Polycyclic aromatic hydrocarbons (PAHs) are a group of organic compounds consisting of two or more fused benzene rings (linear, cluster or angular arrangement), or compounds made up of carbon and hydrogen atoms grouped into rings containing five or six carbon atoms. They are called “PAH derivatives” when an alkyl or other radical is introduced to the ring, and heterocyclic aromatic compounds (HACs) when one carbon atom in a ring is replaced by a nitrogen, oxygen or sulphur atoms. PAHs originate mainly from anthropogenic processes particularly from incomplete combustion of organic fuels. PAHs are distributed widely in the atmosphere. Natural processes, such as volcanic eruptions and forest fires, also contribute to an ambient existence of PAHs (Suchanova et al., 2008). PAHs can be present in both particulate and gaseous phases, depending on their volatility. Low molecular weight PAHs (LMW PAHs) that have two or three aromatic rings (molecular weight from 152 to 178g/mol) are emitted in the gaseous phase, while high molecular weight PAHs (HMW PAHs), molecular weight ranging from 228 to 278g/mol, with five or more rings, are emitted in the particulate phase, (ATSDR, 1995) . In the atmosphere, PAHs can undergo photo-degradation and react with other pollutants, such as sulfur dioxide, nitrogen oxides, and ozone. Due to widespread sources and persistent characteristics, PAHs disperse through atmospheric transport and exist almost everywhere. There are hundreds of PAH compounds in the environment but in practice PAH analysis is restricted to the determination of six (6) to sixteen (16) compounds. Human beings are exposed to PAH mixtures in gaseous or particulate phases in ambient air. Long term exposure to high concentration of PAHs is associated with adverse health problems. Since some PAHs are
considered carcinogens, inhalation of PAHs in particulates is a potentially serious health risk linked to lung cancer (Philips, 1999).
1.2. Physical and Chemical Characteristics of PAHs.
PAHs are a group of several hundred individual organic compounds which contain two or more aromatic rings and generally occur as complex mixtures rather than single compounds. PAHs are classified by their melting and boiling points, vapour pressure, and water solubility, depending on their structure. Pure PAHs are usually coloured, crystalline solids at ambient temperature. The physical properties of PAHs vary with their molecular weight and structure (Table1). Except for naphthalene, they have very low to low water solubilities, and low to moderately high vapour pressures. Their octanol-water partition coefficients (Kow) are relatively high, indicating a relatively high potential for adsorption to suspended particles in the air and in water, and for bioconcentration in organisms (Sloof et al., 1989). Table 1 shows physical and chemical characteristics of few selected PAHs from the sixteen (16) priority PAHs, listed by the US EPA. (see appendix). Most PAHs, especially as molecular weight increases, are soluble in non-polar organic solvents and are barely soluble in water (ATSDR, 1995).
Most PAHs are persistent organic pollutants (POPs) in the environment. Many of them are chemically inert. However, PAHs can be photochemically decomposed under strong ultraviolet light or sunlight, and thus some PAHs can be lost during atmospheric sampling. Also, PAHs can react with ozone, hydroxyl radicals, nitrogen and sulfur oxides, and nitric and sulfuric acids which affect the environmental fate or conditions of PAHs (Dennis et al., 1984; Simko, 1991).
PAHs possess very characteristic UV absorbance spectra. Each ring structure has a unique UV spectrum, thus each isomer has a different UV absorbance spectrum. This is especially useful in
the identification of PAHs. Most PAHs are also fluorescent, emitting characteristic wavelengths of light when they are excited (when the molecules absorb light). Generally, PAHs only weakly absorb light of infrared wavelengths between 7 and 14µm, the wavelength usually absorbed by chemical involved in global warning (Ramanathan, 1985).
Polycyclic aromatic hydrocarbons are present in the environment as complex mixtures that are difficult to characterize and measure. They are generally analyzed using gas chromatography coupled with mass spectrometry (GC-MS) or by using high pressure liquid chromatography (HPLC) with ultraviolet (UV) and fluorescence dectetors (Slooff et al., 1989)
Table 1 Physical and Chemical Characteristics of Some Popular PAHs
Names of PAHs
276.3 10x10-16mmHG 6.58
Sources: (ATSDR, 1995)
Source and Emission of PAHs
PAHs are mainly derived from anthropogenic activities related to pyrolysis and incomplete combustion of organic matter. Sources of PAHs affect their characterization and distribution, as well as their toxicity. Major sources of PAH emissions may be divided into four classes: stationary sources (including domestic and industrial sources), mobile emission, agriculture activities, and natural sources (Wania et al, 1996).
1.3. Stationary Sources
Some PAHs are emitted from point sources and this is hardly shifted (moved) for a long period of time. Stationary sources are further subdivided into two main sources: domestic and industrial.
1.3.1. Domestic Sources
Heating and cooking are dominant domestic sources of PAHs. The burning and pyrolysis of coal, oil, gas, garbage, wood, or other substances are the main domestic sources. Domestic sources are important contributors to the total PAHs emitted into the environment. Difference in climate patterns and domestic heating systems produce large geographic variations in domestic emission. PAH emissions from these sources may be a major health concern because of their prevalence in indoor environments (Ravindra et al., 2008). According to a recent World Health Organization (WHO) report, more than 75% of people in China, India, and South East Asia and 50-75% of people in parts of South America and Africa use combustion of solid fuel, such as wood, charcoal for daily cooking.
Main indoor PAH sources are cooking and heating and infiltration from outdoors. PAH emissions from cooking account for 32.8% of total indoor PAHs (Zhu et al., 2009). LMW PAHs which originate from indoor sources are the predominant proportion of the total PAHs identified
in residential non-smoking air. Toxicity of PAH mixtures from indoor sources is lower than mixtures which contain large amounts of high molecular weight PAHs. Cigarette smoke is also a dominant sources of PAHs in indoor environments. In many studies, PAHs in the indoor air of smoking residences tend to be higher than those of non-smoking residences.
1.3.2. Industrial Sources
Sources of PAHs include emission from industrial activities, such as primary aluminum and coke production, petrochemical industries, rubber tire and cement manufacturing, bitumen and asphalt industries, wood preservation, commercial heat and power generation, and waste incineration (Fabbri and Vassura , 2006).
1.3.3. Mobile Sources
Mobile sources are major causes of PAHs emissions in urban areas. PAHs are mainly emitted from exhaust fumes of vehicles, including automobile, railways, ships, aircrafts, and other motor vehicles. PAHs emissions from mobile sources are associated with use of diesel, coal, gasoline, oils, and lubricant oil. Exhaust emissions of PAHs from motor vehicles are formed by three mechanisms: (1) synthesis from smaller molecules and aromatic compounds in fuel; (2) storage in engine deposits and in fuel; (3) pyrolysis of lubricants (Baek et al., 1991). One of the major influences on the production of PAHs from gasoline automobiles is the air-to-fuel ratio. It has been reported that