What is Air Pollution?
Air pollution is somewhat difficult to define because many air pollutants, at low concentrations, are essential nutrients for the sustainable development of ecosystems. So, air pollution could be defined as: A state of the atmosphere, which leads to the exposure of human beings and/or ecosystems to such high levels or loads of specific compounds or mixtures thereof, that damage is caused. With very few exceptions, all compounds that are considered air pollutants have both natural as well as human-made origins.
Air pollution is not a new phenomenon; in Medieval times, the burning of coal was forbidden in London while Parliament was in session. Air pollution problems have dramatically increased in intensity as well as scale due to the increase in emissions since the Industrial Revolution.
All reports on air pollution in the nineteenth and early twentieth centuries indicate that the problems were local, and concentrated in or around industrial centers and big cities. Even the infamous environmental catastrophes in the area of Liege in the 1930s — the first recorded occurrence of death by air pollution — or in London in the 1950s, were essentially local phenomena. In the London smog episode, stagnant air accumulated dangerously high sulfur dioxide and sulfuric acid concentrations, killing several thousand inhabitants. Epidemiological research has recently shown several damaging effects of aerosols on the respiratory tract, inducing asthma, bronchitis, and early demise. See Impact of local air pollution for more information.
Only in the second half of the 20th century were the effects of air pollution detected on regional (>500 km), continental, and global scales. Around 1960, acid deposition, commonly referred to as acid rain, caused the first observed effects on regional to continental scales.
Lakes in Scandinavia, as well as in North America, lost their fish populations as the lakes were acidified by acid deposition to the point that fish eggs would not produce young fish anymore. About 10 years later, acid deposition was found to cause damage to forests and loss of vitality of trees. See Impact and abatement of acid deposition and eutrophication for more information.
Smog episodes in U.S. cities, such as Los Angeles, were reported during the same period. Reactions of volatile organic compounds (VOCs) and nitrogen oxides (NOx) produced high concentrations of ozone and peroxides that are harmful to human and ecosystem health. Around the same period, the first high oxidant concentrations (the complex mixture of ozone, peroxides and other products of the reactions of organics and nitrogen oxides are called oxidants) were more and more frequently occurring in Europe during stagnant meteorological conditions.
At the same time, severe eutrophication (damage and changes in ecosystems due to the availability of excessive amounts of nutrients) was encountered in Europe and the U.S. Deposition of ammonium and nitrates were shown to contribute substantially to high nutrient concentrations in soil and groundwater. Nitrates and ammonium are beneficial, even essential, for development of vegetation, but in too high concentrations they lead to the loss of diversity, especially in oligotrophic (adapted to low nutrient availability) ecosystems.
The next scale of air pollution is its effect on global dimensions, such as the destruction of stratospheric ozone due to emissions of CFC’s (chlorofluorocarbon compounds). This issue was given a lot of attention in the period 1985–1995, as it was revealed that the destruction of stratospheric ozone leads to higher UV-light intensities and a higher incidence of skin cancer. For more information, see Antarctic ozone hole.
From 1990 onwards, the increase in the concentrations of radiative active substances (compounds which alter the radiative balance of the Earth; greenhouse gases; but also aerosols, and water in liquid form, as clouds) and the connected climatic consequences brought about new research in air pollution.
Historic Ozone Trends
The above time sequence of problems related to air pollution could give the impression of sudden increases in air pollution concentrations. However, that is probably not the case, as can be explained in the case of ozone. By carefully characterizing old methodologies, Voltz-Thomas and Kley have been able to reconstruct ozone concentrations in the free troposphere (the air not directly influenced by processes taking place at the Earth's surface).
The ozone concentrations in Europe slowly increased at a rate of 1 to 2% per year from 10 ppb (parts per billion) to over 50 ppb. It is well-documented that the effects of ozone start at levels of about 40 parts per billion (ppb is a mixing ratio of 1 molecule ozone in a billion molecules of air). Thus, it is not surprising that the effects of ozone were detected in the 1970s, as background continental ozone was already 30 ppb, and additional oxidant formation would increase the ozone concentrations locally or regionally. However, the increase in the continental background concentrations of ozone had already been occurring over a long period of time. In general, the effects of pollution, and of air pollution in particular, are a function of the degree of transgression from the limits over which effects can be expected.
Transformation and Deposition
Pollutants that affect human and ecosystem health in the form in which they are initially emitted are called primary pollutants. Sulfur dioxide is a good example of a primary air pollutant.
Ozone, on the other hand, is a good example of a so-called secondary air pollutant; ozone is a product of a large set of atmospheric, chemical reactions involving nitrogen oxides (NOx) and volatile organic compounds (VOCs). These precursors (compounds which lead to a certain product, in this case, ozone) are emitted by a wide variety of sources. In the U.S. and Europe, they are mainly produced by the transportation sector. These precursors are then transported by air movement and involved in a large and intricate set of atmospheric reactions under the influence of sunlight; ozone is one of the products.
The next stage is the deposition of primary and secondary air pollutants. Deposition can take place in two different ways: wet deposition and dry deposition. In wet deposition, air pollution is first incorporated in clouds or precipitation, and is transported by way of precipitation (acid rain) to the Earth's surface. In dry deposition, air pollutants are deposited directly on vegetation and the Earth's surface. Effects caused by acid deposition and eutrophication are directly linked to the deposition loads.
The removal of pollutants by chemical transformations and deposition processes is essential. If, for instance, sulfur dioxide emitted by natural sources such as volcanoes were not removed through these processes, a concentration of 1,000 ppb or more would be reached in the atmosphere in only a few months, endangering the survival of ecosystems and the human population.
Once the effects of air pollutants are related to the transgression of critical loads or concentrations, it is possible to derive the necessary emission reductions. Models describing the complete process from emissions via transport, through chemical conversions, to deposition and exposure plus their effects are used for this purpose.