1.1 The Earth’s Atmosphere

The Earth’s atmosphere is composed mostly of molecular nitrogen (N₂, 78 % of dry air) and molecular oxygen (O₂, 21 % of dry air). It holds also a fair amount of water vapor (H₂O), which varies greatly in concentration (ranging from negligible in dry regions to a few % in humid regions) and leads to the formation of clouds and fogs in case of supersaturation. The Earth’s atmosphere also contains carbon dioxide (CO₂), which has an average concentration of about 0.04 %. H₂O and CO₂ are gases that absorb infrared (IR) radiation, but let ultraviolet (UV) and visible solar radiation go through. Since they partially absorb IR radiation emitted by the Earth toward space, these species are called “greenhouse gases” (GHG). As a result, the temperature of the Earth’s atmosphere is on average about 34 °C (61 °F) warmer than it would be without those GHG. However, an increase of GHG atmospheric concentrations leads to climate change and its associated consequences in terms of extreme meteorological events, changes in precipitation patterns, and increase of sea level.

Molecular oxygen undergoes photolysis by UV solar radiation, which leads in the upper atmosphere to the formation of oxygen atoms (O) and the formation of triatomic oxygen, i.e., ozone (O₃). Ozone absorbs part of UV solar radiation and, therefore, protects organisms on the Earth’s surface from some of those harmful rays (which may lead to skin cancer).

In addition, gaseous chemical species and atmospheric liquid or solid particles are present in the atmosphere. They originate either from natural sources or from human activities (also called anthropogenic activities). Some of those gases and particles may be harmful to human health and the environment (e.g., ecosystems, buildings, atmospheric visibility). Those gases and particles are then considered to be air pollutants. Some of those chemical species are not necessarily harmful when present in the air, but may become harmful after their transfer to other environmental media and their possible transformation and/or bioaccumulation in those media (soil, water, and the food chain).

1.2 Air Pollution

One may distinguish several aspects of the perturbation of the atmospheric environment. On one hand, some phenomena are global in nature, because of the long lifetime (several years or decades) of the chemical species involved. This is the case in particular for:

1. 
   

   – Climate change due to GHG

   
   
2. 
   

   – The destruction of the stratospheric ozone layer

On the other hand, some phenomena pertain mostly to the lower atmosphere and their impacts occur at the Earth’s surface. We group those phenomena here under the general term of air pollution. Air pollution may occur at spatial scales that are local, regional or global, and for time periods that may be short (on the order of an hour) or long (on the order of a year). Some air pollutants are emitted directly into the atmosphere, whereas others are formed in the atmosphere via chemical processes (chemical or photochemical reactions) and/or physical processes (gas-to-particle conversion). Therefore, one distinguishes:

1. 
   

   – Primary air pollutants, which are emitted directly into the atmosphere

   
   
2. 
   

   – Secondary air pollutants, which are formed in the atmosphere from other chemical species, called precursors

If all precursors of secondary air pollutants are not necessarily directly harmful to human health or the environment, they are nevertheless considered to be pollutants, because they contribute to air pollution.

As mentioned above, one may distinguish air pollutants that have adverse effects due directly to their concentrations in the air and those that lead to adverse effects following their deposition to the Earth’s surface and subsequent transformation in other media, such as soil, water, and the food chain.

Historically, air pollution started with the discovery of fire and human exposure to the chemical species produced during biomass burning (combustion of wood and vegetation) in poorly ventilated locations where concentrations could reach levels that are harmful to human health (e.g., Hardy et al., 2012). During the 19th century, the Industrial Revolution led to the emission of significant amounts of air pollutants via the combustion of a variety of fossil fuels (coal, oil, and gas). In particular, the air pollution in London during the 19th and 20th centuries became particularly problematic. The most significant, documented London air pollution episode occurred during the winter of 1952 (Wilkins, 1954; Bell et al., 2004). Smoke from combustion mingled with London fog to become “smog,” a term coined in the early 20th century by Harold Antoine des Vœux, a member of the Coal Smoke Abatement Society, at a conference of the American Medical Association in London (JAMA, 1905). The stagnant atmosphere during the 1952 event, which contained high concentrations of sulfur dioxide (SO₂), nitrogen oxides (NO_x), and atmospheric particles (including sulfate), contributed to the deaths of several thousands.

During the 1950s, a new type of air pollution appeared in the Los Angeles basin in California. The use of big cars to move around an area characterized by widespread urban planning, combined with industries (power plants, refineries, port activities, etc.) located on the Pacific coast, led to the formation of secondary pollutants such as ozone, oxygenated organic compounds (e.g., acrolein), and fine particles. This type of air pollution was the result of atmospheric chemical reactions. It was more troublesome during summer when solar radiation was intense. Arie Jan Haagen-Smit, a biochemistry professor at the California Institute of Technology (Caltech), proposed in 1952 that emissions of nitrogen oxides and volatile organic compounds (VOC) led, via chemical reactions in the presence of sunlight, to the formation of ozone. Solar radiation in the ultraviolet and visible range has sufficient energy to break the bonds of some molecules. This process, called photolysis, creates chemical species that are very reactive and can initiate a series of chemical reactions leading to the formation of the pollutants mentioned previously. Therefore, this form of air pollution was called photochemical smog, because sunlight was needed to initiate the photochemistry (chemical reactions involving the photolysis of molecules) of this type of air pollution.

In 1962, Rachel Carson published Silent Spring, a book that alerted readers to the harmful effects of pesticides, such as dichlorodiphenyltrichloroethane (DDT), on birds. Scientific studies followed, which confirmed the harmful effects of chlorinated organic compounds not only on the avian fauna, but also on other animals and aquatic ecosystems. Furthermore, their bioaccumulation in the food chain was shown to lead to potentially harmful effects (including carcinogenic ones) on humans. Since then, these organic compounds have been grouped under the term persistent organic pollutants (POP), because their lifetime in the environment is very long (on the order of several years).

During the 1970s, the destruction of forests in Europe and North America suggested the presence of another type of pollution. Atmospheric deposition of acidic species, such as sulfuric acid (H₂SO₄) and nitric acid (HNO₃), modified significantly the chemical equilibrium of soils and surface waters (e.g., Likens and Bormann, 1974; Likens et al., 1979). As a result, nutrients were mobilized and washed away, whereas some toxic metals became available to the roots, as well as to terrestrial and aquatic flora and fauna. This type of pollution was called “acid rain,” because the load of those acidic species was associated in great part with rain events. Actually, dry deposition and snowfall also contribute to atmospheric deposition and the subsequent increase in acidity of soils and surface waters. Therefore, acid deposition is a better, more scientific and precise term than acid rain.

During the 1950s, the contamination of fish by mercury was identified in Minamata, a fishing village on the Island of Kyûshû in Japan, as the possible source of neurologic diseases affecting local people who consumed large amounts of fish (Smith and Smith, 1975). During the 1990s, atmospheric mercury was identified as a major source of contamination of the aquatic fauna, because it can be transformed by bacteria into organic mercury after entering an ecosystem. Next, organic mercury bioaccumulates in the aquatic food chain up to the higher trophic levels, where its concentrations may become harmful to humans who consume fish.

More recently, diesel particles have been listed as carcinogenic to humans by the International Agency for Research on Cancer (IARC) of the World Health Organization (WHO; Benbrahim-Tallaa et al., 2012). Later, IARC listed air pollution, as a whole, as carcinogenic to humans (Loomis et al., 2013).

Some pollutants, such as ozone, not only have harmful health effects, but may also impact vegetation, thereby leading to significant economic loss for agriculture (e.g., Rich, 1964). Furthermore, there are negative aesthetic impacts of air pollution. For example, buildings and statues may be deteriorated by acidic species, and soiled by soot particles (e.g., Kucera and Fitz, 1995). In addition, sunlight is scattered by fine particles and may be absorbed by some particles and gaseous pollutants. As a result, the human eye does not see as far during an air pollution episode (smog or photochemical smog) as under pristine conditions, because the visual range has been degraded by the presence of those fine particles and light-absorbing gases (e.g., Trijonis, 1979).

This brief historical summary highlights the fact that the term air pollution involves a large number of pollutants and adverse health and environmental effects. Furthermore, air pollution occurs at a variety of spatial and temporal scales. The objective of this book is to explain in technically simple, but fundamentally precise, terms the processes that lead to air pollution, to identify the main sources, and to describe its main health and environmental impacts. Possible solutions, both in terms of public policy and available air pollution control technologies, are also presented.