Primary and secondary particles in the air

Liquid and solid particles enter the air predominantly from natural sources only 11% comes from human activity. From these particles, so-called secondary particles are formed directly in the atmosphere, either during a change in the state of matter or in interactions of imissions connected with a formation of liquid and solid products. 

The scale of natural sources of particles is very wide. They enter into the air during volcanic activity, via wind transport from the earth s surface, during forest fires, by sea water spray, and they can also arrive from the interplanetary space in the form of meteorites. The group of particles of the natural origin also includes microorganisms, spores and pollen. 

Among the various anthropogenic sources, the transport and domestic sources are most important, energy production and industry being second to them as regards contributions to emissions of particles. Relative contributions vary considerably depending on the locality (industrial centres, 

The chemical compositions of particles may be very different, depending on their origin. Of the natural particles, sulphates, nitrates, different silicates, carbonates, chlorides, fluorides and also particles of metals and metal oxides, hydrocarbons, and bacteria, moulds and pollen are most frequently represented. Particles from anthropogenic sources are soot, fly ash, and in cities with a dense traffic hydrocarbons, lead compounds and metal oxides. Large amount of particles from human activity is formed in the atmosphere as a result of the interactions of primary polluting components. 

The size of atmospheric particles ranges from molecular dimensions up to a diameter of above 10 m. Since particles with diameters below 0.1 fim (molecules and Aitken s nuclei) pre.sent no important problem and particles with diameters exceeding 10 fim are simply removed from the atmosphere by gravitational sedimentation, particles with diameters between 0.1 and 10 m are the main focus of interest. They can affect the reflection and scattering of the incident solar light, the local cloudiness and precipitation. 

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