You should also consider agriculture

Following from the above suggestion of Agriculture, Land use change can have a big impact, as can mix of Electricity Generation sources. 

Its one of them also.

Natural sources can also contribute to total pollution, eg.:

- sea spray (particulate matter=PM)

- volcanoes (H2S, SO2, PM)

- dust storms (PM)

- stratospheric intrusions (ozone)

Yes thank you your additional valuable inputs. 

It is interesting to distinguish between natural and anthropogenic sources. These are the main classical sources of primary and secondary pollutants (I don't include here typical GH gases such as CO2, etc):

Natural sources:

Oceans (PM, precursors of SOx)

Soils (PM by resuspension, percursors of SOx, NH3)

Vocanoes (SO2 and its precursors)

Vegetation (PM, VOCs)

Lightning (NOx)

Biomass burning (PM, NOx, SO2 and precursors, VOCs)

Stratospheric contributions (O3)

Anthropogenic sources

Industry (PM, NOx, SO2 and their precursors, heavy metals, VOCs, PAHs, gaseous Hg, Cl....)

Traffic (NOx, primary PM and gaseous precursors, PAHs, VOCs, NH3,  heavy metals)

Biomass burning (PM, NOx, SO2 and precursors, VOCs)

Agriculture and livestock sector (PM, VOCs, NH3)

Thank you very much. the topic almost covers these factors with pollutants!

I agree with everything that has been written so far. As with any medium accepting 'foreign' material, this material only becomes a pollutant as such when the medium can no longer deal with the material loading. So although there are many sources that emit pollutants to the atmosphere they will only lead to an air pollution event if the loading to the atmosphere exceeds what it can readily deal with (i.e. dilute). Therefore the factors leading to a high pollution event will also include the state of the atmosphere as well as how much and what type of the pollutants are being emitted.

Good, That is also valuable answer to explore more about the question. Thank you very much.

Also consider emissions from Thermal Power Plants and Cement industries.

BVOC's - Biogenic Volatile Organic Compounds- are mostly emitted by vegetation (especially by some functional type of plants) .

The total amount is really huge. These compounds may have big relevance in pollutant formation and destruction, for instance for ozone and particulate matter.

Please deepen this issue here, if of your interest:

https://www.researchgate.net/publication/222109646_Emissions_of_Volatile_Organic_Compounds_From_Vegetation_and_the_Implications_for_Atmospheric_Chemistry

http://www.apis.ac.uk/overview/pollutants/overview_VOCs.htm

http://www.epa.gov/ord/sciencenews/scinews_trees-and-air-pollution.htm

http://www.nrs.fs.fed.us/units/urban/local-resources/downloads/Tree_Air_Qual.pdf

Regards

Valerio

Article Emissions of Volatile Organic Compounds From Vegetation and ...

 Thank you very much Valerio. It is useful paper for me. It supports to explore new pollutant, formation and destruction. Thank you

Don't forget agricolture, in particular livestock and fertilizers, which are responsable of emissions of ammonia, methane and N2O.

Also, the main factor for air pollution is  the toxic elements or short lived radon decay products (Po-218, Pb-214, Bi-214, Po214) and different types of aerosol and bioaerosol in the atmosphere. 

Thank you very much sir for your valuable additional inputs. Now i have complete package of air pollutants released in the atmosphere; produced/generated from different sources in the earth.

There is a classic text called Air Pollution that is in its 5Th edition by Dr. Daniel Vallero. It is a superb start and after you master the concepts you will be able to examine the current literature wisely

The importance and speed of mechanisms like settling and washing out (together called deposition), rates of chemical transformations and dispersion in the air, kinetics of diffusion and sorption at the soil and liquid surfaces are the factors determining pollutant concentrations as well as their residence times in the air.

Treshold air concentrations defining if a substance is an air pollutant are either exceeded or met through one or more of these processes beyond their emission rates. Thresholds are set according to certain criteria of effects (on health, economy, ecosystem properties, etc.). They are backed up by legal procedures.

However, to be able to define if a molecule in the air causes pollution we need to take other more scientific factors into consideration. Obviously modern analytical techniques are near to excellent and are capable of determining extremely low levels of chemicals in the air. However, depending on which chemical we are talking about, only if their concentration is above a respective critical level we call it a pollutant. These critical concentrations are mostly taken as the average natural background concentrations of the compounds. Some compounds do not naturally exist in the air, so thier presence is pollution. But many compounds exist at very low levels in the composition of average clean air, so their increase due to human activities causes air pollution by them. 

Thank you Aysen for your valuable analytic views. There is some criteria for human health effect to determine the severity of the pollutant e.g. noise pollution 78db etc. For micro pollutants as you said study at molecule level is necessary to find out such pollutants and their identification.

Thank you 

In developing countries, a major driver of air pollution and direct GHG emissions is unregulated, non-planned urban sprawl. It leads to longer trips to work/school/business, using private cars or inefficient public transport, saturation of roads, lower speeds, many stop&go, you may keep counting other effects.

Don't forget to analyse Black carbon (BC) emissions from those sectors. BC has been identified as a short lived climate pollutants (SLCPs) and has very high global warming potential value as compared to CO2.

Off course, we are suffering from these problems. In my experience the main root cause is unplanned interventions in this sector and non regulated policy and regulations. All these are undermined by the government due to strong bureaucracy and political instability.

Thank you very much

https://www.conserve-energy-future.com/causes-effects-solutions-of-air-pollution.php

Vehicle Emissions

Vehicle emissions are another source of fossil fuel emissions and air pollution. Private transportation accounts for about 10 percent of your carbon footprint, or the amount of carbon dioxide your lifestyle and activities contribute to the atmosphere, reports Carbon Footprint Ltd.

Industry

Industry is a major contributor to air pollution. Industrial processes release pollutants such as nitrous oxide and hydrofluorocarbons into the air. Agricultural practices, livestock rearing and landfills also contribute to atmospheric methane concentrations. The overall effect is an increase in the global warming potential.

Deforestation

Deforestation affects the atmosphere in several ways. Forests act as sinks for carbon dioxide through a process called carbon sequestration. Trees store carbon dioxide in their plant tissue as they take in this gas to undergo food-making. In effect, this action removes carbon dioxide from the air. When forests are burned and destroyed, this storage area for carbon dioxide is removed, increasing the concentrations of atmospheric carbon dioxide.

Wood Fires

Wood fires cause air pollution by releasing particulate matter into the air. These particles can become lodged in your respiratory system, causing irritation to tissues. The particles can also aggravate existing health conditions such as asthma, warns the U.S. Environmental Protection Agency.

Smoking

You are still at risk of the dangers of smoking even if you are a nonsmoker. The University of Minnesota estimates that up to 90 percent of the American population is routinely exposed to secondhand smoke. Tobacco smoke contains 40 carcinogens, making it an especially lethal form of air pollution.

https://www.livestrong.com/article/217888-seven-causes-of-air-pollution/

Yes It is quite useful

wind velocity, unplanned construction and improper construction materials management also plays a major part in pollution

https://www.researchgate.net/publication/230738894_Peri-Urban_to_Urban_Landscape_Patterns_Elucidation_through_Spatial_Metrics

https://www.researchgate.net/publication/229826816_Empirical_patterns_of_the_influence_of_Spatial_Resolution_of_Remote_Sensing_Data_on_Landscape_Metrics