Enhancing near-road exposure assessment

Non-tailpipe particle mass (PM) emissions are formed mostly from mechanical processes, in contrast with particles from tailpipe emissions, which are formed during fuel combustion. Because of the significant reduction of tailpipe PM emissions from new technology vehicles, interest in non-tailpipe emissions of motor vehicles is increasing. The main sources of non-tailpipe PM emissions from on-road vehicles include as follows: generation by abrasion of brakes and tires, generation by abrasion of the road surface, and re-suspension from the road surface (called road dust). Particles from non-tailpipe sources differ from those associated with tailpipe emissions in both composition and size distribution. Their size is generally larger than tailpipe particles, although such particles have less carbonaceous material and a higher metallic content (Liati et al. 2019; Nosko, Vanhanen, and Olofsson 2017). Specific chemical species considered to be tracers of non-tailpipe emissions include barium (Ba), copper (Cu), antimony (Sb), iron (Fe), and zinc (Zn) that are derived mostly from brake lining, pads, and rotors, as well as Zn and benzothiazoles for tire wear emissions (Grigoratos and Martini 2015; Denier van der Gon et al. 2013). Study of non-exhaust emissions is challenging because the composition of brakes and tires is highly variable, generally proprietary, constantly changing, and differs by application (e.g., light vs. heavy duty vehicles, traditional vs. low rolling resistance tires, new vs. retreaded tires). For example, Cu and certain other components are being phased out of brake pads as the result of a Memorandum of Understanding among the United States Environmental Protection Agency and various affiliated industry organizations (US EPA 2015). Furthermore, the composition of materials used to build roads, their wear, and the dispersal of dust from surrounding areas is variable.

Because regulations have so far been targeted almost exclusively at tailpipe emissions and vehicle miles traveled continue to increase, it has been predicted that non-tailpipe PM emissions and their contribution to near-road PM concentrations may exceed tailpipe PM emissions at some point in the future (Grigoratos and Martini 2015; Rexeis and Hausberger 2009), and may have already done so in some locations (Southern Ontario Centre 2019). Though there is currently no non-exhaust emission standard, recent work by the UN Particle Measurement Programme (PMP) has sought to design a standard method for testing brake-wear particles; a novel particle measurement setup for on-road investigation of brake wear particles (Farwick Zum Hagen et al. 2019) and a real-world braking cycle (Mathissen et al. 2019) have been reported. Non-tailpipe PM emissions include transition metals, which are of toxicological interest because they may cause pro-inflammatory effects (Gerlofs-Nijland et al. 2019). Limited epidemiological studies have also indicated that exposure to non-tailpipe emissions is associated with adverse health effects (Stafoggia and Faustini 2018). Thus, there is growing interest in understanding how these trends would affect exposures of individuals living near major roads.

Like tailpipe emissions, the impact of non-tailpipe emissions near the road is influenced by meteorological conditions, road surface characteristics, vehicle type, traffic composition and conditions, and local dispersion characteristics. Interactions among the different non-tailpipe sources make the identification of unique markers extremely challenging; for example, brake or tire particles may deposit on the road surface and subsequently be resuspended as part of road dust. They may also interact with tailpipe emissions or other ambient pollutants. Road dust also includes biological components (such as pollen), chemicals from various sources, and debris of various types from myriad non-transportation sources. Previous studies have sought to distinguish the different non-tailpipe PM emissions by identifying unique chemical markers of non-tailpipe particles in both ambient PM and road dust in some locations, such as mobile, near-roadway, or tunnel sampling; other studies have used laboratory-based approaches, such as dynamometers, to study PM generation by specific non-tailpipe sources under controlled conditions. In addition, previous studies have attempted to determine the contribution of the individual sources to ambient PM. However, a lack of detailed source profiles makes the conclusions from these studies uncertain.

The Health Effect Institute (HEI) has had a long interest in non-tailpipe emissions as relatively overlooked components of traffic-related air pollution (Health Effects Institute 2010; HEI Panel on the Health Effects of Traffic-Related Air Pollution 2010; HEI Special Committee on Emerging Technologies 2011). In 2014, HEI issued its request for preliminary applications (RFPA) 14-1A focused on “Enhancing Near-Road Exposure Assessment”. The overall objective of this RFPA was to characterize non-tailpipe PM emissions in proximity to roads (up to 500 m) that could be applied to future exposure and health assessment studies. Specific objectives included characterizing the size distribution and composition of tailpipe and non-tailpipe particle emissions; improving and standardizing sampling methodologies to characterize non-tailpipe emissions under real-world conditions; identifying the most important variables influencing spatial and temporal variability of non-tailpipe emissions; and identifying exposure surrogates for the different types of non-tailpipe PM emissions. As part of its Strategic Plan 2020–2025 (Health Effects Institute 2020), HEI continues to explore this area through a combination of funding research (e.g., an ongoing study by Meredith Franklin at the University of Southern California), an update to its earlier systematic review on epidemiological studies of traffic-related pollution, and a recent multi-expert workshop it convened to identify the most important next steps for research.

This special issue of the Journal of the Air & Waste Management Association includes four articles reporting on research from a study selected in response to the HEI RFPA 14–1A from the Harvard T. H. Chan School of Public Health, in which the authors proposed to characterize coarse and fine ambient PM and road dust near major roads in the Greater Boston area. Twenty-seven locations were investigated and the study roads were selected based on the following parameters: traffic density and composition, slope and elevation, number of traffic lights nearby, type of structures/buildings present, condition and cleanness of the pavement, number of lanes and traffic directions, width of road, posted speed limit, nearby point sources (minimal), and having a safe and convenient place for sampling. At each site, ambient PM and road dust samples were collected at a series of 3 distances away from the road: < 25 meters, 50–200 meters, and >500 meters. Most road sites were sampled once, a subset of sites were sampled multiple times (including different seasons). For this study, a mobile unit with ambient particle concentrators was developed to measure hourly concentrations of elements as a function of distance to road. In addition, a road dust sampler was developed to collect size fractionated road dust particles from the road surface. Together, the articles resulting from this study provide a useful case study on measurements of non-tailpipe particulate matter and its association with distance from road and ambient dust composition. They also present valuable examples of the gradients of traffic related particles as a function of the distance from road, which may be useful in future efforts for assessing exposures and the effects of tailpipe and non-tailpipe emissions. These four articles, presented in this special issue, are as follows:

• Martins, M., J. Lawrence, S. Ferguson, J. M. Wolfson, and P. Koutrakis. Development and evaluation of a mobile laboratory for collecting short-duration near-road fine and coarse ambient particle and road dust samples.

• Silva, E., S. Huang, J. Lawrence, M. Martins, J. Li, and P. Koutrakis. Trace element concentrations in ambient air as a function of distance from road.

• S. Huang, P. Taddei, J. Lawrence, M. A. G. Martins, J. Li, and P. Koutrakis. Trace element mass fractions in road dust as a function of distance from road.

• J. Lawrence, M. Martins, M. Liu, and P. Koutrakis. Measurement of the gross alpha activity of the fine fractions of road dust and near-roadway ambient particle matter.

Looking ahead, the results of these studies, and the conclusions of HEI’s recent workshop, should help point the direction toward the next generation of targeted research on non-tailpipe emissions and exposures, and ultimately analyses of potential health effects.