The Measurement and Characterisation of Aerosol in the Urban Atmosphere (PM10) and an Evaluation of the Sources of these Particles by Number

Andrew L. Dye

October 1998

Department of Environmental Sciences, University of Plymouth

The link between human health and the mass of fine particulate matter below 10 mm (PM10) in air is well documented. Current research suggests that the number, size and shape of particles may be of most concern and that in the urban atmosphere combustion sources of PM10, especially diesel engine sources, dominate the fine (< 1 mm) and ultra-fine (< 0.1 mm) particles. Despite this, the number, size and shape of particles in urban air has not been reported to any great extent or detail, and the percentage contribution to the numbers of particles from different sources is largely unknown. The objectives of this research were to characterise fine particles with respect to their morphology and thus apportion the sources of particles by number.

Urban aerosol above 1 mm was initially examined to study the fluctuations in PM10 number and make retrospective analysis of periods of elevated PM10 for source identification in Plymouth, UK. Aerosol was collected via a Burkard spore trap and examined using light microscopy with image analysis between 16 March 1995 and 31 August 1996, at a background site in Plymouth, UK. Two periods, 19 January-4 February and 10-25 March 1996, identified as UK wide PM10 episodes, were retrospectively studied and compared with PM10 mass measurements. The mean number count for the whole period was 10.5 x 104 ± 7.9 x 104 particles m-3. The two PM10 episodes had elevated average number concentrations of 13.5 x 104 ± 7.6 x 104 particles m-3 for 19 January - 4 February 1996, and 13.0 x 104 ± 9.7 x 104 particles m-3 for 10-25 March 1996. During the periods of elevated PM10 the tapered element oscillating microbalance (TEOM) mass of particles had a low correlation with the particles less than 5 mm and an increased correlation to the particles greater than 5 mm in size. Outside of these peak periods the PM10 TEOM mass was most closely correlated with the number of particles less than 5 mm in size. This work shows the difference in urban aerosol during periods of air quality guideline exceedence. These findings agree with literature that an aged continental aerosol source has a key role in the generation of UK wide PM10 mass exceedances.

Further analysis of the fine urban aerosol (< 1 mm) was made using direct sampling of urban aerosol on to porous carbon films (PCF) developed in this research. The efficiency of collection was low (ca. 5%) but the samples were representative and enabled transmission electron microscopy (TEM) for sub-micron particle analysis. Measurement was made of the fractal dimensions and diameter of particles. This was used to identify any ageing and ultimately the sources of aerosol.

PCF were used in the simultaneous collection of urban roadside and background aerosol, on seven dates between December 1996 and August 1997 in Plymouth, UK. The average perimeter fractal dimension (PFD) of aerosol was consistently significantly greater at the roadside than the background (+ 0.02), indicative of a smoother, aged aerosol at the background site.

The sampling of a variety of combustion engines was made for source identification purposes. The particle morphology produced from the diesel engines showed great uniformity of particle morphology with varying speed and load; no consistent significant differences were found. The morphology results were comparable to other density fractal dimensions and perimeter fractal dimension values found in other studies for diesel. A natural log relationship between the median particle size and the median PFD was found for the diesel engine sources but not in petrol samples. This natural log trend was considered as a tentative 'fingerprint' of diesel engine combustion and was in harmony with literature values of PFD for diesel engine particles.

Using the fractal measures, size and particle classification the bulk of aerosol was identified as from hydrocarbon combustion sources; ca. 88-92% of the roadside and ca. 77-86% of background. A component of carbon ceno-spheres were identified contributing ca. 6-12% of both the roadside and background aerosol. Non-combustion particles increased from ca. 1-4% of the roadside to ca. 7-9% of the background, as did the proportion of aged combustion particles, from 0-1% of roadside to 2-3% of the background aerosol. A strong correlation for the median size vs. PFD morphology curve between, the roadside and diesel sources (0.93 - 0.95) and the background and petrol sources was found (0.95). The roadside aerosol was significantly different to the petrol source in the 120-220nm size range (p=0.007) and there was a low correlation of the petrol and the roadside size vs. morphology curve (0.66). This suggests the domination of roadside aerosol by diesel engine particles. The background aerosol was similar to both diesel and petrol engine sources, especially from a dilution tunnel, thus indicative of a mixture of sources and an aged combustion aerosol. Roadside sources thus dominate the fine and ultra fine urban aerosol by number as compared to most other studies which have only apportioned the sources of particles in the air by mass.

© 1998 by Andrew L. Dye. All Rights Reserved

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