were < 0.1 μm and > 10 μm (bimodal), and Fe, Cu, Sb, and Ba were the most abundant elements. Sanders et al. (2003) performed similar experiments in an open system, and reported that the mode diameter of BWPs was 6 μm for those produced by mild braking and close to 10 μm for those produced by harsh braking.

These laboratory measurements cannot, however precisely reflect the characteristics of real-world on-road particles, because laboratory measurements, by definition, are conducted in a more controlled environment than actual driving conditions (Camatini et al., 2001, Kreider et al., 2010). To address this issue, on-road experiments in which particles are measured during actual travel using sampling devices mounted on a vehicle have been performed (Kuhns et al., 2001, Etyemezian et al., 2003, Hussein et al., 2008, Pirjola et al., 2010, Kreider et al., 2010, Mathissen et al., 2011).

To the best of our knowledge, however, no attempt has been made to simultaneously characterize tire wear and brake wear particles in both on-road and laboratory measurements. In this study, we designed an isokinetic sampling strategy to minimize sampling losses of coarse particles. On-road measurements were conducted to estimate the source profile of PM₁₀ and PM_2.5 produced by tire wear, brake wear, and road dust under different driving situations (constant speed, cornering, and braking) as the basis for a PM emission inventory in Korea. On-road measurements were compared to those obtained from a road simulator study to investigate the effects of road surface wear and resuspended road dust on PM emissions. In addition, size-resolved chemical composition of PM fractions collected on the proving ground under different driving conditions were analyzed to estimate the distributions of major and trace elements.

Section snippets

Mobile sampling vehicle

A schematic of the mobile sampling vehicle employed in this study is provided in Fig. 1. Our mobile sampling vehicle was based on the TRAKER designed by Kuhns et al. (2001). The vehicle had an unloaded weight of 1400 kg and was powered by a 2000 cc gasoline engine. Because studded tires are not used in South Korea, all experiments were conducted with summer tires (Kumho KH25 225/45/R18, UTQG (Uniform Tire Quality Grade Standards) tread wear 480). Sampling inlets were installed in front of the

Constant speed driving condition

The average PM₁₀ and PM_2.5 concentrations of RWPs and TWPs measured behind the front tire at vehicle speeds of 50, 80, 110, and 140 km/h are shown in Fig. 3(a) and (b), respectively. Note that RWPs were measured on the proving ground and TWPs on the chassis dynamometer of the road simulator. The y-axes have different scales for RWPs and TWPs (RWP concentrations were higher), and the variations (error bar: standard deviation) in the PM concentrations of RWPs were larger than those of TWPs. RWP

Constant speed driving

The PM mass concentrations and particle size distribution showed that the concentration of RWPs increased with increasing vehicle speed but started to decrease at vehicle speeds higher than 110 km/h. This may be because under high-speed driving conditions, high air resistance by upwinds or downwinds on the proving ground may reduce the contact area between the road surface and the tire, while in a road simulator hall, ambient air has no influence. However, further studies are required to confirm

Conclusions

In this study, we used a mobile sampling vehicle for on-road and road simulator measurements to determine the physical and chemical properties of coarse and fine particles emitted from non-exhaust sources under different driving conditions. In particular, we designed an isokinetic sampling method to minimize sampling losses of coarse particles in the size range of 2.5–10 μm by increasing aspiration efficiency and decreasing settling loss, and bend loss. Our findings indicate that RWPs consist of

Acknowledgments

The present study was supported by the Center for Environmentally Friendly Vehicles (CEFV) under the project “Development of the eco-friendly tire for reduction of carbon dioxide and tire wear particles” through the Ministry of Environment (ME, Republic of Korea).

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Characterization of non-exhaust coarse and fine particles from on-road driving and laboratory measurements

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Mobile sampling vehicle

Constant speed driving condition

Constant speed driving

Conclusions

Acknowledgments

J Aerosol Sci

Mater Characterization

J Aerosol Sci

Atmos Environ

Atmos Environ

Atmos Environ

Atmos Environ

Atmos Environ

Atmos Environ

Lancet

Atmos Environ

Atmos Environ

Sci Total Environ

Atmos Environ

Environ Res

Atmos Environ

Atmos Environ

Geochim Cosmochim Acta

Atmos Environ

Sci Total Environ

J Aerosol Sci

Standard Operating Procedure (SOP) for the trace elemental analysis of low-volume sample using Inductively Coupled Plasma–Mass Spectrometry (ICP/MS)