Polycyclic aromatic hydrocarbon

The aromatic hydrocarbons polycyclic , commonly called HAP, are a family of chemical compounds made up of carbon atoms and hydrogen of which the structure of the molecules includes/understands at least two aromatic cycles digests.

Attention not to be confused, in the same field, with the Anglo-Saxon abbreviation Hazardous Air Pollutants, whose very long list also, gathers chemicals on the basis of their half-life under normal conditions of evaporation.

Some HAPs:

(1) Phenanthrene; (2) Anthracene; (3) Pyrene; (4) Benzanthracene; (5) Chrysene; (6) Naphthalene; (7) Benzophenanthrene; (8) Benzofluoranthene; (9) Dibenzophenanthrene; (10) Benzoperylene; (11) Triphénylène; (12) o-téphényl; (13) m-téphényl; (14) p-téphényl; (15) Benzo (A) pyrene; (16) Tétrabenzonaphthalène (TBN); (17) Phenanthrophenanthrene (PhPh); (18) Coronène

Since many years, the HAP are very studied because they are compounds present in all the environmental mediums and which show strong a Toxicité. They belong to the persistent Polluants organics (POPs). Moreover, it is one of the reasons which led to their addition in the list of the priority pollutants by the agency of environmental protection of the the United States (US EPA Environmental Protection Agency), since 1976. Today, they also form part of the lists of WHO (the World Health Organization) and of the European Community.

General information

According to the number of cycles, they are classified in light HAP (up to three cycles) or heavy (beyond three cycles), and have physicochemical and toxicological characteristics very different.

The number of HAP likely to be met is unbounded. Indeed, not only there is no limit with the number of joined cores, but the number of isomers increases considerably with the number of aromatic cycles. Moreover, they can be alkylated.

The pure HAP are substances coloured and crystalline with room temperature. The physical properties of the HAP vary according to their molecular mass and their structure. Except for naphthalene, the HAP are very Hydrophobe S, and thus their solubilities in water are low. In parallel, their coefficients of division octanol/water (Kow) are relatively high, testifying to an important potential of Adsorption on the particulate organic matters suspended in the air or in water, as well as strong potential of bioconcentration in the organizations.

Sources of the HAP

The formation of the HAP can have many origins which can be gathered in three categories. One can distinguish the pyrolytic, diagenetic and petrogenic origins. However, the two last are negligible in the atmosphere, compared to pyrolytic source.

Pyrolytic HAP of origin

Before the use of the coal, Oil and Natural gas like energy sources, the contribution of pyrolytic HAP of origin was mainly due to natural phenomena such as fires of Forêt S and meadows. Today, it is the anthropic pyrolytic origin which is regarded as the major source of HAP in the environment, in particular because of the domestic and industrial emissions. The pyrolytic HAP are generated by processes of incomplete combustion of the organic matter at high temperature. The mechanisms concerned during their formation utilize the production of free radicals by Pyrolyse at high temperature (≥ 500 °C) of the fossil matter (oils, Fioul, organic matters…) under defective conditions in Oxygen. The pyrolytic HAP of origin come from the combustion of the fuel Automobile, of combustion domesticates (coal, wood…), of the industrial production (steel-works…), of the energy production (powerplants running with oil or coal…) or of the incinerators.

Also, part of the HAP present in the environment comes from natural processes such as the ic eruptions Volcan.

According to the origin, certain HAP will be formed preferentially. This makes it possible to use them as indicators of origin. Thus, the reports/ratios of concentration in various HAP make it possible to calculate molecular indices. For example, if the report/ratio of the anthracene concentration on that of phenantrene is higher than 10, then the origin will be petrogenic while if it is lower than 10, it will be of pyrolytic origin.

In France, in 2002, the anthropic emissions of HAP were dominated by the domestic sector, because of energy consumption. Then come the sector of road transport, in particular of the vehicles diesel, then that of manufacturing industry.

Atmospheric concentrations

In the atmosphere, the concentrations of particulate HAP are very variable. Indeed, the concentrations can be very different between a very moved back place, like the the Antarctic, where one finds a few tens of pg/m ³, and a place very urbanized, where the concentrations can reach a few hundreds of ng/m ³, as with Santiago of Chile.

In urban environment, the range of concentrations very broad and lies between 2 and 300 ng/m ³. However, these values tend to decrease, taking into account the projection of technologies on the limitation of the automobile emissions (reduction of consumption, use of catalytic pots, development of the particle filters…). One also notices a seasonal variability marked by more important concentrations in Hiver. This phenomenon is explained mainly by four processes:

- increase in the emissions due to the domestic heating; - processes of degradation of the HAP which are less important the winter than the summer (less photodegradation); - the lower temperature which makes than the HAP will tend to remain in particulate phase; - a greater stability of the atmospheric column in winter, thus limiting the dispersive phenomena.

Studies highlighted the variability of the concentrations in HAP in several European cities, according to the seasons. It was especially shown that the concentrations were stronger in winter for the particulate HAP. However, this study also showed that there were variations between the various places of taking away and the distribution of the various HAP.

While moving away from the sources of combustions, the concentrations in HAP decrease very quickly. In rural environment, the concentrations are very weak. However, the HAP are present on all planet because of atmospheric transport.

Toxicity

One of the reasons having led to the classification of the HAP in the list of the priority pollutants of the EPA is the toxic character of some of them. In fact biologically active molecules, once absorptive by the organizations, lend themselves to reactions of transformation under the action of enzymes leading to the epoxy formation of S and/or hydroxylated derivatives. The Métabolite S thus formed can have a toxic effect more or less marked while binding to fundamental biological molecules such as proteins, ARN, DNA and cause cellular dysfunctions.

benzo (A) pyrene

The Benzo (A) pyrene (B (A) P) is one of the most toxic HAP. Indeed, he is recognized like Cancérogène by the IARC (International Association for Research one Cancer). This is related to its capacity to form adduits with the DNA. There exist several ways of activation of B (A) P, but most important is that of the epoxy diols because it leads to the formation of adduits stable. B (A) P will be oxidized by the enzymatic systems of the P450 cytochrome, for finally forming an epoxy. This last product, Benzo (A) Pyrene-7,8-dihydrodiol-9,10-epoxy (BPDE), is likely to react with the DNA. Thus, the toxicity of the benzo (A) pyrene is partly directly related to the carcinogenic potential of the one of its metabolites, the BPDE, which is fixed at the level of the DNA of the cells and involves changes in the long term being able to lead to the development of Cancer S.

In addition to their carcinogenic properties, the HAP present a mutagen character depend on the chemical structure of the metabolites formed. They can also involve a reduction in the answer of the immune system increasing the risks of infection thus.

Partition gas/particle

At the conclusion of the primarily pyrolytic formation processes, the HAP are emitted in the atmosphere, compartment from which they can then disperse in the other compartments of the environment. During their time of residence in the atmosphere, the HAP coexist at the same time in gas phase and particulate phase.

What determines the distribution of the HAP between the gas phase and the particulate phase, it is the Steam pressure saturating with the compounds. Indeed, the lightest HAP, with which steam pressures saturating are raised, will be mainly present in the gas phase whereas the heaviest HAP, whose saturating steam pressures are lower, are rather related to the particulate phase. In general, the compounds having two benzene cycles will be present in gas phase whereas those having more than six cycles are rather met in the particulate phase.

For the intermediate compounds (between three and six cycles), the distribution is done between the particulate phase and the gas phase. When the molecular mass of the HAP increases, and thus that the number of aromatic cycles grows, the partition is done in favor of the particulate phase. Up to four benzene cycles, the HAP are mainly present in gas phase, and that beyond, they are rather found in particulate phase. The room temperature also influences the partition gas/particle. Indeed, the saturating steam pressure depends on the temperature. Thus, more the temperature increases, more the HAP will tend to be present in gas phase. Moreover, it was observed that in summer the percentage of the HAP in gas phase is higher than in winter. However, this variation according to the seasons is much less important than that of the degree of aromaticity of the HAP.

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