Roadside measurements of fine and ultrafine particles at a major road north of Gothenburg

Abstract

Particle measurements were conducted at a road site 15 km north of the city of Gothenburg for 3 weeks in June 2000. The size distribution between 10 and 368 nm was measured continuously by using a differential mobility particle sizer (DMPS) system. PM_2.5 was sampled on a daily basis with subsequent elemental analysis using EDXRF-spectroscopy. The road is a straight four-lane road with a speed limit of 90 kph. The road passing the site is flat with no elevations where the vehicles run on a steady workload and with constant speed. The traffic intensity is about 20,000 cars per workday and 13,000 vehicles per day during weekends. The diesel fuel used in Sweden is low in sulphur content (<10 ppm) and therefore the diesel vehicles passing the site contribute less to particle emissions in comparison with other studies. A correlation between PM_2.5 and accumulation mode particles (100–368 nm) was observed. However, no significant correlation was found between number concentrations of ultrafine particles (10–100 nm) and PM_2.5 or the accumulation mode number concentration. The particle distribution between 10 and 368 nm showed great dependency on wind speed and wind direction, where the wind speed was the dominant factor for ultrafine (10–100 nm) particle concentrations. The difference in traffic intensity between workday and weekend together with wind data made it possible to single out the traffic contribution to particle emissions and measure the size distribution. The results presented in combination with previous studies show that both PM_2.5 and the mass of accumulation mode particles are bad estimates for ultrafine particles.

Introduction

There is an increased concern about human exposure to air particles originating from vehicle traffic. In an urban atmosphere the sources of particles could be divided into two major source categories, one that contributes mostly to the number concentration, while the other is related to the mass loading of particles (Ruuskanen et al., 2001). The traffic is without doubt one of the most dominant source of particles in urban areas. Parallel to this the research related to environmental problems associated with urban aerosols have been focused on respirable particles regarding health problems and on even larger particles regarding soil and deterioration of surfaces on buildings and other objects. The PM₁₀ and PM_2.5 concepts, measuring mass of particles with a diameter <10 and 2.5 μm, respectively, have been the dominant way to monitor particles (Harrison et al., 2000). Furthermore, the majority of available epidemiological and exposure studies of airborne particles are related to PM₁₀ or PM_2.5 with or without subsequent chemical analysis (McMurry, 2000; Harrison and Yin, 2000).

From numerous studies a link between elevated PM₁₀ particle concentrations and respiratory diseases has been found (e.g. Brunekreef and Dockery, 1995). Lately, there have been a few studies that do not focus on the inhaled mass but rather on the number or the total surface of particles inhaled (e.g. Oberdörster, 2001). One of the reasons for this approach is that the larger particles, carrying the bulk of the mass, are deposited in the upper airways and are more easily removed. Particles smaller than about 2.5 μm, on the other hand, can travel into the lungs and subsequently be deposited in the alveolar region. It has been shown that in the alveolus, the presence of a number of particles is more important to the adverse effect than total mass (e.g., Seaton et al., 1995). Donaldson et al. (1998) suggested that ultrafine particles (particles with an aerodynamic diameter <100 nm) could penetrate into pulmonary interstitial spaces and thus provoke inflammation.

Known urban sources of ultrafine particles, which contribute significantly to the number concentrations, are combustion processes and in particular traffic-related emissions. As traffic density is increasing rapidly in most parts of the world the problems with the emissions of traffic-related pollution is one of the hardest to solve, even if technical development can reduce the emissions from each vehicle substantially. At several urban sites there have been a decline in average mass concentrations while the number concentrations of particles are still increasing (Morawska, 2001). Nowadays, more measurements are available on number distributions and corresponding number concentrations at urban sites (e.g. Ruuskanen et al., 2001; Ruellan and Cachier, 2001; McMurry, 2000). Most of the results from such studies reveal some inconsistency in monitoring PM₁₀ as a substitute for number concentration measurements. Still, one must keep in mind that environmental and health problems linked to aerosols is a field of research where there are many aspects that are not yet known or fully understood. Consequently, many different types of studies are needed in order to be able to describe the role of aerosols.

The aim of this study is to evaluate and quantify the contribution of ultrafine particles from traffic close to a road with respect to the size distribution and major meteorological parameters. The site chosen was located outside the main city thereby limiting other nearby sources. The results are discussed and compared with previous studies focused on particles at urban sites.