Secondary component, atmospheric aerosol

Atmospheric aerosols are important nuclei for the condensation of water droplets (cloud, rain, fog). The dissolution of the water-soluble aerosol components contributes to the composition of the aqueous phase [e.g., NH4NO3, (NH4)2S04]. Aerosols may contain, in addition to the absorbed gases, a substantial fraction of atmospheric components that return ultimately to the earth surface by dry or wet deposition. The particle diameter ranges from 0.01 nm up to a few hundred micrometers. Primary atmospheric aerosols consist of dust and smoke particles while secondary aerosols are made up of constituents of the gas phase. 

The secondary component results from atmospheric chemical reactions that produce inorganic ionic species of which the most important are NH. and NOJ. Organic vapors also react in the atmosphere to form condensable products. For example, cyclic olefins react with ozone to form less volatile dicarboxyiic acids. The secondary chemical species nomially reported in studies ofaliiiospheric aerosol composition are relatively stable reaction products they have usually survived in the atmosphere and on filter or impactor... 

Particles in the atmosphere arise from natural sources, such as windborne dust, sea spray, and volcanoes, and from anthropogenic activities, such as combustion of fuels. Whereas an aerosol is technically defined as a suspension of fine solid or liquid particles in a gas, common usage refers to the aerosol as the particulate component only (Table 2.17). Emitted directly as particles (primary aerosol) or formed in the atmosphere by gas-to-particle conversion processes (secondary aerosol), atmospheric aerosols are generally considered to be the particles that range in size from a few nanometers (nm) to tens of micrometers... 

It has been established that the principal growth mechanism for urban atmospheric aerosols in the 0.1-1.0 ym diameter size range (the so-called Accxamulation Mode) is gas-to-particle conversion (68,71) The major secondary components in atmospheric aerosols have been identified as sulfates, nitrates and particulate organic species (68,71,72,73). The qualitative picture of polluted... 

In this section we describe the methodology used to study the effect of emissions from the local sources of atmospheric pollution on the processes of secondary aerosol formation. The basic admixtures selected as the main objects of investigations were dust and oxides of sulphur and nitrogen which are the main components of die emissions from the industrial... 

Noninteracting Secondary Organic Aerosol Compounds The simplest case is that of a compound that does not interact with already existing aerosol components. Such a compound does not adsorb on the aerosol surface or form solutions with the existing aerosol and gas-phase species. Let us assume that a reactive organic gas (ROG) undergoes atmospheric oxidation to produce products, P1 P2, and so on... 

Deviations from the theory outlined above may occur if the organic solution is not ideal. In this case the mole fraction x, should be replaced by the product y,jc where y is the activity coefficient of the component i in the given solution. Saxena et al. (1995) have shown that a series of organic aerosol compounds can interact also with the aqueous phase in atmospheric particles. Therefore, for high relative humidities, the secondary organic aerosol compounds may exist in three phases—namely, the gas, the organic aerosol material, and the aqueous phase. 

Some of the substances mentioned are concerned in the formation of secondary harmful species ozone, peracyl nitrate, singlet oxygen and solid aerosols in the atmosphere, which are components of the white photochemical smog of the California variety. Sulphur dioxide in the air is partially further oxidized to sulphur trioxide and hence sulphuric acid. These substances, together with soot are components of the black smog of the London type. 

Sea spray, volcanic eruptions, soil dust, as well as some industries (cement manufacturing) produce the so called primary aerosols, i.e. the material is emitted directly in particulate state (Klockow, 1982), and they are both line and coarse. Secondary aerosols are produced in the atmosphere usually by eondensation after emission from high temperature sources, and they are fine as a rule. Considering the difference in the chemical composition it is recognized that the major components of the fine aerosols are toxie substances of anthropogenic origin such as As, Cd, Pb, Se, Zn etc. while the course aerosols are enriched in elements like Ca, Fe, Si coming from erosion, sea aerosols and other natural sources. 

Inorganic and organic aerosols that are directly emitted into the atmosphere are known as primary aerosols, for example, aerosols from ocean spray, volcanic eruptions or combustion. Those aerosols that are formed in the atmosphere through a sequence of chemical reactions are referred to as secondary aerosols, and they account for a major component of tropospheric particulates. 

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