Comparative Study of Single Particle Characterisation by Transmission Electron Microscopy and Time-of-Flight Aerosol Mass Spectrometry in the London Atmosphere

Abstract

Size fractionated airborne particle samples were collected from the top of the BT Tower and a ground-based site in Regents Park in London by deploying MOUDI instruments at the two locations. Particles were examined by Transmission Electron Microscope (TEM), Energy Dispersive X-Ray Analysis (EDX) and selected area electron diffraction patterns to determine the morphology, crystallinity and composition of individual particles in aggregated and non-aggregated forms. Eight different types of particles were observed which were common both to the ground-based location and some 160 m above the conurbation of London at the top of the BT Tower. Of these different types, amorphous carbonaceous aggregates (soot-like) containing C and O and often extending to the inclusion of Ca, K and Fe were the most common type of particles that occurred across all stages of the impactors but were most common in the fractions <1.2 mm. Other types of particles included beam-sensitive sulphur-rich particles containing Na and Cl that were present in crystalline and amorphous forms. Iron and titanium were common elements in other types of particles. Concurrent ATOFMS measurements were taken at the Regents Park site and 13 particle types were found. An intercomparison of the two techniques found three types of particle (out of eight) identified by the TEM, which had analogues (but not direct equivalents) in the ATOFMS clusters. Many of the particle types identified by the ATOFMS are vacuum-volatile (e.g. nitrates) and are therefore not seen by the TEM. The relative strengths and weaknesses of the two techniques are considered and it is concluded that both have major weaknesses but that they tend to complement one another.