Primary and Secondary Pollutants

A substantial portion of fhe gas and vapors emitted to the atmosphere in appreciable quantity from anthropogenic sources tends to be relatively simple in chemical structure carbon dioxide, carbon monoxide, sulfur dioxide, and nitric oxide from combustion processes hydrogen sulfide, ammonia, hydrogen chloride, and hydrogen fluoride from industrial processes. The solvents and gasoline fractions that evaporate are alkanes, alkenes, and aromatics with relatively simple structures. In addition, more complex 

Primary pollutants are those emitted directly to the atmosphere while secondary pollutants are those formed by chemical or photochemical reactions of primary pollutants after they have been admitted to the atmosphere and exposed to sunlight. Unbumed hydrocarbons, NO, particulates, and the oxides of sulfur are examples of primary pollutants. The particulates may be lead oxide from the oxidation of tetraethyllead in automobiles, fly ash, and various types of carbon formation. Peroxyacyl nitrate and ozone are examples of secondary pollutants. 

Some pollutants fall in both categories. Nitrogen dioxide, which is emitted directly from auto exhaust, is also formed in the atmosphere photochemically from NO. Aldehydes, which are released in auto exhausts, are also formed in the photochemical oxidation of hydrocarbons. Carbon monoxide, which arises primarily from autos and stationary sources, is likewise a product of atmospheric hydrocarbon oxidation. 

It has been well established that if a laboratory chamber containing NO, a trace of N02, and air is irradiated with ultraviolet light, the following reactions occur  

The net effect of irradiation on this inorganic system is to establish the dynamic equilibrium 

However, if a hydrocarbon, particularly an olefin or an alkylated benzene, is added to the chamber, the equilibrium represented by reaction (8.4) is unbalanced and the following events take place  

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Fig. 4-6. Primary and secondary pollutants. Reaction can occur without catalysis (HC hydrocarbons).

From a detailed knowledge of the emissions, topography, meteorology, chemistry, and deposition processes, one can develop mathematical models that predict the concentrations of primary and secondary pollutants as a function of time at various locations. Depending on the particular model, these may describe pollutant concentrations over a variety of scales ... 

Because of the gaseous nature of many of the important primary and secondary pollutants, the emphasis in kinetic studies of atmospheric reactions historically has been on gas-phase systems. However, it is now clear that reactions that occur in the liquid phase and on the surfaces of solids and liquids play important roles in such problems as stratospheric ozone depletion (Chapters 12 and 13), acid rain, and fogs (Chapters 7 and 8) and in the growth and properties of aerosol particles (Chapter 9). We therefore briefly discuss reaction kinetics in solution in this section and heterogeneous kinetics in Section E. 

FIGURE 16.7 Typical primary and secondary pollutant profiles in a propene-NO, irradiation in a smog chamber (adapted from Pitts et al., 1975). 

However, without knowledge of the source of the increased OH flux, extrapolation of the concentration-time profiles of both the primary and secondary pollutants observed in such smog chamber studies to real atmospheres becomes less certain. For example, the reactions leading to the unknown precursor(s) to OH may occur only in smog chambers. Extrapolation to ambient air would thus require subtracting out this radical source. On the other hand, the same reactions may occur in ambient air where surfaces are available in the form of particulate matter, buildings, the earth, and so on if this is true, then the rates would be expected to depend on the nature and types of surfaces available and may thus differ quantitatively from the smog chamber observations. 

Aldehydes are primary and secondary pollutants in the urban atmosphere, and since laboratory studies indicate that they photo-oxidize to form peroxides, the presence of peroxides in the atmosphere seems evident. 

The distinction between primary and secondary pollutants is conceptually useful, because primary and secondary species usually show distinctly different patterns of diurnal and seasonal variation in polluted regions of the atmosphere. The ambient concentrations of primary species are controlled largely by proximity to emission sources and rates of dispersion. The highest concentrations of these species tend to occur at nighttime or early morning and in winter in northerly locations, because atmospheric dispersion rates are slowest at these times. By contrast, high concentrations of... 

Due to the contrasting diurnal patterns of primary and secondary pollutants, ozone and other secondary pollutants appear to anticorrelate with their precursor species (NO and VOCs) if measured concentrations are plotted for a full diurnal cycle. Peak ozone occurs during and after the time of maximum sunlight, which is also the time of maximum vertical dilution. Concentrations of primary pollutants reach their diurnal minimum at this time. 

Kuebler J., van den Bergh H., and Russell A. G. (2001) Longterm trends of primary and secondary pollutant concentrations in Switzerland and their response to emission controls and economic changes. Atmos. Environ. 35, 1351-1363. 

In urban environments there are pollutant compounds emitted to the atmosphere directly and these are called primary pollutants. Smoke is the archetypical example of a primary pollutant. However, many compounds undergo reactions in the atmosphere, as we have seen in the section above. The products of such reactions are called secondary pollutants. Thus, many primary pollutants can react to produce secondary pollutants. It is the distinction between primary and secondary pollution that now governs our understanding of the difference... 

Air pollutants are also categorized, like the pollution sourees. The two eategories are primary and secondary pollutants. Primary pollutants are the ehemieals that are direetly emitted to the atmosphere from known sources, while secondary pollutants are the speeies that are formed by the chemical reactions of primary pollutants (Wayne, 2000). Primary pollutants include sulphur oxides (SOx), nitrogen oxides (NOx), carbon monoxide (CO), hydroearbons (HC), and particulate matter (PM). One of the most important secondary pollutants is ozone. 

At local and urban levels, it is essentially a question of vicinity pollution. Several air pollutants are concerned sulphur dioxide, SO2 (primary pollutant) nitrogen oxides NO (primary or secondary pollutants) particulate matter PM (primary and secondary pollutants) carbon monoxide, CO, (primary pollutant) (volatile) organic compounds, HC (or VOCs) (primary and secondary pollutants), and photochemical oxidants, O3, PAN (secondary pollutants). Organic compounds account for a wide range of hydrocarbons and are found in solid, liquid, and gas forms. 

Figure 17.25 shows typical variations with time of primary and secondary pollutants. Initially, the concentration of NO2 is quite low. As soon as solar radiation penetrates the atmosphere, more NO2 is formed from NO and O2. Note that the concentration of ozone remains fairly constant at a low level in the early morning hours. As the concentration of unbumed hydrocarbons and aldehydes increases in the air, the concentrations of NO2 and O3 also rise rapidly. The actual amounts, of course, depend on the location, traffic, and weather conditions, but their presence is always accompanied by haze (Figure 17.26). The oxidation of hydrocarbons produces various organic intermediates, such as alcohols and carboxylic acids, which are all less volatile than the...

When further harmful substances enter the atmosphere together with hydrogen sulphide, then the number of possible reactions is extended by combinations with primary and secondary pollutants. For example, there are the emissions from plants for the production of viscose fibres, where particularly H2S and CS2 are released into the atmosphere. The following reactions probably occur in the atmosphere ... 

The combustion of coal produces both primary and secondary pollutants. Primary pollutants include all species... 

Distinguish between primary and secondary pollutants in our atmosphere. 

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. 

Briefiy rationalize in your own words the daily variations of concentrations of primary and secondary pollutants in photochemical smog as shown in Figure 16.12. 

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