Atmospheric Structure and Pollution Sources

The atmosphere is the thin layer of gas surrounding earth. Half of the mass of the atmosphere is below a height of 5.5 km, 90% is below 16.5 km, and 99% is below 30 km.

This is what the atmosphere looks like viewed edge on from space. The image is of a small cross-sectional area, note the small curvature of the surface, yet the atmosphere is a small part of the whole. Looking closely, you can see tall thunderstorm clouds silhouetted against an orange layer of atmospheric gases backlit by the sun just below the horizon. Above this layer is the clear blue of the stratosphere and the blackness of space.
From NASA Space Shuttle Flight 6 on 4 April 1983.

Composition

The atmosphere is composed of 78.08% nitrogen and 20.95% oxygen with small amounts of other gases: 0.93% argon, 0.038% carbon dioxide, 0.002% neon, and yet smaller concentrations of helium, methane, krypton, and hydrogen. Both nitrogen and oxygen exist in large quantities only because of life on earth, especially life in the ocean.

It would seem that the composition of the atmosphere would be stratified with different chemical composition at different heights. In fact, mixing in the atmosphere causes the composition to be nearly uniform up to about 80 km.

Ozone is a very important trace gas in the atmosphere. It exits in two places:

1. In the stratosphere at heights around 20-30 km. This is good ozone. It protects all life on earth from dangerous solar ultraviolet radiation (energy).
2. Close to the surface due to pollution. It is produced from nitrogen oxides and volatile carbon-based compounds when there is intense solar radiation (energy), above all in the spring and summer. This is bad ozone. It causes respiratory illness; it damages plants; and it attacks rubber.

Remember "Good Uphigh, Bad Nearby"

Average ozone concentration in June (red line) and ozone pressure (in nano bars, where one bar is approximately atmospheric pressure at sea level) on 23 June 2006 (black line), as a function of height in kilometers above the Swiss Payerne station.
From Swiss Federal Office of Meteorology and Climatology, ozone page.

Many other gases are found in trace amounts in the atmosphere. Most are short lived pollutants. Most are local or regional in extent.

The space and time scales of trace gases in the atmosphere. The moderately long-lived species contribute to regional and urban air pollution and smog. The long-lived species contribute to the ozone hole and greenhouse warming.
Redrawn from

Structure

The density and pressure of the atmosphere drop nearly exponentially with height up to a height of 100 km. Temperature decreases with height in the troposphere up to a height of 10-15 km, then it increases with height in the stratosphere. The stratosphere is defined as that region above the troposphere where temperature increases with height. Because temperature increases with height, the layer is stable and stratified, hence the name. The stratosphere is heated from above by the absorption of solar ultraviolet radiation by ozone in the stratosphere.

Left: Density of the atmosphere as a function of height. Click on the image for a zoom.
From Joel Michaelsen, University of California Santa Barbara.
Right: Temperature as a function of height in the atmosphere measured by radiosondes at three cities. Click on the image for a zoom. More plots are available from the University of Wyoming's Department of Meteorology. and from the University Center for Atmospheric Research RAP Real-Time Weather Data.
From Don Collins, Texas A&M University.

If you look closely at the lower right corner of the temperature plot, you will notice a small increase of temperature with height above San Diego. This is called an inversion.

Inversion in the atmosphere above San Diego, red curve. The air at a height of one kilometer is more than 5° C warmer than the air at the surface. There is also a much weaker, and shallower inversion above Dallas.

Inversions strongly influence atmospheric pollution. Inversions inhibit vertical convection, trapping pollutants close to the surface. Strong, persistent, inversions over urban areas lead to much greater concentrations of pollutants in the urban air.

Inversions:

1. Limit vertical mixing. This traps pollutants close to the ground.
2. Limit cloud formation. This leads to more sunlight, which drives chemical reactions in the polluted air.
3. Limits precipitation. This increases the lifetime of the pollutants in the atmosphere.

Inversions are common along west coasts of continents. There winds blowing toward the equator (purple arrow in figure below left) cause water at the ocean's surface to move away from the coast (red arrow). The water is replaced by colder water upwelled from deeper in the ocean (blue arrow). The cold water cools the lower kilometer of the atmosphere, producing the inversion. Strong inversions are common along the California coast. They are responsible for the warm, dry, cool climate of Los angeles, San Francisco, and San Diego. They are also responsible for smog commonly found in these cities, especially Los Angeles. For more, read at Applications of Ekman Theory.

Inversions also occur above inland lakes and rivers on days when the water is cooler than the air, and winds are weak, and in valleys at night when colder air drains off surrounding hills.

Right: Sea-surface temperature along the US west coast on 16-18 July 2006 measured by the Advanced High Resolution Radiometer AVHRR on the NOAA polar-orbiting, meteorological satellites. The cold (blue) areas are upwelled water caused by north winds offshore of the coast. Click on the image for a zoom with color scale.
From NOAA CoastWatch.

Left: Schematic diagram showing equatorward winds along the California coast (purple arrow) push water offshore (red arrow), leading to upwelling of colder water along the coast (blue arrow) shown in the figure on the right above. The cold water not only influences the atmosphere, but it also carries nutrients that increase the productivity of the area. Click on the image for a zoom.
From Bay Nature: A Moveable Feast: The Ups and Downs of Coastal Upwelling. Drawing by Fiona Morris.

Pollution Sources

Many processes contribute to atmospheric pollution and trace gases.
From US Strategic Plan for the Climate Change Science Program, Final Report July 2003: Chapter 3 Atmospheric Composition.

The important sources of atmospheric pollution on a global or regional scale are:

1. Automobiles. According to the U.S. Environmental Protection Agency (EPA), driving a car is the single most polluting thing that most of us do. Motor vehicles emit millions of tons of pollutants into the air each year. In many urban areas, motor vehicles are the single largest contributor to ground-level ozone, a major component of smog.
   
   The primary pollutants produced by automobiles are:
   1. Hydrocarbons. They come from the evaporation of fuel, especially on hot days, leaking fluids, and during refueling at gas stations.
   2. Nitrogen oxides. Produced by high heat during the burning of fuel.
   3. Carbon monoxide. Produced by the incomplete burning of fuel.

   Modern automobiles pollute much less than older models thanks to emission controls including catalytic converters. But the number of cars is so large, and they are driven so much, they are still major sources of pollution.

2. Urban activity. The world's population is concentrated more and more in mega cities, with five urban areas having more than 20 million people: Tokyo, Japan (34,997,000); Mexico City (22,800,000); Seoul, South Korea (22,300,000); New York (21,900,000); and São Paulo, Brazil (20,200,000). Urban air pollution is now common in all large cities, worse on some days, better on others, but never gone.
   
   
   From Air Pollution in Mexico City by Pierre Madl of Salzburg University Sound and Video Studio.
   
   It is caused not only by emissions from cars, trucks, buses and lawnmowers (operating a lawn mower for one hour produces as much pollution as driving a car 100 miles), but also by fumes from drying paint, charcoal fires (grills), and dry cleaners.
   
   
   A huge traffic jam backs up the streets in Bangkok. High population density in large urban areas leads to air pollution. From Patagonia.
   
   Urban activity leads to photochemical smog in many areas such as Los Angeles, Houston, Mexico City, and London, the archetype of a smoggy city (The term smog was coined by Dr. Henry Antoine Des Voeux in 1905, when he combined the words smoke and fog).
   
   In London, the smoke came from the burning of coal to heat thousands of houses. The London smog began in the middle ages, and extreme smog events led to periodic attempts to reduce air pollution. The great smog of 5-9 December 1952 killed more than 4,700 people during the event, and led to an additional 8,000 deaths in the year following the event. During the event, visibility was reduced to 20 m over an area of 20 by 40 km (Boubel et al, 1994) and deaths reached 900 per day. To ensure that such an event would never happen again, parliament passed the UK Clean Air Act of 1956.
   
   Photochemical smog is formed when sunlight acts on volatile carbon-based molecules and nitrous oxides trapped below inversions above cities. The sunlight powers chemical reaction that form harmful pollutants such as tropospheric ozone, aldehydes, and peroxyacyl nitrates (PAN). Here is an outline of some important chemical reactions leading to smog:
   
   The high temperature in automobile and diesel engines converts nitrogen gas to nitrous oxide.
   N₂ + O₂ -----> 2 NO (nitric oxide)
   
   In the atmosphere, nitric oxide is converted to nitrogen dioxide NO₂, a brown gas which gives smog its characteristic color.
   2 NO + O₂ ------> 2 NO₂
   
   When nitrogen dioxide concentrations are high, sunlight leads to the formation of ozone.
   NO₂ + sunlight ----------> NO + O
   O + O₂ ---------> O₃.
   
   NO₂ + O₂ + hydrocarbons + sunlight ----------> CH₃CO-OO-NO₂ (peroxyacetylnitrate).
   
   
   Los Angeles smog on 29 January 2004. The top of the inversion layer is easily seen against the backdrop of distant mountains. Hilltops above the layer are visible at great distances, urban areas below the layer are obscured.
   Photo by Alan Clements, Middlesbrough, England.
   
   
   This is what the smog in Los Angeles looks like from the ground, a thick brown or slightly orange haze with a strong smell of ozone. In this scene, the inversion is below the top of the highest buildings. The exhaust from more than a million cars driven in the morning rush hour is trapped below this level.
   From Larvalbug article Choking on Air.
   
   
3. Agricultural burning
   
   Crop burning, Alberta Canada. From Ag-Info Centre, Government of Alberta, Canada.
   
   
4. Forest fires.
   
   Wildfires and smoke on 23 October 2007 in southern California. The fires burned 800 square miles, and area almost 2/3 the size of Rhode Island. The smoke from the fires is clearly visible over the Pacific ocean on the left of the image. Red spots mark the location of the fires. Click on image for zoom.
   From NASA California Wildfires. Other similar images are at the NOAA site: Operational Significant Event Imagery.
   
   NOAA issues daily Fire Products, including a map showing all forest fires in North America, and information of Fire Events worldwide.
   
   
5. Industrial activity
   Smelters, steel mills, oil refineries and chemical palnts, paper mills, manufacturing plants, and power plants, especially coal-fired plants are the major sources. But even relatively clean industries such as semiconductor fabrication plants, which make computer chips, also contribute. Many of the worst polluters were in the format Soviet Union. Fortunately, industrial emissions are being greatly reduced as nations become richer.
   
   V.I. Lenin Steel mill, Magnitogorsk, 1991. From Monroe Gallery of Photography, photographed by Shepard Sherbell.
   
   
6. Dust storms. Strong winds blowing across desert regions lift dust high into the troposphere. The higher-level winds then carry dust great distances. The Sahara, the Aral Sea, and Mongolia are notorious sources.
   
   Dust blown from the Sahara across the Atlantic on 24 July 2005. Dust is colored yellow-brown in the image.
   From NOAA Dust Storm site. NASA has a catalog of images of dust storms.
   

Revised on: 4 June, 2008