Characterisation of airborne particles in London by computer-controlled scanning electron microscopy

Abstract

This study assessed the personal exposure of cyclists and Underground train users in London to particulate matter below 5 μm in diameter (PM 5) and provides evidence of the number, shape, size distribution and elemental composition of collected particles. Samples were analysed using computer-controlled scanning electron microscopy (CCSEM) and energy dispersive X-ray detection (EDX), including analysis of samples for low energy elements (carbon) by open window detection. Results were processed and classified using a custom written software package (MIDAS). A total of 33 938 particles were analysed for size and 12 568 particles were classified for size and elemental composition. Samples were also collected for gravimetric analysis. Thirty volunteers cycling commuter routes into central London were selected and monitored according to particulate matter for 1 week during November 1995–February 1996. Samples were also collected by three commuters using London Underground during their daily commuter journeys as a comparison. Cassella personal sampling pumps fitted with cyclone heads incorporating filters were used to collect particles. Carbon particles are clearly the dominant particle type in the road traffic samples with mean particle fractions of 66% carbon. The size distribution of the aerosol sampled by cyclists — high numbers of the smallest sized particles — is typical of vehicle emissions. Samples from the Underground show a distinctly different size distribution and elemental composition. Samples exhibited a higher loading of coarse mode particles with a more even distribution across the particle sizes collected. The most abundant particles in the Underground are Fe/Si-rich particles with 53% (56% in the 20-kV range) of the total number of particles. The average Fe concentration in this particle class was 22.8% and the Si concentration 17.4% together with C, Ca and K. The particle mass concentration in the London Underground trains proved to be almost 10 times higher than those measured by cyclists in traffic generated aerosol.