Concentrations of submicrometre particles from vehicle emissions near a major road

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Abstract

As part of a program of study to assess the exposure risks related to particulate matter in the outdoor environment, number concentrations of particles from vehicle emissions were measured at increasing distances from a major road. Particles in the size range from 0.015 to 0.697 μm were measured with the scanning mobility particle sizer (SMPS) and in the size range from 0.5 to 20 μm, with the aerodynamic particle sizer (APS). In addition to number concentration measurements, an approximation of PM2.5 fraction was obtained using a DustTrak (simple photometer). The measurements conducted at distances from the road ranging from 15 to 375 m showed, that for conditions where the wind is blowing directly from the road, the concentration of fine and ultrafine particles decays to around half of the maximum (measured at the closest point to the road) at a distance of approximately 100–150 m from the road. For the wind blowing parallel to the road, the reduction to half of the concentration occurs at 50–100 m. There is no effect on total particle number concentration at a distance greater than 15 m from the road when the wind is blowing towards the road and away from the sampling points. Total number concentrations of larger particles measured were not significantly higher than the average values for the urban environment, and decrease with distance from the road, reaching about 60% at 150 m from the road for wind from the road. PM2.5 levels also decrease with distance to around 75% for wind from the road and to 65% for wind parallel to the road, at a distance of 375 m.

Introduction

There is an increasing recognition that adverse health effects due to the exposure to airborne particulate matter could be more significant than due to the exposure to many other airborne pollutants. Throughout the last 30 years, there have been a number of scientific studies indicating that particulate air pollution can have an acute effect on human health, (review by Vedal, 1997). Standards for monitoring this pollution have been established; however, as technology improves in measurement techniques and more information becomes available on the relationship between exposure and health effects, it has become more apparent that existing standards may need to be revised to include monitoring of a finer particle size range.

Currently, there are standards in the USA for the mass of particulate matter less than 10 μm in aerodynamic diameter (PM10, – coarse particles), and less than 2.5 μm (PM2.5, – fine particles). PM2.5 was only recently introduced as a standard to the US in 1997, following the findings from epidemiological studies on the relationship between exposure to PM2.5 and health effects, and thus reflecting the growing recognition of the importance of fine particles in relation to health effects.

In an urban environment over 80% of particulate matter in terms of number is related to ultrafine particles (Morawska et al., 1998b), although their total mass is usually insignificant in comparison with the mass of the small number of larger particles. Ultrafine particles are considered here as those below 0.1 μm. Fine and even more ultrafine particles, typically contain a mixture of components including soot, acid condensates, sulphates and nitrates, as well as trace metals and other toxins. The information about these very small particles is not recorded when mass samples are taken during conventional PM10 and PM2.5 measurements; in fact, a majority presence of finer particles can be completely obscured by only a few larger, more massive particles. This can lead to a misrepresentation of the quantity of pollutants present and hence of the air quality and the potential health effects.

There is still very little information available on the production rate, airborne concentrations and fate of fine and ultrafine particles in the air. In an urban environment, motor vehicle emissions usually constitute the most significant source of fine and ultrafine particles. Thus, for exposure assessment, it is necessary to quantify particle emission levels, and also to determine particle behaviour after emissions, as they are transported away from the emission source – the road. There have been some studies conducted on behaviour of gaseous emissions at an increasing distance from the road, as well as on behaviour of mass concentrations of particles in terms of PM10 or PM2.5, but very limited studies on particle number concentration. A study focused on the nitrogen oxides and ozone close to a motorway was conducted by Kuhler et al. (1994). In the study the dispersion of NO, NO2, CO, and O3 concentrations was modelled, and concentrations of the same gases were measured at three distances from a motorway (50, 100, and 600 m). The model predicted gas levels (except for ozone), to increase to a maximum at a distance of 50 m, and to decrease from 50 to 600 m. This was confirmed by the data measured at 100 and at 600 m, however, the accuracy of the model at 50 m was not shown, as those concentrations were not included in the concentration versus distance graph. Another study examined PM10, PM2.5, NO2, black smoke, and benzene at four different distances from a major motorway (Roorda-Knape et al., 1998). In this study, monitoring sites were set up at approximately 50, 100, 150, and 300 m from a major motorway at two different locations. It was concluded that NO2 and black smoke concentrations decrease with increased distance from a road, whereas there is no significant decrease in concentrations of PM10, PM2.5, and benzene.

Janssen et al. (1997) compared the mass concentration and elemental composition of particles sampled near major roads and at background locations. The authors concluded that PM10 and PM2.5 concentrations were on average only 1.3 times higher near the road compared with the background readings, and black smoke (elemental carbon) readings were 2.6 times higher. These findings indicate that black smoke is more closely related to motor vehicle emissions than PM10 or PM2.5 fractions. In addition, the levels of Fe and Si (both elements are associated mainly with natural emission sources) were significantly higher in PM10, and to a lesser extent in PM2.5 samples near the road. This finding indicates a contribution from suspended road dust, which means that vehicle exhaust emissions may not be the main source for PM10 and PM2.5 in proximity to a busy road.

Another study (Clairborn et al., 1995) evaluated the PM10 emission rates from paved and unpaved roads out to distances of approximately 80 m using the tracer gas SF6. The assumption was that SF6 simulates particulate matter transport for the distances used in the experiment. There was a clear decreasing trend for concentrations of SF6 as distance increased.

In a pilot study, horizontal and vertical profiles of submicrometre particulates in relation to busy roads were examined by Morawska et al. (1999). The study selected two sites within the city area of Brisbane, one at a distance from the freeway of up to 210 m, and the other located at the junction of several major transport routes in the central business district (distance from freeway up to 73 m). Concentration measurements were made at a reference point close to the freeway and at increasing distances with two SMPS’ for the first site, and with one SMPS for the second site. There was no significant decrease in particle number concentration with distance from the road for the first site, but there was a decrease in concentration at the second site. This difference was thought to be due to the topography at the sites. Further investigation into a more open topography was recommended.

In view of the increasing importance of fine and ultrafine particles for exposure assessment, the aim of this study was to measure particle size distribution and concentration in the size range from 0.015 to 20 μm, at increasing distances from a road, and to conclude on the trends in particle characteristics for different wind conditions in relationship to the road.

Previous major studies at the Environmental Aerosol Laboratory (EAL) of the Queensland University of Technology (QUT) focused on direct vehicle emissions and included comprehensive examination of the gaseous and particle phases of city council bus exhaust (diesel fuel) (Morawska et al., 1998a), and of car exhaust (petrol and LPG fuel) (Ristovski et al., 1998). These studies presented information on the concentration and size distributions of particles in the exhausts of both cars and buses for different load and speed conditions for the test fuel used in the measurements. The majority of particles from the exhaust were found to be in the range 0.02–0.13 μm (diesel) and 0.04–0.06 μm (petrol). Thus, the lower particle size range chosen for the measurements presented here ensured that most of the particles which are directly related to motor vehicle emissions were detected. The upper particle size range enabled detection of particles which have little contribution to the total particle number, but a significant contribution to particle mass, and thus to PM10 and PM2.5 fractions.

Section snippets

Experimental techniques and procedures

The study design for the project included measurements of particle number concentration in the range from 0.015 to 0.697 μm, and from 0.5 to 20 μm taken using a scanning mobility particle sizer, and an aerodynamic particle sizer, respectively, at increasing distances from a major road. An approximation of PM2.5 was also measured at the same distances using a DustTrak. Measurements were made under different recorded wind conditions while temperature and relative humidity were also recorded.

Results

The results presented here relate to measurements of total particle concentration in the SMPS and the APS size ranges for different wind characteristics, and to changes in the particle size distribution for wind from the road. Wind categories on the presented figures represent the range of wind speeds which occurred on the day of measurement.

Discussion and conclusions

The horizontal distributions of particles from vehicle emissions near a major road were investigated and total number concentrations of particles were measured for three wind directions in relation to the road. There is a clear decrease in fine and ultrafine particle number concentration (in the range 0.015–0.697 μm) as distance from the road increases which indicates that these particles are related to vehicle exhaust emissions. For conditions where the wind is blowing directly from the road,

Acknowledgements

This project was funded by the Built Environment Research Unit, Queensland Department Housing and thanks to Chris Greenaway, Keith Eigeland and Brian Stockwell. Members of the QUT EAL, in particular, Milan Jamriska, are appreciated for their discussions and assistance with fieldwork.

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