1.1 Background of Study
Polycyclic Aromatic Hydrocarbons refers to a large class of organic compounds which contains two or more fused aromatic rings made up of carbon and hydrogen atoms. PAHs have the following general characteristics common to them; high melting and boiling points, low vapour pressure and very low water solubility which tends to decrease with increasing molecular mass (Ahland and Mertens, 1980). Most of the PAHs with low vapour pressure in the air are adsorbed on particles. When dissolved in water or adsorbed on particulate matter, PAHs can undergo photodecomposition when exposed to ultraviolet light from solar radiation. In the atmosphere, PAHs can react with pollutants such as ozone, nitrogen oxides and sulfur dioxide, yielding diones, nitro-and dinitro-PAHs, and sulfonic acids, respectively. PAHs may also be degraded by some microorganisms in the soil (WHO, 1987; USDHHS, 1994).
PAHs are important priority organic pollutants. They may be released during incomplete combustion or pyrolysis of organic matter. This is a major source of human exposure. PAHs emanate primarily as combustion products from vehicle and generator emissions, coal and oil burning plants. They are conveyed by rainfall into the aquatic environment after being absorbed onto smoke particles settling in all kinds of surfaces. (Brookes and Lawley, 1964).
Toxicological studies have shown that some of these PAHs have the potential for tetratogenesis or carcinogenesis in human beings. As a result of their highly hazardous nature, they are monitored in waste waters, soils and atmosphere. Sensitive
detection and accurate reproducible quantification of organics is very important in minimizing the health risks of PAHs. (Larsson, 1982).
PAHs are formed mainly as a result of pyrolytic processes, especially the incomplete combustion of organic materials during industrial and other human activities, such as processing of coal and crude oil, combustion of natural gas, including for heating, combustion of refuse, vehicle traffic, cooking and tobacco smoking, as well as in natural processes such as carbonization. There are several hundred PAHs; the best known is benzo[a]pyrene (BaP). In addition a number of heterocyclic aromatic compounds (e.g. carbazole and acridine), as well as nitro-PAHs, can be generated by incomplete combustion (WHO, 1987).
The emissions of BaP into the air from several sources in the Federal Republic of Germany in 1981 were estimated to amount to 18 tonnes: about 30% was caused by coke production, 56% by heating with coal, 13% by motor vehicles and less than 0.5% by the combustion of heating oil and coal-fired power generation. Other BaP sources were not taken into consideration (WHO, 1987). However, the present contributions from the different important sources, such as residential heating (coal, wood, oil), vehicle exhausts, industrial power generation, incinerators, the production of coal tar, coke and asphalt, and petroleum catalytic cracking, are very difficult to estimate. These figures may also vary considerably from country to country. In the USA, the residential burning of wood is now regarded as the largest source of PAHs (USDHHS, 1994). Stationary sources account for a high percentage of total annual PAH emissions. However, in urban or suburban areas, mobile sources are additional major contributors to PAH release to the atmosphere (Baek et al., 1991).