Elsevier

Atmospheric Environment

Volume 59, November 2012, Pages 533-539
Atmospheric Environment

Concentration and oxidative potential of on-road particle emissions and their relationship with traffic composition: Relevance to exposure assessment

https://doi.org/10.1016/j.atmosenv.2012.05.039Get rights and content

Abstract

Particles emitted by vehicles are known to cause detrimental health effects, with their size and oxidative potential among the main factors responsible. Therefore, understanding the relationship between traffic composition and both the physical characteristics and oxidative potential of particles is critical. To contribute to the limited knowledge base in this area, we investigated this relationship in a 4.5 km road tunnel in Brisbane, Australia.

On-road concentrations of ultrafine particles (<100 nm, UFPs), fine particles (PM2.5), CO, CO2 and particle associated reactive oxygen species (ROS) were measured using vehicle-based mobile sampling. UFPs were measured using a condensation particle counter and PM2.5 with a DustTrak aerosol photometer. A new profluorescent nitroxide probe, BPEAnit, was used to determine ROS levels. Comparative measurements were also performed on an above-ground road to assess the role of emission dilution on the parameters measured.

The profile of UFP and PM2.5 concentration with distance through the tunnel was determined, and demonstrated relationships with both road gradient and tunnel ventilation. ROS levels in the tunnel were found to be high compared to an open road with similar traffic characteristics, which was attributed to the substantial difference in estimated emission dilution ratios on the two roadways. Principal component analysis (PCA) revealed that the levels of pollutants and ROS were generally better correlated with total traffic count, rather than the traffic composition (i.e. diesel and gasoline-powered vehicles).

A possible reason for the lack of correlation with HDV, which has previously been shown to be strongly associated with UFPs especially, was the low absolute numbers encountered during the sampling. This may have made their contribution to in-tunnel pollution largely indistinguishable from the total vehicle volume. For ROS, the stronger association observed with HDV and gasoline vehicles when combined (total traffic count) compared to when considered individually may signal a role for the interaction of their emissions as a determinant of on-road ROS in this pilot study. If further validated, this should not be overlooked in studies of on- or near-road particle exposure and its potential health effects.

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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