Secondary atmospheric constituents

The air emissions of fossil fuel combustion are dispersed and diluted within the atmosphere, eventually falling or migrating to the surface of the Earth or ocean at various rates. Until recently, most attention was focused on the so-called primary pollutants of fossil fuel combustion that are harmful to human health oxides of sulphur and nitrogen, carbon monoxide, suspended particles (including soot), heavy metals, and products of incomplete combustion. These pollutants are most concentrated in urban or industrialized areas close to large or multiple sources. However, the primary pollutants may interact with each other, and with atmospheric constituents and sunlight, forming secondary pollutants that disperse far beyond the urban-... 

A further complication is the basic assumption of the statistical methods that source profiles neither change during air transport nor with time. Therefore they cannot be applied strictly to secondary aerosol constituents formed in the atmosphere by gas-to-particle conversion processes. Still, the secondary aerosol constituents tend to be grouped into one source group since they have a common source , i.e. formation in air triggered by solar irradiation. 

Oxidants present in the atmosphere thermochemically capable of oxidizing SO2 or NO2 include not only molecular O2 but also the trace, highly reactive constituents O3 and H2O2 that are the products of secondary atmospheric photochemical reactions. Despite... 

Primary and secondary pollutants Pollutants in the from in which they are emitted from sources. Primary pollutants may or may not react in the atmosphere with other pollutants or atmospheric constituents to form secondary or higher order pollutants. Primary pollutants are not necessarily chemically or physically simple. They can be emitted from their sources in quite complex chemical and physical form. 

Secondary pollutants are produced by interaction of primary pollutants with another chemical or by dissociation of a primary pollutant, or by other effects within a particular ecosystem. Again, using the atmosphere as an example, formation of the constituents of acid rain is an example of the formation of secondary pollutants (see above). 

Review of the literature provides ample evidence that aerosol formation is an important part of the atmospheric chemistry linked with photochemical-oxidant production. The important chemical constituents of concern include sulfate, nitrate, and secondary organic material. 

Tropospheric 03 is a secondary constituent formed by chemical reactions in the atmosphere involving several precursors (NOx, hydrocarbons and CO). The concentrations of these precursors are controlled by the atmospheric oxidation processes, which are regulated by hydrogen radicals OH and H02. 

Electron transfer from methane is significant because it seems likely that the radical cation and secondary intermediates derived from it (CH3 , CH3+) played a significant role in the chemical evolution preceding the origins of life. Methane is a probable constituent of early planetary atmospheres and its radical cation has potential significance as an interstellar species. 

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. 

Atmospheric pollution cannot be controlled so long as the nature and the mechanism of formation of its deleterious constituents remain unknown. While many chemical constituents of polluted atmospheres have been identified, their presence or concentration does not seem to follow a regular pattern. On the other hand, ozone is always present in polluted outdoor atmospheres. Its concentration consistently rises from a normal value of a few parts per hundred million to many times this value during periods of severe contamination. Whether ozone is the primary cause of pollution or is a secondary effect of the reaction of other substances is not entirely clear, but it appears to be an important link in the chain of chemical reactions which produce atmospheric pollution. Very likely, a knowledge of the variations of ozone concentration in atmospheres would permit a study of the influence of the various parameters, and this knowledge may eventually furnish a lead to an explanation of the mechanism of formation and the effects of pollutants. 

On the other hand, the effect of the wet removal can be practically neglected here.3 It is thus understandable that the residence time of trace constituents is greater in the stratosphere than in the troposphere. Above the tropopause the horizontal wind speed first decreases then increases with height. Consequently, a secondary maximum in the wind speed can be observed in this atmospheric layer. The increase of the temperature ends approximately at an altitude of SO km (stratopause), where the temperature is around 0 °C (see Fig. 1). Above this level, in the mesosphere, the temperature again decreases (third layer in the homosphere). For this reason the stratopause can be considered as an active heat-supplying surface similar to the Earth s surface. In this atmospheric region the distribution of the temperature makes possible the convection which, in favourable cases, results in a formation of so-called noctilucent clouds at an altitude of about 80 km (mesopause) where the temperature is only around — 80 °C. This is the coldest level of our atmosphere. 

After the dissipation of cosmic gases (approximately 4 x 109 years ago) several gaseous materials were liberated from the solid Earth. The substances formed in this way are termed secondary constituents. These atmospheric components were due either to volcanic, desorption and thermal processes or to chemical reactions. 

In the biosphere, rain that is not lost back to the atmosphere by evaporation from the ground or from trees may pass deep underground, only to emerge at a much later date (Table 2.27) in a river or lake. Water coming into contact with rocks (and derived soils) reacts with primary minerals contained in them. The minerals dissolve to varying extents, and some of the dissolved constituents react with one another to form new, secondary minerals. Dissolution is mainly controlled by the water acidity provided from plant mineralization (humic acids), atmospheric carbonic acid and acid rain . The overall process is called chemical weathering (see Chapter 2.2.2.5, Eqs. 2.62 and 2.62 Berner and Berner 1996). 

In what sense is acid rain a secondary air pollutant What is the chemistry behind the formation of most acid rain constituents Explain how particles in the atmosphere may be either primary or secondary air pollutants ... 

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