Paper on Polycyclic Aromatic Hydrocarbons (PAHs)

What is Polycyclic Aromatic Hydrocarbon?

Polycyclic aromatic hydrocarbons are hydrocarbons—organic compounds containing only carbon and hydrogen—that are composed of multiple aromatic rings. The simplest such chemicals are naphthalene, having two aromatic rings, and the three-ring compounds anthracene and phenanthrene. They are also a group of more than 100 chemicals that are also called polynucleararomatichydrocarbons. PAHs are released from burning coal, oil, gasoline, trash, tobacco, and wood.

Sources of Polycyclic Aromatic Hydrocarbon

Natural sources: Examples of natural sources of PAHs formation include: forest and brush fires, volcanoes, bacterial and algal synthesis, petroleum seeps, erosion of sedimentary rocks containing petroleum hydrocarbons, and decomposition of vegetative liter fall.

Man-made sources: Examples of man-made or anthropogenic sources of PAHs range from:

• Large point sources include incomplete combustion (such as incinerators and some industrial processes.
• Smaller point sources include , dispersed sources (such as automotive emissions, smoke from wood-burning stoves, jet aircraft exhausts, cigarette and cigar smoke, and backyard barbecues).
• Other anthropogenic sources of PAHs include petroleum product spills, sewage sludge, and tarry or creosote waste materials.

Structures of Polycyclic Aromatic Hydrocarbon

By definition, polycyclic aromatic hydrocarbons have multiple cycles, precluding benzene from being considered a PAH. Naphthalene is considered the simplest polycyclic aromatic hydrocarbon by the US EPA and US CDC for policy contexts. Other authors consider PAHs to start with the tricyclic species phenanthrene and anthracene. PAHs are not generally considered to contain heteroatoms or carry substituents.

The general characteristics common among PAHs are high melting and boiling points, low vapor pressures, and very low water solubility which tends to decrease with increasing molecular mass. With low vapor pressures, the majority of PAHs, and especially those with three or four rings, are semi-volatile (i.e., compounds with vapor pressures between 10⁻⁶ and 10⁻² Pa) and readily undergo long-range transport via the atmosphere. PAHs may be emitted in particulate form or in the gas-phase where they may remain, or sorb to particles such as black carbon, mineral dust and sea salt.

Atmospheric PAHs are subject to wet and dry deposition to surface waters and soils, where they have longer lifetimes; models suggest that atmospheric half-lives of 3–5 ring PAHs are of the order of hours or days, whereas their persistence in soils can be on the order of decades. However, their semi-volatile properties allows many PAHs to re-volatilize from terrestrial compartments and surface seawater and re-enter the atmosphere to undergo enhanced long-range transport potential via multi-hopping.

Although most PAHs are sufficiently lipophilic to bioaccumulate in organisms, they are also subject to biotransformation processes and so do not appear to biomagnify through food chains, including those in the Arctic.

PAHs with five or six-membered rings are most common. Those composed only of six-membered rings are called alternant PAHs, which include benzenoid PAHs. The following are examples of PAHs that vary in the number and arrangement of their rings:

16 PAHs classified under the United State Environmental Agency as priority PAHs

Name	CAS#	Rings	MW
Naphthalene	91-20-3	2	128.17
Acenaphthylene	208-96-8	3	152.19
Acenaphthene	83-32-9	3	154.21
Fluorene	86-73-7	3	166.22
Phenanthrene	85-01-8	3	178.23
Anthracene	120-12-7	3	178.23
Fluoranthene	206-44-0	4	202.26
Pyrene	129-00-0	4	202.26
Benzo[a]anthracene	56-55-3	4	228.29
Chrysene	218-01-9	4	228.29
Benzo[b]fluoranthene	205-99-2	5	253.32
Benzo[k]fluoranthene	207-08-9	5	253.32
Benzo[a]pyrene	50-32-8	5	253.32
Dibenzo[a,h]anthracene	215-58-7	6	278.35
Benzo[g,h,i]perylene	191-24-2	6	276.34
Indeno[1,2,3-cd]pyrene	193-39-5	6	276.34

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