Pollutants (sources, distribution, effects)

Particularly, in the case of air pollution, the dispersal of pollutants play an important role in the seen effects: in certain cases, pollution from a specific origin will disperse itself over a large area with a minimum impact. In other cases widely distributed pollution (for example transportation exhaust) will be concentrated by winds and geography to have a significant impact on the population.

Ozone 

Ozone is said to be a secondary pollutant; it is not emitted directly by car engines or by industrial operations, but formed by the reaction of sunlight on air containing hydrocarbons and nitrogen oxides that react to form ozone directly at the source of the pollution or many kilometers down wind. It is a contributing factor in the production of smog. Ozone is produced mostly during the summer months due to abundant sunlight and high heat. We are talking about tropospheric ozone or low level ozone which is known to cause respiratory problems and eye irritation. To the contrary ozone in the upper atmosphere caused buy the reaction of UV rays reaction with oxygen, gives us the ozone layer which protects us from UV rays.

Nitrogen oxides

Nitrogen oxides, generally  labeled as N0x, regroups :
nitric oxide (NO),
nitrogen dioxide (NO₂)
Sources :
N0x is mainly produced by man. All combustion at high temperatures and at high pressure produce N0x; automobile engines, particularly diesel engines due to their higher compression levels produce 2 to 3 times the amount of N0x than gasoline engines do. An aircraft on take-off generally produces 14kg of nitrogen dioxide, the approximate equivalent of 2000 diesel cars driving 25km.
Health risks : during peak pollution periods, N0x's can cause respiratory problems, inflammation and obstruction of  the airways and increased risk to microbial attack.
People at risk : smokers, people with respiratory problems (asthma and allergies), weak people and the elderly with cardiovascular problems.


Volatile organic compounds (VOC)

VOC's are hydrocarbons that can be anthropogenic (petroleum transformation, fumes from solvents, burning plastic etc....) and also from vegetation.


Greenhouse Gases

Carbon dioxide (CO₂)

Carbon dioxide is not toxic, it is necessary in plant growth. Environmentalists in the 1990's found evidence that an excess of carbon dioxide is a form of pollution because of its role in global warming (see greenhouse gases).This is the Kyoto Protocol, that came into effect in 2005, established a timeline for the reduction of this gas.
Although  questioned by a small minority, the consensus on the report from the IPCC of 2007 states that there is a 90% probability that man is responsible for climatic change since the industrial age to now.

Methane

Methane (CH₄) is harmful because of its large contribution to global warming. Its potential for global warming for a period of 100 years is 25 times that of CO₂ . Since methane degrades quickly in the atmosphere the impact is greater over a period of 20 years, be it 72 times that of CO₂.
)
Sources :
  Fermentation, decomposition
  Digestion gases from livestock.(this type of pollution represents an 18% contribution to climate change compared        
  to 13.5% for all modes of transport combined.
  Rice farms
  Natural gas

Nitrous oxide

Nitrous oxide N₂O,  is an important greenhouse gas despite low concentrations its potential for global warming over a 100 years is 298 times more than that of CO₂ . The production of N₂O is essentially a consequence of using nitrogen rich fertilizers in agriculture, and has nothing to do with other nitrogen oxides that are produced from combustion such as internal combustion engines.

CFC's

In the 1980's , chlorofluorocarbons (CFC's also known as freon)  were shown to have a potentially negative effects: such as the destruction of the ozone layer  thereby greatly contributing to global warming. The Montreal protocol is phasing out such manufactured compounds.
Sources :
Used in refrigeration and climate control systems for their heat transfer properties, they are released due to leaks from appliances and from the destruction of various equipment and appliances. It was also used as an aerosol propellant and was partly released with each use. Aerosols are now propelled by compressed air or carbon dioxide (CO₂)


Other Gases

Carbon monoxide (CO): is a byproduct of incomplete combustion. Dangerous because it attaches itself to  hemoglobin in the blood and reduces oxygen transport throughout the organism. It is odorless and transparent, it dissipates quickly in the ambient air but in closed areas its concentration becomes toxic, even deadly. Each year there are dozens of cases of deadly carbon monoxide poisoning due to combustion equipment (ie. Generators) placed in a poorly ventilated area.   

Sulfur dioxide (SO₂) : is the main byproduct produced by burning fossil fuels. Originates from anthropogenic sources(residential heating, transportation, industry...) and also natural sources : swamps, oceans, and volcanic activity. SO₂ is a respiratory tract irritant. SO₂ is also one of the compounds found in acid rain which harms ecosystems like forests and lakes. In 2006 China was the planets largest producer of sulfur dioxide emissions, which augmented 27% between 2000 and 2005.

Polycyclic aromatic hydrocarbons, are an assembly of compounds, of which some are carcinogenic, produced during combustion mostly domestic but also in diesel engines.

The BTX process:A mixture of Benzene, Toluene and Xylene.

Particulates

Particulates suspended in the air are mainly composed of:

Soot, carbon rich carcinogenic particulate due to incomplete combustion ( from diesel engines, industrial activities, residential wood heating, for example).

Dust ( from erosion or volcanic activity).

Particulates of biological origins; pollen, viruses, bacteria, spores...  Pollens may become allergenic from oxidizing pollutants( particularly ozone ), after a long while in the air or even after being exposed to UV 5...

Industrial production such as  lumber

The weight and size of these particulates ranging from a micrometer to hundreds of micrometers in diameter allows them to be dispersed by the wind. Once dispersed they can stay suspended from a couple of hours to many months.

Particulates can penetrate deep into the lungs the smaller the particulate the deeper it can penetrate. ( fine particulates smaller than 2.5 µm. Depending on their composition , their concentration and the duration of exposure to them  particulates can cause allergies, respiratory problems or even lesions leading to cancer  in some cases. 

Rainfall  brings back  many airborne pollutants to the ground, but some of them when dried will again become airborne. Pollutants that are lighter than water or water soluble are tentatively caught by the oceans in the surface biofilm, but can be released through evaporation or in sea spray and transported by the wind tens or even hundreds of kilometers during storms.

Among the emerging questions :

Those of climate change appear to preoccupy more and more. Pollution is making its way higher up into our atmosphere because of jet aircraft. Their vapor trails, not considered by the Kyoto Protocol has an environmental impact that seems to have been underestimated. 

Light pollution, which is on the rise is indirectly linked to air pollution. The reflection of light off suspended particulates augments the  size of the halo.