Spatial & temporal variations of PM10 and particle number concentrations in urban air

Abstract

The size of particles in urban air varies over four orders of magnitude (from 0.001 microm to 10 microm in diameter). In many cities only particle mass concentrations (PM10, i.e. particles <10 microm diameter) is measured. In this paper we analyze how differences in emissions, background concentrations and meteorology affect the temporal and spatial distribution of PM10 and total particle number concentrations (PNC) based on measurements and dispersion modeling in Stockholm, Sweden. PNC at densely trafficked kerbside locations are dominated by ultrafine particles (<0.1 microm diameter) due to vehicle exhaust emissions as verified by high correlation with NOx. But PNC contribute only marginally to PM10, due to the small size of exhaust particles. Instead wear of the road surface is an important factor for the highest PM10 concentrations observed. In Stockholm, road wear increases drastically due to the use of studded tires and traction sand on streets during winter; up to 90% of the locally emitted PM10 may be due to road abrasion. PM10 emissions and concentrations, but not PNC, at kerbside are controlled by road moisture. Annual mean urban background PM10 levels are relatively uniformly distributed over the city, due to the importance of long range transport. For PNC local sources often dominate the concentrations resulting in large temporal and spatial gradients in the concentrations. Despite these differences in the origin of PM10 and PNC, the spatial gradients of annual mean concentrations due to local sources are of equal magnitude due to the common source, namely traffic. Thus, people in different areas experiencing a factor of 2 different annual PM10 exposure due to local sources will also experience a factor of 2 different exposure in terms of PNC. This implies that health impact studies based solely on spatial differences in annual exposure to PM10 may not separate differences in health effects due to ultrafine and coarse particles. On the other hand, health effect assessments based on time series exposure analysis of PM10 and PNC, should be able to observe differences in health effects of ultrafine particles versus coarse particles.

The Policy Relevance of Wear Emissions from Road Transport, Now and in the Future—An International Workshop Report and Consensus Statement

“…Mobile measurements on pavements with different maximum stone size but the same mineral composition indicate that smaller stone size gives rise to higher emissions if studded tires are used, but the differences are small and more tests are needed (Johansson et al, 2007b). Laboratory studies using a road simulator showed that there are large differences between different pavement rock materials and maximum stone sizes, especially when using studded tires in the tests.…”

Transport impacts on atmosphere and climate: Land transport

“…Brake wear and wear of the road surface is an important factor for the highest concentrations in the coarse mode (Manoli et al, 2002). The contribution to particulate mass ranges from place to place from a few percent up to 80% (Almeida et al, 2005;Johansson et al, 2007). It was also found that the traffic contribution in the coarse size fraction (1.9e72 mm) was approximately 80% up to 150 m from the road, it dropped abruptly by a factor of 2 over a distance of 150e200 m and declined further to 20% at 1500 m from the road (Wrobel et al, 2000).…”

Properties and toxicological effects of particles from the interaction between tyres, road pavement and winter traction material

“…During dry periods in winter and early spring, abraded pavement and sand are ejected into the air by vehicle turbulence and cause particle concentrations to vastly exceed the environmental quality standard set for inhalable particles (Johansson et al, 2007). This standard is valid for PM 10 and stipulates that the daily mean concentration must not exceed 50 µg m -3 more than 35 days a year and the yearly mean must not exceed 40 µg m -3 (EU, 1999).…”