The Aero Select was employed to sample aerosol particles in the air at a semi-rural site in the USA, and SEM images of particles in the aerosol samples were subsequently obtained by the RJ-Lee Group. The images show the complex morphology and chemical composition of particles in the air that we breathe every day (see figures 2.1-2.3).
Figure 2.1 shows a crystalline particle formed mainly from iron oxide. It is interesting that on the surface of the particle many small nanoparticles can be seen. These nanoparticles most likely became attached to the surface of the big crystal while the particle was airborne. The origin of the iron oxide particle is not known but it was most likely formed by nucleation from a gas phase.
Fig 2.1: An SEM image accompanied by and XRF element spectrum below.
Figure 2.2 shows an agglomerate of many spherical particles containing mainly carbon. The chemical composition confirms that the particles originated from a combustion process, for example they might have been generated by a diesel engine.
Fig 2.2: An SEM image accompanied by an XRF spectrum below.
Figure 2.3 is an example of the complexity of real atmospheric aerosol particles. The particle shown here is an agglomerate of many smaller spherical or compacted particles. The image shows the results of gas-to-particle interactions that have caused the shape of constituent particles to be smoothed, or possibly covered with a film formed by adsorption or by chemical reactions. The chemical composition is mainly C, S and O. Images were taken in narrow energy bands corresponding to emissions of the separate elements C and S. It was found that both elements are present in various particles but in different proportions, indicating that particles are internally mixed. These particles are most likely the result of burning biomass.
Fig 2.3: An SEM image obtained in the energy band corresponding to a single element. C (top left) and S (bottom left) accompanied by an XRF element spectrum (bottom right).
REFERENCE: RJ-Lee Group (2009)