Sources of pollution

The list of atmospheric pollutants is long. To the classic names of well-known but nevertheless real pollutants such as particles, SO₂, NOx, CO, heavy metals, volatile organic compounds, fluorine, hydrochloric acid, etc. have been added a series of other substances such as greenhouse gases : CO₂, CH₄, N₂O, CFCs, HFCs, PFCs, SF₆, as well as other substances such as ozone, organochlorates (dioxins and furans), PAHs (Polycyclic Aromatic Hydrocarbons), etc.

Of course, there are many different sources of pollutants in the atmosphere and every sector of human activity is involved (the domestic sector, industry, agriculture, transport, etc.) as well as nature. It will be seen that the proportion of emissions which can be attributed to each sector varies considerably according to the constituents, particularly when it comes to the major energy-consuming sectors.

Solid particles
For both physiological and psychological reasons, pollution by dust was felt by populations in early times, and gave rise to regulations long ago, though the toxicity of dust is often less than many gaseous constituents. This last generalisation must of course be qualified according to the nature of the dust and the size of the particles (fine particles < 2.5 µm, asbestos fibre and dust, etc.).

The principal sources are combustion plants and industrial processes such as mineral extraction, cement works, steel works, foundries, glass-works, gypsum quarries, fine chemistry, etc. Emissions of dust have greatly diminished in the last 20 years. Solid particles act as vectors of different toxic or even carcinogenic or mutagenic substances (heavy metals, PAHs,...) and for this reason remain a major issue of concern.

SO₂ - Sulphur dioxide
Emissions of SO₂ are largely due to the use of fossil fuels containing sulphur (coal, lignite, petroleum coke, heavy fuel oil, domestic heating oil, diesel oil). All users of these fuels are involved. Certain industrial processes also emit oxides of sulphur (production of H₂SO₄, production of paper pulp, oil refining, etc.). Even nature emits sulphur compounds (volcanoes).

The biggest emitters are generally power stations, refineries, large combustion plants, etc. Natural gas, liquid petroleum gas (LPG) and wood are fuels with very little, if any, sulphur .

Quantities of SO₂ emitted into the atmosphere

NOx - Nitrogen oxides
Nitrogen oxides (commonly defined as NOx = NO + NO₂), like SO₂, come essentially from the combustion of fossil fuels and from a fewindustrial processes (production of nitric acid and fertiliser, surface treatment, etc.). They are mainly formed in combustion chambers where:
N₂ (air) + O => NO + N when the temperature exceeds 1400°C (thermal NO)
N₂ (fuel) + O => NO + N (fuel NO) if N₂ combined
N₂ + CH => HCN + N and then NO after different stages even at lower temperatures (prompt NO)

In the presence of oxygen, the NO is transformed into NO₂ (from 0.5 to 10%) in the furnace. This reaction continues slowly in the atmosphere and explains the brownish colour of the layers of polluted air a few hundred metres above towns with heavy traffic (action combined with dust). The oxides of nitrogen also play a part in the formation of photochemical oxidants and have an indirect effect on increasing the greenhouse effect.

The principal emitters are major combustion plants and internal combustion engines in particular (hence the policy of reducing emissions with catalytic converters for example). Volcanoes, thunderstorms and forest fires also contribute to emissions.

Quantities of NOx emitted into the atmosphere

CO - Carbon monoxide
Carbon monoxide is produced by incomplete combustion, generally due to badly-adjusted equipment (especially in the smallest installations). It is also present in the by-products of certain industrial processes (agglomeration of minerals, steel works, incineration of waste) but also and especially in motor vehicle exhaust fumes.

Quantities of CO emitted into the atmosphere

VOCs - Volatile organic compounds
The term VOC includes a multitude of substances and is a very loose definition. VOCs include hydrocarbons and the two groups are often wrongly assumed to be synonymous. This is no doubt due to the fact that VOCs are often measured in methane or propane equivalent total hydrocarbons, or by comparison with another hydrocarbon of reference. Methane (CH₄), which is one particular VOC, naturally present in the air, is frequently dealt with separately from other VOCs which are then termed NMVOCs (non-methane volatile organic compounds).

There are many sources of VOC emissions. These are due to certain industrial processes involving the use of solvents (basic chemistry and fine chemistry, parachemistry, degreasing of metals, paint application, printing, glues and adhesives, rubber, etc.), or not (oil refinery, use of CFCs, production of alcoholic drinks, bread, etc.).
The use of fuels in furnaces contributes somewhat to emissions but this cannot be compared to the proportions indicated for SO₂ and NOx.
One of the major sources of emission is transport (particularly motor vehicles). Note also that biomass is a significant producer (forests), not forgetting emissions from household products (paint, cleaning materials, perfumes and cosmetic, newspapers, tobacco, etc.).

Quantities of NMVOCs emitted into the atmosphere

CO₂ - Carbon Dioxide
The importance attributed to CO₂ comes from the rapid increase in the concentration of this gas in the atmosphere as a result of increased consumption of fossil energy in the world and a major reduction in the area covered by forests (a tropical forest absorbs from 1 to 2 kg of CO₂ per m² each year, whereas a European forest or a cultivated field absorbs only 0.2 to 0.5 kg of CO₂ per m² each year). In addition, the oceans play an essential role in maintaining the general carbon balance. The increase of CO₂ in the atmosphere, in the proportions that we observe, would probably not pose any problem to mankind for a very long time if it were not for the phenomenon of the enhanced greenhouse effect and its potential socio-economic consequences, which could prove dramatic according to the experts.

Compared with emissions by other countries, France is very well placed, with slightly under 2 tonnes of carbon per inhabitant per year.

Quantities of CO₂ emitted into the atmosphere

CH₄ - Methane
This occupies a distinct place among VOCs. It has a direct influence on the increase of the greenhouse effect (its contribution is estimated at about 18%) and its concentration in the ambient air is rising even more quickly than CO₂. Principal sources are: coal mining activities, domestic waste landfill sites, livestock, gas distribution, etc.

Quantities of CH₄ emitted into the atmosphere

N₂O - Nitrous oxide
Although not traditionally included among NOx, N₂O or nitrous oxide is an oxygenated compound of nitrogen. It contributes to the increasing greenhouse effect along with CO₂, CH₄, etc. N₂O is emitted during the combustion of fossil fuels, by certain industrial processes, motor vehicles and the soil (particularly when treated with nitrogen fertiliser). The oceans and the natural soil also contribute to emissions.

Quantities of N₂O emitted into the atmosphere

CFCs - Chlorofluorocarbons
These are totally artificial (with the exception of methyl chloride which is of marine origin). CFC emissions used to be due to the use of these products in everyday consumer goods (aerosol propellants, foams, fire extinguishers, refrigerants, etc.). Under an international agreement, the production of these substances, which contribute to the greenhouse effect, is now greatly reduced and in fact largely banned, but given their atmospheric lifetime (about 60 to 110 years depending on the substances), the quantities present in the atmosphere will continue to have an effect for some time.

HFC - Hydrofluorocarbons
Synthesised exclusively by chemical means, HFCs had no major applications before the adoption of the 1987 Montreal Protocol banning CFCs and HCFCs, used as CFC replacement gases. These compounds which also contribute to the greenhouse effect are today used as propellants in aerosols, as refrigerating fluids, solvents, foam expanding agents, etc.

Quantities of HFCs emitted into the atmosphere

PFC - Perfluorocarbons
Synthesised exclusively by chemical means, PFCs are largely used in certain stages in producing semi-conductors. They are also by-products of the electrolysis of aluminium. The improvement of processes has resulted in a noticeable decline in emissions of these greenhouse effect gases since 1990.

Quantities of PFC emitted into the atmosphere

SF₆ - Sulphur hexafluoride
Synthesised exclusively by chemical means, SF₆ is used in a great number of technical applications: as an insulating and cut-off agent in electrical equipment, and as a protective gas in magnesium foundries. SF₆ also contributes to the greenhouse effect.

Quantities of SF₆ emitted into the atmosphere

HF - Hydrofluoric acid
Fluorine is emitted in large quantities during the first fusion of aluminium. Electrolysis requires the addition of cryolite (Na₃AlF₆) which gives rise to HF. Because fluorine is present in numerous minerals (clays, natural phosphates, coal, dolomite, bauxite, etc.), the users of these minerals are therefore potential emitters. The best known are brickworks, manufacturers of glass fibre, enamel, iron and steel, etc. which generally remove pollutants from their waste gases before releasing them into the atmosphere.

O₃ - Ozone
This is an important intermediate element in the formation and development of photochemical oxidants. The ozone layer (stratospheric or 'good ozone') protects us from UV rays from the sun, whereas tropospheric ozone ('bad ozone') is a very toxic pollutant because it is in direct contact with humans and other ecosystems. There are very few industrial sources of ozone.

The combination of solar radiation, NOx and VOCs encourages the formation of tropospheric ozone in this part of the world and on a larger scale in the southern hemisphere. This is a complex problem because reducing nitrogen oxides can, as can be seen above, cause an increase in the concentration of ozone.

H₂S - Hydrogen sulphid
Well known for its smell and its toxicity which is much greater than SO₂. It is worth knowing that the smell disappears before it reaches the threshold of toxicity (the threshold of olfactory perception of H₂S is 0.1 ppm).
H₂S is mainly produced in factories making paper pulp by the Kraft process, and by the Claus process in oil refineries.

Heavy metals
There are various sources of heavy metals which contaminate the atmosphere.

Arsenic (As) comes either from traces of this metal in solid mineral fuels and in heavy fuel oil, or else from certain raw materials used mainly in processes such as glass-making, non-ferrous and ferrous metal working.
Quantities of As emitted into the atmosphere

Cadmium (Cd) is formed mainly through the manufacture of zinc and the incineration of waste. Burning solid mineral fuels, heavy fuel oil and biomass accounts for a significant proportion of emissions.
Quantities of Cd emitted into the atmosphere

Chromium (Cr) comes essentially from the production of glass, cement, ferrous metallurgy and foundries.
Quantities of Cr emitted into the atmosphere

Copper (Cu) comes mainly from the erosion of overhead cables by railway traffic. In addition, as for the other heavy metals, ferrous and non-ferrous metal production processes, the treatment of waste, and combustion are all, to varying degrees, major sources of copper emissions.
Quantities of Cu emitted into the atmosphere

Mercury (Hg) ) is emitted in small but still excessive quantities, by the combustion of coal and oil, the production of chlorine, and also by the incineration of household, hospital and industrial waste. Preventative action has considerably reduced mercury emissions in recent years.
Quantities of Hg emitted into the atmosphere

Nickel (Ni) is emitted essentially by the combustion of heavy fuel oil which contains traces of this metal.
Quantities of Ni emitted into the atmosphere

Lead (Pb) was mainly emitted by automobile traffic until leaded petrol was banned (as of 1 January 2000). Other sources of lead are the first and second fusion of lead, the manufacture of electric batteries and certain types of glass (crystal), etc. Lead is very toxic, which can result in lead poisoning, so emissions are severely restricted to very low levels.
Quantities of Pb emitted into the atmosphere

Selenium (Se) mainly comes from glass production. The use of heavy fuel oil also contributes to emissions, due to the traces of this metal that it contains.
Quantities of Se emitted into the atmosphere

Zinc (Zn) comes from the combustion of coal and heavy fuel oil but also from certain industrial processes in the fields of ferrous and non-ferrous metals, and the incineration of waste.
Quantities of Zn emitted into the atmosphere

Other pollutants
Other pollutants include:

• Ammonia (NH₃), of which agricultural activities form by far the major source,
  Quantities of NH₃ emitted into the atmosphere
  
• Polycyclic Aromatic Hydrocarbons (PAHs) due to certain phenomena of combustion. Some of these, such as benzopyrene, are recognised as being highly carcinogenic,
  Quantities of PAHs emitted into the atmosphere
• Dioxins and furans (PCDD-F) which can be formed in certain specific conditions and certain varieties of which are very toxic,
  Quantities of PCDD-F emitted into the atmosphere
• etc. This list is far from exhaustive.