The quantitative morphology of roadside and background urban aerosol in Plymouth, UK

Analysis of the fine urban aerosol (<1 μm) was made using direct sampling of urban aerosol on to porous carbon films (PCF). The efficiency of collection was low 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. PCF were used in the simultaneous collection of urban roadside and background aerosol, on four dates between March and August 1997 in Plymouth, UK. The aerosol was classified in to agglomerate and non-agglomerate groups. At the roadside agglomerate particles accounted for 94% of the particles analysed, this fell to 89% of the particles in the background samples. The analysis of agglomerate particles by energy dispersive X-ray spectroscopy confirmed their carbonaceous nature. A variety of agglomerate particles were found having coatings and mixed morphologies. The morphology of particles was analysed using two fractal analysis techniques to derive a density fractal dimension and a perimeter fractal dimension. These measures quantitatively describe the space filling quality and the roughness of the boundary of the two dimensional projection of the particle. The average perimeter fractal dimension (PFD) of aerosol was consistently significantly greater at the roadside than the background (+0.02), in analysis both including and excluding the non-fractal particles. There is evidence of a change in the average morphology of aerosol between roadside and background aerosol which is still the case when non-fractal particles are removed. This morphology change may be due to the inclusion of particles from other non-agglomerate sources, which have a low fractal dimension or it may be indicative of a smoother, aged roadside aerosol at the background site. The consistency of the difference between the roadside and the background aerosol morphology suggests that there is some morphology change that occurs in particles between the roadside and the background site.