September 23, 2011, 11:50 am

In general the term smog has become connected to a situation where visibility is significantly impaired as a result of local air pollution. This loss of visibility is generally caused by the formation of fine particles.

The term smog, a contraction of smoke and fog, was introduced in 1905 by Dr. H.A. des Vœux to describe the mixture of soot, sulfuric acid, and other pollutants in emissions from coal furnaces in London. The city has been affected by smog since the times of King Edward I (1272-1307), who was the first to try to forbid the use of coal in London. During the winter of 1952, London experienced a very severe smog event during a unusual cold spell that resulted in approximately 12,000 additional deaths in the city. This episode resulted in significant legislation to regulate and limit the underlying causes of the smog. This case highlighted the potential health impacts of smog.

Smog is a common problem in many large urban areas where human activity gives rise to particulates in the air. It can be exacerbated significantly by weather conditions (e.g. temperature inversions) and a local geography (e.g., surrounding mountains) that can prevent particulates from being disbursed naturally by winds. While coal burning was the major cause of smog in years prior to the 1960s, today it is more closely associated with automobile and industrial emissions that interact frequently to create the particles that cause smog.

Nearly all major cities and many smaller ones have a smog issue. Large cities that have very notable smog issues include Beijing, China, Mexico City, Mexico, Tehran, Iran and Los Angeles, California. Significant progress in addressing smog in such cities as London, England and Los Angeles illustrate how the problem can be tackled.

Indicative of the direct and indirect importance of local weather conditions, several scientific authors have distinguished two types of smog based on season: summer and winter smog.

Indicative of the importance of different chemicals interacting to cause smog, is the term photochemical smog. The most common cause of photochemical smog is the interaction of nitrogen oxides (or NOx) and Volatile Organic Compounds (or VOCs in sunlight resulting in particulates and ground-level ozone.

Fine particles and visibility

caption Figure 1. Interaction between aerosols and light
Underlying smog is the interaction of light with fine solid or liquid particles in the air, known as aerosols.

Aerosols have a large impact on visibility, as we see objects by their reflected light. Aerosols of a certain size, between 0.1 and 1.5 ?m in diameter, scatter visible light. Not only is the amount of light emitted by the object we are looking diminished due to the presence of aerosols, but sunlight is scattered by the particles to form a background haze that limits visibility.

Two types of scattering, "Mie" and "Rayleigh" scattering are observed:

  • Rayleigh scattering is scattering that occurs primarily as a result of gas molecules, limiting the horizontal visibility. It is a function of the wavelength of the light and limits the visibility to approximately 130-260 km (80-160 miles) depending on the color of the light.

  • If the wavelength of the incoming light and the size of a particle are about the same, the light can be scattered in all directions, including backward (back scatter). This is referred to as Mie scattering. Particles between 0.1 and 1.5 micron, with about the same diameter as the wavelength of incoming solar visible light, scatter the visible light and reduce visibility

caption Figure 2. Connection between aerosol mass and visibility, in km.
caption Figure 3. increase in size of aerosol particles with increasing relative humidity.

In addition, particles can absorb visible light (such as carbon-based particles). Some light may pass through the particles or aerosols if no absorption takes place.

Very small particles have minimum influence on the light. For a given size distribution of particles and relative humidity, a |linear relation exists between the visibility and the aerosol mass (Figure 2.)

Pollution episodes lead to much a higher number of small particles, so visibility is reduced. This reduction of visibility is worse under conditions of high relative humidity: Aerosol particles contain hygroscopic compounds like ammonium sulfate, ammonium nitrate and sodium chloride. At increasing relative humidity water is attracted by these hygroscopic compounds, leading to increased size of aerosol particles, see Figure 3. As many more very small particles are available compared to the larger ones, this phenomenon will lead to an increase of particles in the critical range of 0.1 and 1.5 ?m and hence to a reduction in visibility.

Winter smog

The London smog disaster of December 5-10, 1952, is a classical example of a winter smog episode and may have caused the untimely death of 4,000–12,000 people—and was the direct reason for the enactment of the first clean air act in England. The disaster occurred during a particularly cold period when the residents of the city were burning large amounts of coal. A temperature inversion occurred when cold, moist, still air settled at ground level, absorbing soot and other pollutants causing what is known as a "pea soup" smog. Visibility dropped to below 500 meters for 114 hours, below 50 meters for 48 hours and below 10 meters for 48 hours at Heathrow Airport. Sickness and deaths from chronic respiratory and cardiovascular problems, pneumonia, bronchitis, tuberculosis, and heart failure rose above normal levels rapidly. Estimates at the time were that approximately 4,000 deaths were related to the smog, however, later estimates increased this number to approximately 12,000. The main compounds in the aerosol particles were undoubtedly sulfuric acid, ammonium hydrogen sulfate and ammonium sulfate, all originating from conversion of sulfur dioxide. At wintertime this conversion is quite slow but the period of stagnation was so long that nonetheless extremely high sulfate concentrations were formed. Though this London episode is the most extreme example of a winter smog episode, pollution episodes of this kind were encountered quite frequently in Europe and in the United States (a smog episode in Donora, Pennsylvania—near Pittsburgh—between October 30 and 31, 1948, is a good example).

Summer smog

In Los Angeles, California, the combination of stable atmospheric conditions, due to a temperature inversion, and emissions from a large number of automobiles, led to a different kind of air pollution, first recognized in 1943.

Ozone and related compounds formed by the reaction of organic compounds and nitrogen oxides have long been a problem for the residents of Los Angeles. The reactions that form ozone, also lead to a very high OH radical concentrations, see the article on Ozone. The combination of high OH radical concentrations, intense solar radiation and high temperatures, lead to a very high conversion rate of sulfur dioxide to sulfate and of nitrogen oxides to nitrate and hence to increased aerosol concentrations.

Particles are formed in large numbers that severely restrict visibility, analogous to the classical fog due to the emissions of burning coal, thus visibility is strongly reduced during summer smog episodes. This kind of air pollution is known as smog, though the causes of visibility reduction are quite different from winter smog episodes.

Roles of geography and weather in the formation of smog

Geography and weather can have significant impacts on smog formation by creating stagnant air over a particular area. The chemicals that can lead to smog have greater time to interact and form the aerosols that constitute the smog which then accumulate.

Cities surrounded by mountains are frequently exposed to a stagnant air masses where the mountains "trap" the precursors to smog. Barcelona, Spain, Beijing, China, Los Angeles, California, Mexico City, Mexico and Salt Lake City, Utah are good examples.

Temperature inversions, where the normal cooling of air with great altitude is reversed and a layer of water air traps a layer of cold air below it, limit the movement air and helps the formation of smog. Temperature inversions can occur on clear nights when the ground quickly radiates heat away and cools the air near the ground so that it is colder than the air farther from the ground. Another way for temperature inversions to occur is for cold air over a lake, sea, or ocean to roll in low over a coastal region. A third way is for a "cell" of warm high pressure air to move over an area and 'trap" the cold air below.

caption Figure 4. Conversion of sulfur dioxide to sulfate and deposition of precursor and products

A city like Los Angeles is impacted by nearly all these effects. A coastal city and surrounded by mountains, it is subject to high pressure cells of air forming above cool ocean air below. These factors combine with a large population emitting chemicals that combine under sunlight to form aerosols. However, significant improvements in Los Angeles' air quality since the 1960s (a period marked by increases in population and cars, demonstrate that smog can be addressed with appropriate policies).

Finally, weather plays an important role in the rate at which smog precursors are converted into aerosols. For example, the speed of the conversion of sulfur dioxide to sulfate and the formation of ozone is dependent on temperature, relative humidity and a number of chemical parameters. Figure 4 gives an overview of the atmospheric conversion of sulfur dioxide to sulfate. Conversion is faster at higher temperatures, greater relative humidity and at higher OH radical concentrations, so smog is formed faster in hot humid climates and conversion speeds will accordingly vary between less than 1% and 10% per hour. But even at high conversion speeds time is needed to complete smog formation.

Health Effects of Smog

Increased concentrations of particles and ozone have a clear impact on human health, for details see impact of local air pollution. The impact of aerosols on human health has been documented very well over the last years. It should be pointed out, however, that exposure to aerosols over long periods seems to have greater effects then to short-lived smog episodes. Also, ozone affects human health, but the main impact is on ecosystems and agricultural crops, see article on ozone.

Policies to Address Smog

Such measures such as fuel switching (use of natural gas instead coal), and desulfurization of flue gases of coal-fired power plants have contributed very significantly to reducing sulfur dioxide concentrations to the extent that winter smog episodes have more or less ceased to be a problem in the US and Europe. In some developing countries sulfur dioxide-induced smog is still a problem, e.g. in some cities in Western China like Urumqi. Low-nitrogen oxide-emitting gas and oil burners, car exhaust catalysts, expansion of public rail transport and low-emission application of fertilizer in agriculture have significantly contributed to the abatement of photochemical smog problems—and the frequency of summer smog episodes has gone down in Europe and the US. Strong economic growth in e.g. East Asian countries has induced a large increase in emissions of nitrogen oxides and Volatile Organic Compounds, leading to more air pollution problems due to summer smog. 

caption Figure 5. View of Hong Kong obscured by haze.

Present situation

Sulfur dioxide concentrations have decreased in most areas, therefore winter smog is less of a problem compared now than it was thirty years ago. Increased traffic has led to an increase in problems with summer smog. Photochemical smog is a prime air pollution problem in all mega-cities, in both developed and developing countries, see figure 5. But abatement measures can indeed lead to a better situation, see the UN assessment report on environmental quality in Beijing during the Olympics in the further reading section below.



Further reading

Descriptions of the 1952 London smog episodes can be found in:

A description of smog in Urumqi, China is given in:

  • Juan Lia,b, Guoshun Zhuanga, Kan Huanga, Yanfen Lina, Chang Xu, Shulong Yu. Characteristics and sources of air-borne particulate in Urumqi, China, the upstream area of Asia dust. Atmospheric Environment Volume 42 (2008) 776–787, PDF for purchase here

More information on photochemical smog in the US can be found at a relevant EPA site: 

An rather complete overview about smog related air pollution and its effects in Europe is given in the report:

The evaluation of UN-UNEP of abatement measures in connection with the 2008 Beijing Olympic Games is made in:



Slanina, S. (2011). Smog. Retrieved from


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