The three main air pollutants are:

1. Suspended Particles – made up of soot, smoke, dust and liquid droplets.
2. Associated health hazard: particles and soot exposure over a long period of time is related to a wide range of chronic respiratory illness such as asthma and chronic obstructive pulmonary diseases as well as worsening heart conditions and other conditions.
3. Sulfur Dioxide – produce by the combustion of fossil fuels, on the whole, motor vehicles and small and varied sources (such as boilers and stoves) contributing the most.
4. Associated health hazard: causes acid rain and can be extremely detrimental to the health of the young and elderly.
5. Nitrogen Dioxide – caused by fuel combustion, aerobic decomposition and nitrogenous fertilizers.

Associated health hazard: causes acid rain and can damage lung tissue by forming into acid in the presence of water

Associated with the considerable but temporary spikes of air pollution related deaths is Winter smog which is made up of mainly of dust, soot and sulfur dioxide. A good example of a severe case of Winter smog which was related to a large number of deaths of the London Smog of December 1952 which was thought to be responsible for up to 4000 deaths.

Summer smog is also very common in cities and is caused by ground level ozone occuring due to actions of the sun mixed with particles and volatile organic compounds. Summer smog causes extreme breathing difficulties for asthmatics as well as having an adverse affect on plant life, often severely reducing numbers. Furthermore, According to the World Bank 1992, at least half a million premature deaths are caused by air pollution worldwide and the significant number of these occur in major cities around the world. ......source http://www.portfolio.mvm.ed.ac.uk/studentwebs/session4/27/citydiff.htm

The greenhouse effect is the term used to describe the role of the atmosphere in insulating the planet from heat loss. Greenhouse gases (GHG) are gases in the atmosphere that give rise to this greenhouse effect. GHGs allow shortwave or solar radiation to penetrate down to the atmosphere and the Earth's surface. This radiation is absorbed and the Earth's surface warms (Figure 1). GHGs function as a blanket that traps heat in the lower atmosphere - termed the "greenhouse effect."

The radiative balance of the atmosphere, termed the greenhouse effect. The solar energy passes through the atmosphere and is absorbed by the Earth. This energy is emitted as long-wave radiation and is absorbed by the greenhouse gases in the atmosphere. Source: Government of Canada.
This "natural greenhouse effect" is an important phenomenon to biological life on Earth. The average surface temperature is maintained at approximately 15°C because of this effect. If all the radiant energy were to escape out of the atmosphere, the global temperature would be -18°C, a difference of 33°C.

The Earth's temperature and climate system can be thought of as a heat engine driven by energy from the sun. There is an energy balance between incoming and outgoing radiative energy that is partially regulated by the concentrations of GHG gases in the atmosphere. Climate change occurs when the total amount of the sun's energy absorbed, does not equal the amount of energy released, causing an imbalance in the radiative exchange.

Consequently, humans can also cause temperatures and the climate system to change. Human activities such as the burning of fossil fuels, deforestation or land surface change, industrial processes, etc., are increasing the concentration of GHGs in the atmosphere at an alarming rate. This additional increase of GHG is known as the "enhanced greenhouse effect", where more incoming energy is trapped within the atmosphere. This can have serious impacts on the physical and chemical processes, and biological life on Earth.

The proportion of GHGs amounts to less than 1% of the total gases in the atmosphere. However, these gases collectively increase the Earth's surface temperature by 33°C. Because the concentrations of these gases in the atmosphere are so low, it is possible for human emissions to have a drastic effect on the radiative budget and resulting surface temperature. ..........source http://www.msc.ec.gc.ca