Secondary pollutant

Air pollution can be considered to have three components sources, transport and transformations in the atmosphere, and receptors. The source emits airborne substances that, when released, are transported through the atmosphere. Some of the substances interact with sunlight or chemical species in the atmosphere and are transformed. Pollutants that are emitted directiy to the atmosphere are called primary pollutants pollutants that are formed in the atmosphere as a result of transformations are called secondary pollutants. The reactants that undergo transformation are referred to as precursors. An example of a secondary pollutant is O, and its precursors are NMHC and nitrogen oxides, NO, a combination of nitric oxide [10102-43-9] NO, and NO2. The receptor is the person, animal, plant, material, or ecosystem affected by the emissions. 

Over the past decade, water pollution control has progressed from an art to a science. Increased emphasis has been placed on the removal of secondary pollutants, such as nutrients and refractory organics, and on water reuse for industrial and agricultural purposes. This in turn has generated both fundamental and appHed research, which has improved both the design and operation of wastewater treatment faciUties. 

Air pollution (qv) problems are characteri2ed by their scale and the types of pollutants involved. Pollutants are classified as being either primary, that is emitted direcdy, or secondary, ie, formed in the atmosphere through chemical or physical processes. Examples of primary pollutants are carbon monoxide [630-08-0] (qv), CO, lead [7439-92-1] (qv), Pb, chlorofluorocarbons, and many toxic compounds. Notable secondary pollutants include o2one [10028-15-6] (qv), O, which is formed in the troposphere by reactions of nitrogen oxides (NO ) and reactive organic gases (ROG), and sulfuric and nitric acids. 

Atmospheric pollutants released by combustion of fossil fuels fall into two main categories those emitted direcdy into the atmosphere as a result of combustion and the secondary pollutants that arise from the chemical and photochemical reactions of the primary pollutants (see Airpollution). 

There are two different types of air pollution problems in urban areas. One is the release of primary pollutants (those released directly from sources). The other is the formation of secondary pollutants (those that are formed through chemical reactions of the primary pollutants). 

At least three types of problems contribute to air pollution problems on the regional scale. One is the carryover of urban oxidant problems to the regional scale. With the existence of major metropolitan areas in close proximity, the air from one metropolitan area, containing both secondary pollutants formed through reactions and primary pollutants, flows on to the adjacent metropolitan area. The pollutants from the second area are then added on top of the "background" from the first. 

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

An understanding of the transformation of SO2 and NO. into other constituents no longer measurable as SOj and is needed to explain mass balance changes from one plume cross section to another. This loss of the primary pollutant SOj has been described as being exponential, and rates up to 1% per hour have been measured (30). The secondary pollutants generated by transformation are primarily sulfates and nitrates. 

Combustion processes are the most important source of air pollutants. Normal products of complete combustion of fossil fuel, e.g. coal, oil or natural gas, are carbon dioxide, water vapour and nitrogen. However, traces of sulphur and incomplete combustion result in emissions of carbon monoxide, sulphur oxides, oxides of nitrogen, unburned hydrocarbons and particulates. These are primary pollutants . Some may take part in reactions in the atmosphere producing secondary pollutants , e.g. photochemical smogs and acid mists. Escaping gas, or vapour, may... 

Nitrogen Dioxide (NO2) Is a major pollutant originating from natural and man-made sources. It has been estimated that a total of about 150 million tons of NOx are emitted to the atmosphere each year, of which about 50% results from man-made sources (21). In urban areas, man-made emissions dominate, producing elevated ambient levels. Worldwide, fossil-fuel combustion accounts for about 75% of man-made NOx emissions, which Is divided equally between stationary sources, such as power plants, and mobile sources. These high temperature combustion processes emit the primary pollutant nitric oxide (NO), which Is subsequently transformed to the secondary pollutant NO2 through photochemical oxidation. 

Compact design No filter to replace. Generates secondary pollution in form of dusts deposited around the vicinity of the appliance Ineffective against gaseous pollutants Requires constant maintenance. 

Kills bacteria, protozoa, molds, fungi, yeasts, spores and viruses Possible generation of secondary pollutants including ozone and PCO byproducts ... 

The indoor air quality appliance must have better than ninety percent efficient in removing airborne particulates, bioaerosols (i.e., airborne bacteria and fungi spores), carbon monoxide, volatile organic compounds and odors. It must meet or exceed the industry s standards in performance and safety. The product must be able to maintain the pollutants in indoor air at below government IAQ standards and must not emit or produce harmful secondary pollutions during... 

Complex reactions involving radicals occur, giving rise to secondary pollutants such as ozone, aldehydes, peroxyacetyl nitrate (PAN) and particulate matter. 

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. 

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). 

These gases are also classed as primary pollutants because they are emitted directly from the source and then react to produce secondary pollutant, such as acid rain (Speight, 1993). The emissions may include a number of biologically active substances that can pose a major health concern. These gases are classed as pollutants because (1) they may not be indigenous to the location or (2) they are ejected into the atmosphere in a greater-than-natural concentration and are, in the current context, the product of human activity. Thus, they can have a detrimental effect on the environment in part or in toto. 

Gaseous pollutants gases released into the atmosphere that act as primary or secondary pollutants. 

---