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Daly, A. and P. Zannetti. 2007. An Introduction to Air Pollution – Definitions, Classifications, and History. Chapter 1 of AMBIENT AIR POLLUTION (P. Zannetti, D. Al-Ajmi, and S. Al-Rashied, Editors). Published by The Arab School for Science and Technology (ASST) (http://www.arabschool.org.sy) and The EnviroComp Institute (http://www.envirocomp.org/).

Chapter 1

An Introduction to Air Pollution – Definitions, Classifications, and History Aaron Daly and Paolo Zannetti
The EnviroComp Institute, Fremont, CA (USA) daly@envirocomp.com and zannetti@envirocomp.com Abstract: Definitions and classifications used in the study of air pollution are introduced. Also introduced are a brief history of air pollution, its regulation, and trends of its ambient concentrations and emissions. Keywords: Anthropogenic, biogenic, geogenic emissions; primary and secondary pollutants; criteria, hazardous, radioactive, indoor pollutants; sulfur smog, photochemical smog; UN-ECE, US-EPA, Clean Air Act.


The Natural Environment and Some Definitions of Air Pollution

The present-day atmosphere is quite different from the natural atmosphere that existed before the Industrial Revolution (circa 1760 1 ), in terms of chemical composition. If the natural atmosphere is considered to be “clean”, then this means that clean air cannot be found anywhere in today’s atmosphere. The chemical composition of the pre-industrial (i.e., before the 18th century), natural global atmosphere is compared to current compositions in Table 1: 1

T.S. Ashton, The Industrial Revolution, 1760-1830, London: Oxford University Press, 1948. 1

© 2007 The Arab School for Science and Technology (ASST) and The EnviroComp Institute


Ambient Air Pollution Table 1. Atmospheric Chemical Compositions2.

Gas Nitrogen Oxygen Argon Neon Helium Krypton Xenon Carbon dioxide Methane Nitrous oxide Water Vapor

Symbol N2 O2

Ar Ne He Kr Xe CO2 CH4 N2O H2O

Percent by volume (Current Atmosphere) 78.1 20.9 0.92

ppm (Natural Atmosphere)

ppm (Current Atmosphere)

18.2 5.2 1.14 0.09 280.0 0.750 0.270 Variable (0.004 to 4) 370.03 1.774 0.3185

Defining “air pollution” is not simple. One could claim that air pollution started when humans began burning fuels. In other words, all man-made (anthropogenic) emissions into the air can be called air pollution, because they alter the chemical composition of the natural atmosphere. The increase in the global concentrations of greenhouse gases CO2, CH4, and N2O (shown in Table 1), can be called air pollution using this approach, even though the concentrations have not found to be toxic for humans and the ecosystem. One can refine this approach and only consider anthropogenic emissions of harmful chemicals as air pollution. However, this refined approach has some drawbacks. Firstly, one has to define what “harmful” means. “Harmful” could mean an adverse effect on the health of living things, an adverse effect on anthropogenic or natural non-living structures, or a reduction in the air’s visibility. Also, a chemical that does not cause any short-term harmful effects may accumulate in the atmosphere and create a long-term harmful effect. For example, anthropogenic emissions of chlorofluorocarbons (CFCs) were once considered safe because they are inert in the lowest part of the atmosphere called 2

Builtjes, P. (2003) The Problem – Air Pollution. Chapter 1 of AIR QUALITY MODELING – Theories, Methodologies, Computational Techniques, and Available Databases and Software. Vol I – Fundamentals (P. Zannetti, Editor). EnviroComp Institute (http://www.envirocomp.org/) and Air & Waste Management Association (http://www.awma.org/). 3 http://lwf.ncdc.noaa.gov/oa/climate/gases.html#cd 4 http://lwf.ncdc.noaa.gov/oa/climate/gases.html#m 5 http://lwf.ncdc.noaa.gov/oa/climate/gases.html#n2o

1 An Introduction to Air Pollution


the troposphere. However, once these chemicals enter the stratosphere, ultraviolet radiation can convert...
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