The presence of particulate matter in air (small powder particles in air) is relevant for our health. As a general rule, "small powder particles" are all particles with dimensions less than 1 micron (1/1000 mm) suspended in the atmosphere. To evaluate the degree of atmospheric pollution, sampling of suspensoids is performed. The collected particles are analyzed and the result is given as weight per air unit volume. Sometimes the average chemical analysis is also performed. However, no information is available on particle dimension and shape and on the true chemical composition of each particle. Instead, this information is available by SEM. Particles as small as 0.3 microns can be analyzed and the analysis allows identifying the sources of environmental pollution. As a consequence, prevention and air pollution monitoring can be improved. The microstructural approach to environmental pollution is relevant for works of art conservation also. When particulate matter or agents come into contact with frescoes, historical buildings or metal objects, damage mechanisms develop from the surface to the bulk of materials. In this case, the study of particulate matter both inside and outside buildings allows to detect the damage mechanisms and to set up conservation methods.

Here is an example of particulate matter analysis. Sampling was performed in Castiglione Olona, 100 m from the nearest high-traffic road, on January 31, 2001, between 6.00 GMT and 7.00 GMT. Weather was fine, without wind.

Weather conditions, time and place of sampling are fundamental for a correct interpretation of the collected data.
Particles collected on a filter. A few large particles with diameter 2 to 10 microns are visible. Smaller particles can be detected (white dots) as well as the filter pores (round dark features).
Sub-micron particles, harmful to human health, can be observed. The morphology of this fine powder indicates that the pollution source is mainly hydrocarbon combustion (combustion engines, heating systems). Spherical particles are typical of hydrocarbon combustion, are characterized by high carbon content and contain sulfur as well. The spherical particle is about 1 micron large.
Higher magnifications (about 20.000) reveal that the small powders are composed by a few microns particles, building up chains and clusters of larger dimensions. The round dark features are the pores of the sampling filter.
Some particles originate from non-antropic sources. The central particle is an example of that. It contains the chemical elements shown in the microanalysis spectrum below. At the left top of the image there is a large afterburnt conglomerate.
The above microanalysis spectrum is obtained contemporarily to the image, thanks to the microanalysis system attached to the electron microscope. The spectrum shows the chemical elements that constitute the particle at the center of the previous photograph. Each peak refers to an element and peak height is roughly proportional to the elemental concentration. The particle is about 1 micron large. It turns out to be an iron oxide, containing silicon and copper in trace.
The above microanalysis spectrum shows the elements that constitute the particle cluster in the square and in the upper photograph. The main element is carbon, confirming that the cluster is an afterburnt.
Here are some other natural particles, less harmful to health. The chemical analysis of one of them is shown in the image below.
In the previous photograph the arrow-marked particle is a silicon-aluminum oxide, with potassium, iron and magnesium traces.

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Last updated on February.12.2006