Remember when atmospheric contaminants were romantically called stardust?
Lane Olinghouse

Generally, warm air near the surface rises and mixes with cooler air above. Unlike pressure, temperature does not decrease steadily as altitude increases. Instead, it goes through a series of inversions. An inversion is a section of the atmosphere in which the temperature levels out and may even begin to increase. It is these inversions that divide the atmosphere into the many layers. In an atmospheric inversion, as the name suggests, this situation is inverted. Inversions occur when there is a reversal of normal thermal conditions when warmer air prevents cooler, denser surface air from rising and pollutants can become trapped in the cool air. Inversions are part of a daily atmospheric cycle and most commonly occur in the early morning hours when the ground cools the air layer immediately above it. The warm air layer at some distance overhead acts as a blanket, holding down cooler air underneath. Inversions tend to dissipate during the middle of the day as the sun heats the earth causing wind currents to mix the air layers.

Sometimes pollutants trapped in there inversions can travel great distances if the inversion is notably dense and towers high into the atmosphere. Farmers are regularly cautioned to avoid spraying pesticides or adjust the droplet sizes when they disperse the chemicals during inversions to avoid the hazards of these drifts. While wintertime inversions in urban environments trap carbon monoxide, summertime will bring ozone action alerts because smog (the primary component of photochemical smog is ozone) and other pollutants are trapped. This happens frequently in urban areas where nearby mountains form a natural barrier to the movement of air. Los Angeles, which has dense automobile traffic, bright sunshine, and mountains to the east that block winds from the ocean, is particularly subject to smog conditions. Some communities have enacted laws restricting the burning of wood or land clearing on particularly bad days to reduce the haze formed by inversions.

Smog and other air pollutants are particularly damaging when "trapped" at ground level and results can be deadly. In 1948, an inversion over the Pennsylvania town of Donora resulted in the deaths of 20 people and hospitalized hundreds when sulfur, ozone and air particulates were trapped at ground level. The US Congress passed a series of bills to study and research the problem of air pollution as a result of this tragedy, and by 1960 presidential campaigning began calling for a coordinated, national approach to this problem.

The following is a list of air pollution catastrophes resulting from atmospheric inversion events:

  • 1948: Air pollution trapped by a temperature inversion in Donora, Pennsylvania results in more than 6,000 inhabitants becoming sick and 20 deaths.
  • 1952: A five day temperature inversion in London in December causes smoke from open fireplaces and industrial emissions to sicken thousands of Londoners and results in the premature deaths of 4,000 people, many of whom had preexisting heart and lung conditions. To read more about it see the Great Smog of 1952.
  • 1963: The Federal Clean Air Act provides funds to support state and local air pollution agencies, to initiate interstate air pollution research and to enforce interstate pollution laws. Because the main policy thrust is the protection of human health, this act is administered by the Public Health Service of the U.S. Department of Health, Education and Welfare.
  • 1966: A temperature inversion traps urban air pollution for three days in New York City. Epidemiologists attributes the deaths of 168 people to the episode.
  • 1967: A revised Air Quality Act authorizes HEW to establish federal air-quality standards, requires states and local entities to monitor their "airsheds" - the air located above so-called air quality control regions covering 247 metropolitan areas throughout the U.S. - and requires states to establish standards for specific pollutants in these air quality control regions. However, only 21 states submit their required plan to. Department of Health, Education and Welfare by the time the 1967 Act expires in 1970.
  • 1970: Amendments to the Federal Clean Air Act limit emissions from cars, factories and power plants. The allowable level of sulfur dioxide from utility smokestacks is reduced; refineries are required to phase out lead from gasoline; and automakers are required to cut emissions by 90 percent by 1975. Congress later gave the automobile industry until 1978 to meet auto air pollution standards after fierce opposition. The 1970 Act also gives the newly created Environmental Protection Agency (EPA) the power to establish National Ambient Air Quality Standards (NAAQS) to protect human health and secondary standards to protect the environment. States are required to develop and implement plans to reduce air pollution to levels below these national standards by 1975.
  • 1973: The Arab Oil Embargo leads Congress to pass the Energy Supply and Environmental Coordination Act of 1973. To ease pressure on the oil supply, the act requires power plants to switch their fuel sources from oil and natural gas to more polluting coals. It also temporarily suspends emission limits on power plants and other stationary sources like refineries.
  • 1977: Amendments to the Federal Clean Air Act require companies to install new pollution-control equipment or face stiff penalties. The law also allows states to adopt emission limitations more stringent than federal standards and requires that coordinated transportation planning and clean air planning must take place at the local level.
  • 1979: The EPA relaxes the National Ambient Air Quality Standards for ozone, raising the permissible level from 0.08 to 0.12 parts per million.
  • 1983: The EPA decides to allow states additional time to submit State Implementation Plan revisions, where needed, and to negotiate the control strategy and rule provisions required to demonstrate attainment of the NAAQS.
  • 1984: An industrial accident in Bhopal, India (see: the Bhopal Disaster) at a Union Carbide plant spews methyl isocyanatee gas into the local area, killing more than 2,500 people, and hospitalizing tens of thousands. The event leads to international pressure for better emergency response plans at manufacturing plants and information regarding toxic releases.
  • 1985: An accident at a Union Carbide plant in Institute, West Virginia sends a poisonous plume of aldicarb oxime over the town, sending 135 people to the hospital. Only a new chemical batching process, instituted at the West Virginia plant after the Bhopal accident, prevents the release of the more toxic methyl isocyanate.
  • 1986: The Emergency Planning and Community Right to Know Act requires companies to report to the Environmental Protection Agency both regular and accidental releases to the air, water or land of more than 300 toxic chemicals. The act also requires manufacturing companies to develop emergency response plans to use in the event of an accident.
  • 1990: Amendments to the Federal Clean Air Act, the most far-reaching air pollution prevention legislation ever passed, address airborne toxins, acid rain and the depletion of the ozone layer, and toughen regulations on vehicles, utilities and other sources of smog-producing emissions.

Some rather bizarre ideas among scientists have been forwarded over the years before atmospheric inversions and the resulting smog was completely understood.

  • 1940's: If it worked for sewage, so thought the engineer, it might work for smog. One idea was to connect all of the Los Angeles industries to a massive network of concrete exhaust pipes routed to the mountains where pollution could be released above the inversion layer. The idea failed when it was realized that it would cost more energy than the Hoover Dam could supply to move the pollutants through the required 89 miles of duct work.
  • 1950's Another idea made impracticable also due to the lack of energy required since it involved a land mass of 1,600 square miles and over 200 million tons of air aspired to cut holes in the mountains and install large exhaust fans to blow or wash away smog. Another scientist suggested blackening whole sections of the eastern mountains so as to store heat and create thermal currents and westerly winds that would blow smog over the ocean. Another expert recommended seeding clouds to wash the air clean.
  • 1960's:An engineer came up with an idea to install jet engines attached to vertical tubes and propel the smoggy air above the inversion layer. In 1967, a chemistry professor thought he had a better idea. Instead of spending money on tailpipe pollution controls, why not simply fumigate the urban areas each summer smog season with a chemical called diethyl hydroxylamine, or DEHA. The professor postulated that this would interrupt smog formation because DEHA scavenges the short-lived free radicals that fuel the airborne chemistry of smog. The only problem was that the chemical posed an even greater health risk than air pollution.
As silly as they may sound today these ideas did lead to many others that have prove successful at reducing pollution, such as bag houses to control dust from factories, vapor recovery systems with booted nozzles at gas stations, catalytic converters on cars and power plants and reformulated gasoline.

An interesting phenomenon caused by atmospheric inversions is called a green flash. It is a mock mirage that occurs when the sun and is quite rare lasting only a few seconds. Of course an observer would want to study up on and definitely take the necessary precautions when viewing anything related to the sun. It is usually seen when the sky is clear, but this green flash may also be seen when the sun is low and is being obscured by a sharply defined cloud. The green flash is a split second of green light visible during the last speck of the sun at sunset. As green rim forms at the upper part of the solar disk, one can see a piece of green flash appear as the small part of the upper solar disk is separated from the main disk by atmospheric refraction through the different inversion layers. This flash happens in a second and is easiest to see when the last dot of light fades behind the horizon. This very last ray of the setting sun is a bright metallic green, a sahde of such purity that it would be considered completely mendacious if a painter dared to include it in his painting. However, Jules Verne has written an entire novel on the theme of this mysterious green flash Le Rayon vert (1882) or the Green Ray about a young girl who refuses to marry the man her uncles have selected for her until she sees the Green Ray. Legend has it that this is an indication of true love. Surely that alone is a good enough reason to go looking for mirages on the horizon!


Air Quality:

From Tunnels to Turbines: Old Ideas for Ridding Area of Smog:

PBS Teachersource - Scienceline - Air Pollution:

Reducing Spray Drift:

The Green Flash:

Urban Air:

Log in or register to write something here or to contact authors.