VOC’s
Volatile organic compounds (VOCs) include a large group of chemicals containing carbon and hydrogen atoms that can react quickly to form other chemicals in atmosphere. "Volatile Organic Compound" can mean any organic compound (all chemical compounds containing carbon) that is volatile (evaporating or vaporizing readily under normal conditions).

VOC’s are important because they can:

• react with oxides of nitrogen in the presence of sunlight to form ozone and photochemical smog
• be toxic to humans, animals or vegetation.

The major sources of VOCs are vegetation, automobile emissions, gasoline marketing and storage tanks, petroleum and chemical industries, dry cleaning, fireplaces, natural gas combustion and aircraft. VOC emissions from natural sources, such as forests, grasslands and swamps, are estimated to be almost six times greater than human sources. Individual VOCs are also produced from the evaporation of solvents and organic chemicals as well as from leaking valves, flanges, pumps and compressors at industrial facilities. The major source of VOCs in most urban areas is vehicle exhaust emissions.

Aromatic hydrocarbon
In organic chemistry, the structures of some rings of atoms are unexpectedly stable. Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone.

Polycyclic aromatic hydrocarbons (PAH’s)
Polycyclic aromatic hydrocarbons (PAH’s) are chemical compounds that consist of fused aromatic rings and do not contain heteroatoms or carry substituents. PAH’s occur in oil, coal, and tar deposits, and are produced as byproducts of fuel burning (whether fossil fuel or biomass).

Ozone
Ozone is one of the strongest commercially available oxidizing agents, and is commonly used for the treatment of water in municipal and industrial applications. The special advantage of ozone is in the environmentally friendly way in which it works, without the formation harmful by-products. It can also be generated from Oxygen in the atmosphere, making it a renewable resource.

Hydrogen Sulphide (H₂S) and Total Reduced Sulphur (TRS)
Hydrogen sulphide (H₂S) is a colourless gas with a rotten egg odour. Total Reduced Sulphur (TRS) compounds include: hydrogen sulphide, mercaptans, dimethyl sulphide, dimethyl disulphide and other sulphur compounds. Industrial sources of H₂S and TRS include petroleum refineries, tank farms for unrefined petroleum products, natural gas plants, petrochemical plants, oil sands plants, sewage treatment facilities, pulp and paper plants that use the Kraft pulping process, and animal feedlots. Natural sources of H₂S include sulphur hot springs, sloughs, swamps and lakes.

Ammonia (NH₃)
Ammonia (NH₃) is a colourless gas with the well-known pungent odour found in household cleaners. NH₃ is produced both by natural and human sources. Some natural sources of NH₃ include the decay of plant material and animal waste. A small portion is also released during respiration. The fertilizer industry is one of the main industrial sources of NH₃, producing synthetic NH₃ for either direct application to soil, or as a raw material for use in the production fertilizer products. The other major source of NH₃ is commercial feedlots, mainly due to the large amount of animal waste.

Sulphur Dioxide (SO₂O)
Sulphur dioxide (SO₂O) is a colourless gas with a pungent odour. Natural gas processing plants are responsible for a large proportion of SO₂O emissions. Oil sands facilities and power plants are also major SO₂O sources. Other sources include gas plant flares, oil refineries, pulp and paper mills and fertilizer plants.

Total Suspended Particulates (TSP)
Tiny particles of solid material or liquid aerosols, defined collectively as particulates, are present in the air, and at high concentrations, may become an air pollution concern. TSP range in size from 0.001 to 500 micrometres (a human hair is about 70 micrometres in diameter) and, depending on their size and other properties, may remain suspended in the air for a few seconds or indefinitely. TSP are primarily important from a nuisance perspective.

Suspended particles may result from a variety of natural and human sources. These sources include vehicle exhaust emissions, industrial emission sources, soil, road dust, dust resulting from other human activities (i.e. agriculture), smoke from forest fires, smoke from recreational sources (i.e. campfires and fireplaces).

Dimethyl sulfide (DMS)
Dimethyl sulfide (DMS) or methylthiomethane is an organosulfur compound with the formula (CH₃)₂S. Dimethyl sulfide is a water-insoluble flammable liquid that boils at 37°C (99°F).

Dimethyl sulfide has a characteristic cabbage-like smell that becomes highly disagreeble at higher concentrations. DMS has a low olfactory threshold that varies from 0.02 to 0.1 ppm between different persons.

Beetroot, asparagus, cabbage, corn and seafoods produce dimethyl sulfide when cooked. Dimethyl sulfide is also an odorant emitted by wastewater, kraft pulping mills.

Methyl mercaptan (CH₃SH)
Methanethiol (also known as methyl mercaptan) is a colorless gas with a smell like rotten cabbage with an extremely low odor threshold (2 ppb). It is a natural substance found in the blood, brain, and other animal as well as plant tissues.

Methanethiol is mainly used to produce methionine, which is used as a dietary component in poultry and animal feed. It is also used in the plastics industry and as a precursor in the manufacture of pesticides. It is also released as a decay product of wood in pulp mills.