Case Study: No recess today—too much smog!

Photochemical Smog and Health Risks

Photograph of photochemical smog in the Los Angeles area.
Source: United States Geological Survey.

Photochemical smog is a serious environmental concern, and it poses a health problem to people living in many metropolitan regions around the world. In fact, sometimes levels of ozone - a major component of smog - are so high that school children in Los Angeles, California, are kept from going outside for recess because of the potential health risks!

Photochemical smog was first identified in Los Angeles in 1944. Although several other kinds of smog occur, photochemical smog (or Los Angeles-type smog) is a yellow-brown haze produced by the reaction of sunlight with exhaust from automobiles and power plants that burn coal. Ozone, nitrogen dioxide, and other volatile organic compounds that make up this smog irritate eyes and nasal passages. These are particularly dangerous to people who have heart disease, asthma, or other respiratory illnesses, and to anyone who exercises or does manual labor outdoors when smog is heavy.

Photochemistry of Los Angeles-Type Smog

As its name implies, photochemical smog forms in the presence of light, so this type of smog is seen most frequently during the hot and sunny summer months. Though the components of photochemical smog might be in the air, if sunlight does not reach them or they are not concentrated enough, the smog will not form. The worst cases of smog occur when winds are calm and smog is trapped near the surface by a temperature inversion, a condition in which cooler air near Earth's surface has warmer air above it. These ideal smog-forming conditions commonly occur near cities that are adjacent to mountains.

Photochemical smog forms through a series of chemical reactions among compounds in the atmosphere. When nitric oxide (NO), a component of the exhaust from cars and power plants, enters the atmosphere, it reacts with oxygen to produce nitrogen dioxide (NO2). Sunlight can break nitrogen dioxide down. This process initiates other chemical reactions that lead to the formation of low-level ozone. Although ozone (O3) that is high in the stratosphere filters out harmful UV radiation, ozone's presence at the ground level poses a health risk. Also, Volatile Organic Compounds (VOCs), molecules that enter the atmosphere from substances such as gasoline, cleaning solvents, and trees, play a crucial role in forming photochemcial smog.

From Brown, T.L., LeMay Jr., H.E., & Bursten, B.E. (2000). Chemistry: The central science(8th ed.). New Jersey: Prentice Hall.

These equations show a very basic generalization of the chemical reactions that lead to the production of nitric oxide, nitrogen dioxide, and ozone.

Image E



Satellites Track Smog from Space

A depiction of the NASA Aura satellite in orbit around Earth. The sensor on the satellite uses remote sensing techniques to gather data about nitrogen dioxide concentrations. Image Source: NASA Goddard Space Flight Center.

Smog poses a serious threat to human health. It is particularly dangerous to very young children, the elderly, and anyone with respiratory disease. Some constituents of smog can also damage vegetation and agricultural crops. Smog conditions are worse in large cities, but due to atmospheric circulation, smog also affects rural communities that are downwind. Humans can be thankful that thus far, there have been no major human disasters attributed to photochemical smog; however, the problems related to this type of smog continue to grow worldwide as the human population increases and more countries become industrialized.

From orbit, the NASA Aura satellite detects the concentration of nitrogen dioxide in the atmosphere, from the top of the troposphere down to Earth's surface. Since nitrogen dioxide is typically found in photochemical smog, its presence indicates locations on Earth's surface that have high incidences of smog.

In this chapter, you will use Google Earth to import and examine datasets of nitrogen dioxide concentration and population density. You will analyze the images to identify patterns in nitrogen dioxide concentration, population, and geography that will enable you to make general predictions about where photochemical smog might form.

Photochemical Smog and Climate Change

While, NO2 is major component of photochemical smog, and the focus of this Case Study, several other components of smog also deserve mention because of their role in climate change. These gases include Methane, (CH4); Nitrous Oxide, (N2O); and Ozone, (O3).

Methane (CH4); is a potent, although short-lived greenhouse gas. It is largely an agricultural byproduct. It is naturally produced in swamps and wetlands and a byproduct of coal and oil extraction.

Car and truck emissions, pollution from factories, and burning vegetation, produce compounds of carbon and nitrogen that, when acted on by sunlight, produce ozone in the troposphere. This component of photochemical smog is an important greenhouse gas. Concentrations of ozone have risen 30% since the pre-industrial era and it is now considered the 3rd most significant greenhouse gas, behind Carbon Dioxide and Methane.

Nitrous Oxide (NO2) is another significant, but lesser known, greenhouse gas. It is a product of the use of fertilizers, as well as a byproduct of the production of nylon and nitric acid. It is far more potent than CO2.

Chlorofluorocarbons (CFCs) are entirely man-made. They were synthesized for use as refrigerants and cleaning solvents in the 1920s. Their production has been banned since 1978, but they remain in the atmosphere due to their long half-lives.

Greenhouse gases such as Nitrous Oxide, Ozone, water vapor, and chlorofluorocarbons (CFCs) are now being tracked via satellite. One of the NASA satellites presently monitoring these gases is the Atmospheric Infrared Sounder, AIRS, flying on a NASA weather and climate research satellite, Aqua. The Going Further page, gives simple instructions as to where to locate and download these datasets.


    Ahrens, C.D. (2000). Meteorology Today (6th ed.). Pacific Grove, CA: Brooks/Cole.

    Bailey, R.A., Clark, H.M., Ferris, J.P., Krause, S., & Strong, R.L. (2002). Chemistry of the Environment (2nd ed.). San Diego, CA: Academic Press.

    Girard, J.E. (2005). Principles of Environmental Ehemistry. Sudbury, MA: Jones and Bartlett.


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