ENVR S335 Environmental Control, Monitoring And Modeling (Free courseware)

This module introduces you to some issues related to air pollution. We start by defining air pollution and examining the main sources of air pollution. We then classify air pollutants according to origin, chemical composition, state of matter, 1 and area of impact; and examine the effects of air pollution, particularly the immediate health affects. We conclude by explaining Hong Kong’s Air Pollution Index.

What is air pollution?

Air pollution is not a new problem. But with today’s technology, we have added new dimensions to this centuries-old problem. This section defines and describes air pollution. Air is one of the global commons, owned by no one but used by all. It is a source of oxygen, which is vital to most living creatures including us human beings. Air also provides a source of carbon dioxide, a vital ‘food’ for plants. Air consists of 78% nitrogen, 21% oxygen, about 0.04% carbon dioxide and various inert gases including helium, argon (almost 1%), xenon, neon and krypton, and minute amounts of other gases. To fully understand air pollution, you need to also understand the earth’s atmosphere.

More than 80% by mass of the air and virtually all water vapour, clouds and precipitation occur in the lower portion of our atmosphere, the troposphere, which extends to about 10 to 12 km above sea level at mid-latitude. At the poles, the troposphere may go down to about 5 to 6 km above sea level, while at the equator, to about 18 km. The troposphere is very turbulent and has strong vertical air movements. Such turbulence leads to rapid and complete mixing and thus rapid dispersion of pollutants.

This is good on one hand because it thins out the air pollutants emitted, and bad on the other, as it spreads pollutants out and affects other areas. The pollution from forest fires in Indonesia that affected nearby ASEAN countries, and radioactive dust 2 from the nuclear disaster at Chernobyl was spread across Europe by this turbulence in the troposphere.

The stratosphere is a stable layer of very dry air, which extends to about 50 km above the surface of the earth. Little air movement occurs in the stratosphere, and air pollutants that find their way into this layer may stay there for years before they eventually drift back to the troposphere and disperse. The stratosphere is also where ozone absorbs the short-wave ultraviolet energy from the sun and protects us from excessive ultraviolet radiation. This layer, together with the troposphere, accounts for about 99.9% of the total mass of the air; thus when we are talking about air pollution, we refer to the effects in these two layers.

Pollution is an undesirable change in the physical, chemical, or biological characteristics of the air, water, or land that can harmfully affect the health, survival, or activities of humans or other living organisms (Henry and Heinke, 1996, 2 (Henry and Heinke, 1996, 2 – Environmental Science and Engineering, 2nd edn, Upper Saddle River, NJ: Prentice-Hall.)). The term ‘air pollution’ is used in this course to refer to the presence in the atmosphere of any materials or gases that affect human health or damage the natural environment due to human activities. The term ‘environment’ refers to both humans and their surroundings. This broad definition is used to emphasize the interrelations between humans and their surroundings, so they should be treated as one single system.