Characterisation of particle emissions from the driving car fleet and the contribution to ambient and indoor particle concentrations

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Abstract

The population is mainly exposed to high air pollution concentrations in the urban environment, where motor vehicle emissions constitute the main source of fine and ultrafine particles. These particles can penetrate deep into the respiratory system, and studies indicate that the smaller the particle, the larger the health impacts. The chemical composition, surface reactivity and physical properties are also important. However, the knowledge about chemical and physical properties of particles and the temporal and spatial variability of the smallest particles is still very limited. The present study summarises the first results of a larger project with the aims to improve the knowledge.

The concentration and the emissions of ultrafine particles from petrol and diesel vehicles, respectively, have been quantified using Scanning Mobility Particle Sizer of ultrafine particles in the size range 6–700 nm and routine monitoring data from urban streets and urban background in Denmark. The quantification was carried out using receptor modelling. The number size distributions of petrol and diesel emissions showed a maximum at 20–30 nm and non-traffic at ≈100 nm. The contribution of ultrafine particles from diesel vehicles is dominating in streets. The same technique has been applied on PM10, and ≈50% contribution from non-traffic. The technique has also been introduced in relation to elemental and organic carbon, and the first data showed strong correlation between traffic pollution and elemental carbon.

The outdoor air quality has a significant effect on indoor pollution levels, and we spend most of the time indoors. Knowledge about the influence of ambient air pollution on the concentrations in the indoor environment is therefore crucial for assessment of human health effects of traffic pollution. The results of our studies will be included in air quality models for calculation of human exposure. Preliminary results from our first campaign showed, that the deposition rate of particles in the apartment is negligible in the particle size range 100–500 nm. In the size range below 100 nm the deposition rate increases with decreasing particle diameter to a value of approximately 1 h−1 at 10 nm. The penetration efficiency shows a maximum of 60% at 100 nm. More detailed studies of exchange of particles in outdoor/indoor air and the transformation are planned to take place during three next campaigns.

Introduction

Effects of human exposure to air pollution include short term and long term health effects. People suffering from respiratory diseases, especially allergy and asthma, and from cardiovascular diseases have been identified to be especially sensitive to air pollution.

American risk assessment studies (Pope et al., 1995) hypothesise that an increased mortality of around 50,000 deaths per year is associated with the present particle levels in the US, as a comparison in Great Britain the corresponding number is estimated to be around 10,000 deaths per year. In Denmark the authorities for environment and health have evaluated that the number of annual deaths may be reduced by about 400 people per million, if the particle concentrations (given as PM2.5) in Danish cities are reduced by 1/3 (Danish Ministry of Environment/Danish National Board of Health 2000). These estimates have been based on the studies by Dockery et al. (1993) and Pope et al. (1995) in which an increase of 10 μg/m3 PM2.5 (as annual average) was associated with a relative risk for total mortality of 1.14 and 1.07, respectively. In the epidemiological studies conducted over the past ten years a very consistent quantitative picture has emerged between the levels of air pollution (especially fine fraction particles) and increases in morbidity and mortality (Lippmann et al., 2000; Wichmann et al., 2000; Samet et al., 2000a, Samet et al., 2000b).

Particles are often quantified by the mass in terms of PM10 and PM2.5, which expresses mass of particles with a diameter below 10 and 2.5 μm, respectively. Many studies suggest that the correlation between particle concentration and health effect increase with decreasing particle diameter. It is therefore important to determine the concentration, e.g. given as number of particles, in many size intervals in order to have optimum exposure assessment to relate to health effects.

The new EU directive “Council directive 1999/30/EC of 22 April 1999 relating to limit values for sulphur dioxide, nitrogen dioxide and oxides of nitrogen, particulate matter and lead in ambient air” gives limit values for particulate matter (PM10). The directive also includes obligations for the Member States to collect data on smaller particles, PM2.5. However, investigations have shown that the correlation between particle concentration and health effect increases with decreasing particle diameter (Dockery et al., 1993; Pope et al., 1995), which means that future revisions of the EU directives may include smaller size fractions. WHO has not recommended a limit value for particulate matter, because no lower adverse effect level has been identified and more research is necessary.

A major contribution to particulate pollution in urban areas is believed to be attributed to traffic, and especially to emissions from diesel fuelled vehicles. Ultrafine particles emitted from petrol as well as diesel engines are formed at high temperature in the engines, in the exhaust pipe, or immediately after emission to the atmosphere. Some of these particles may be in the so-called nucleation mode (nano-particles <30 nm). The dominating ultrafine particle mode has a number concentration peak in the range 30–100 nm. These particles are often formed by coagulation of primary particles, and by condensation of gases on particles. The fine particles (accumulation mode in the range 0.1–2 μm) are typically secondary particles formed by chemical reactions (e.g. SO2 and NOx to form sulphate and nitrate), or other relatively slow processes in the atmosphere. The coarse mode is the particles >2 μm, which in urban areas typically are formed mechanically by abrasion of road material, tyres and brake linings, soil dust raised by wind and traffic turbulence, etc. These coarse particles may also cause health effects.

The fine and ultrafine particles will be deposited deep in the lungs and the residence time will be very long, up to several months (WHO, 2000). The chemical and physical properties are important for assessment of deposition in the lungs and assessment of the adverse health effects as the deposition is strongly influenced by water uptake of the particle in the humid lungs. The important properties––in addition to size––are state (liquid/solid), volatility, hydroscopicity, chemical composition (content of organics, metals, salts, acids etc.), morphology, and density. These properties are also important for selection of methods for regulation and control of emissions. The test cycles used to measure the emission from engines are based on measurements using dilutions systems, while the car runs on a dynamometer. This method is often not comparable with the real emission conditions in a street. The control parameter is usually the particle mass and that will be dominated by the larger particles. The mass does often only include solid particles, as exhaust gas is heated to e.g. 300 °C, when the measurement takes place. This will remove any semi-volatile compounds, e.g. organic condensates, which would be present in emissions in urban areas. However, these particles/droplets will also be transported and deposited in the lungs.

It is therefore important to measure and determine the particle emission under normal driving conditions in ambient air, in order to establish the relationship between the sources and the exposure of the population. Selected results from our studies of ultrafine particles in urban streets are presented in this paper.

The population spends most of the time indoors. The ambient (outdoor) air quality has a significant effect on indoor pollution levels (Lai and Nazaroff, 2000; Long et al., 2001). Quantitative knowledge about the influence of ambient air pollution on the concentrations in the indoor environment is therefore crucial for assessment of adverse health effects of traffic pollution. Traffic generated particles that penetrate to the indoor environment will be transformed by physical and chemical processes and reaction with other air substances during transport. There is thus a strong need for quantifying the processes governing the particle composition and size distributions also in the indoor environment.

Section snippets

Objectives

A four-year project on particle studies was initiated in 2000/2001. The main objectives are:

  • To characterise the geographic and temporal variability in particle composition and size distributions in Danish ambient air.

  • To determine particle emission factors for various vehicle categories.

  • Determine indoor–outdoor relationships for buildings in busy streets.

  • Determine the role of traffic emissions in formation of indoors particulate irritants.

This paper gives a summary of preliminary results, which

Experimental methods

Measurement sites. Most of the measurements in these studies were performed in central Copenhagen in a street canyon, Jagtvej, which is a 10 m wide main road and during rush hours in practise a four lane road. Both sides of the roadway are bicycle lanes and pavement. In addition, Jagtvej is lined on both sides by 5–6 storey buildings. The traffic density is ≈26,000 vehicles per 24 h, including 6–8% heavy vehicles, i.e. buses, lorries and larger vans. A fixed monitoring station of the Danish Air

Results

Average weekly cycles of the entire measurement periods at the street stations of particles, NOx and CO concentrations were generally used for the analysis (the cycles of CO, NOx and total particles at Albanigade can be seen in Fig. 1). A clear diurnal variation of the three parameters with a sharp rush hour peak in the morning and another rush hour peak especially for CO during the afternoon was observed on workdays at both street stations. The pattern is different on Saturdays and Sundays.

Data analysis and discussion

Receptor modelling of particles contributions from petrol and diesel vehicles and non-traffic. The data analysis was based on long time series of air pollution concentrations measured routinely at the normal air quality monitoring stations and measurement campaigns of the pollutants under investigation, e.g. particulates (Palmgren et al., 2001b). The time series from the campaigns, typically for several months, were analysed using statistical methods. Traffic data for different vehicle

Conclusions and future activities

Traffic is the dominating source of ultrafine particles in busy streets, but also contribution to PM10 is significant. The application of averaged PM data, collected continuously, in combination with routine monitoring data and manually counted traffic rates, is a powerful tool to determine contribution and emission factors of particles from diesel and petrol vehicles from the actual car fleet under normal driving conditions in cities. The method is useful for demonstration of the effect of

Acknowledgements

The Ministry of Energy and Environment and the Environmental Research Programme supported this work. Rita van Dingenen and Fran Raes at JRC, Ispra are acknowledged for providing the design of the SMPS systems. Erik Swietlicki at the University of Lund is acknowledged for production of the DMAs and Ole John Nielsen and Merete Bilde at the University of Copenhagen for the co-operation during the experiments.

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