Concentration and oxidative potential of on-road particle emissions and their relationship with traffic composition: Relevance to exposure assessment
Highlights
► Reactive oxygen species might cause several adverse health effects of particles. ► We measured particle associated ROS on under and above-ground roads. ► Particle associated ROS was best correlated with total traffic volume. ► Non-diesel vehicles shouldn't be overlooked in exposure assessment and modelling.
Introduction
Many studies have demonstrated detrimental health effects due to airborne particles, especially those in the ultrafine size range (Dp < 100 nm, UFP) (Donaldson et al., 1998; Nel, 2005; Li et al., 2003; Oberdörster, 2001). In urban environments, vehicle emissions are the major source of UFPs (Morawska et al., 2008). The toxicity of particles emitted by vehicles is thought to be related to both their chemical composition and size (Donaldson et al., 1998; Lin et al., 2008). A well-promoted mechanism via which the chemical composition of particles may cause cellular damage is oxidative stress (Nel, 2005). Therefore, characterizing and quantifying vehicle particulate emissions and their potential to cause oxidative stress under real world conditions is necessary to better appreciate their human health effects.
Many of the adverse health outcomes associated with exposure to fine (Dp < 2.5 μm, PM2.5) and ultrafine particles have been attributed to oxidative stress caused by the presence of reactive oxygen species (ROS) on these particles, and generation of free radicals and related ROS at their sites of deposition (Dellinger et al., 2001; Nel, 2005; Li et al., 2003). An indication of particle toxicity can be inferred by measuring particle associated ROS, which have been detected in diesel and gasoline emissions (Cheung et al., 2010; Surawski et al., 2009). However, while measurements of ROS in-vehicle emissions under relatively well-controlled conditions in dynamometer studies have established their presence and variability, the relationship between vehicle emitted ROS and road and traffic parameters under real world conditions is not well defined. On-road UFP concentrations are highly dynamic, especially in tunnel environments (Kumar et al., 2011), and on-road measurements can capture the characteristics of concentrations entering vehicle cabins (Gouriou et al., 2004; Knibbs et al., 2009). Road tunnels can be an especially high exposure microenvironment (Knibbs et al., 2009, Knibbs et al., 2011), and are excellent locations in which to measure vehicle emissions, as the influence of meteorological conditions is minimised (Kumar et al., 2011). On-road measurements can also provide information on the influence of tunnel roadway factors, such as gradient and ventilation, on vehicle emissions (Chang et al., 2009; Colberg et al., 2005; Gouriou et al., 2004; John et al., 1999). While several studies have shown that the road gradient in a tunnel can affect the concentrations of gaseous pollutants (Chang et al., 2009; Colberg et al., 2005; John et al., 1999), there have only been limited on-road measurements of the effects on vehicle particle emissions (Gouriou et al., 2004).
This study aimed to contribute towards addressing the knowledge gaps outlined above by quantifying the effect of tunnel characteristics, namely gradient and ventilation, on particle concentrations. In addition, we aimed to assess the oxidative potential of particles present in the tunnel under real world conditions. Finally, we aimed to determine the effects of traffic number and composition on the concentration of UFPs, PM2.5 mass and particle associated ROS in the tunnel. Through this, we sought to better understand the physical and chemical characteristics of particles to which people are exposed in on-road microenvironments.
Section snippets
Setting
Measurements were conducted in a recently opened (March, 2010) road tunnel in Brisbane, Australia. The tunnel runs in a north-south direction and consists of two 4.5 km unidirectional bores, each with two lanes, and with a maximum speed of 80 km h−1. The tunnel is currently Australia's longest. There are two northbound entries to the tunnel, one of which is 1.5 km along the tunnel, and a single exit at the northern portal. The southbound bore has one entry and two exits, with one exit 3 km
Overall results
The average total vehicle counts per bore in the tunnel during campaigns I, II and III were 1728, 1029 and 1112 vehicles h−1, respectively. The decrease in count after campaign I was due to the introduction of a toll. The traffic composition was similar, with short vehicles representing 79, 77 and 74.3%, medium vehicles 15.6, 17 and 19.6%, and long vehicles 5.4, 6.3 and 6.1% of the average total traffic during the 3 campaigns, respectively.
The average time taken to travel through the northbound
Conclusions
Despite the absence of strong relationships between on-road particle concentrations and other parameters, we used a simple technique to estimate concentration profiles along a 4.5 km tunnel roadway. This could be combined with models for predicting in-cabin concentrations for the purposes of exposure assessment.
PCA found total traffic volume was better related to UFP, PM2.5 and ROS concentrations in the tunnel than either HDV or gasoline vehicle volume individually. ROS levels on an open road
Acknowledgements
We thank River City Motorway Group for their involvement and assistance. This project was supported by ARC Linkage Grant LP0882544 “Quantification of Traffic Generated Nano and Ultrafine Particle Dynamics and Toxicity in Transit Hubs and Transport Corridors”. LDK acknowledges an IHBI Human Health and Wellbeing Early Career Researcher Grant.
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