Abstract
Ultrafine particles are formed directly in the atmosphere (in situ). New aerosol forming substances (AFS) are produced as a result of photochemical and catalytic reactions between some atmospheric gases. The vapor saturation pressure for these substances is lower than that of the initial ones. Therefore, condensation of supersaturated vapor of AFS occurs and as a result ultrafine particles are formed, which then can re-interact with gases. All oxygenous gases can generate clusters. Process of AFS vapors conversion into the disperse phase can be derived as a result of homogeneous condensation of the same substance vapor; homogeneous heteromolecular condensation of several substance vapors; adsorption of molecules onto clusters; and heterogeneous condensation. Ultrafine particles are formed in situ from the gas-phase by homogeneous nucleation. They form the basis for production of submicron aerosols, which have the longest lifetime in the atmosphere. Because measurements of the ultrafine particles are very difficult, the information about their behavior is very limited. This chapter presents some measurement results on number concentration of such particles obtained during the April 1995 over Tomsk at the TOR-station.
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