The atmosphere is a thin layer of gas that surrounds the planet and consists of a relatively stable mixture of several hundred different gases. It has a mass of about 5.15e15 tons, held down to the planet's surface by gravitational attraction. Excluding water vapour, the proportions of gases are nearly uniform up to around 80km high. The major components of this region are Oxygen (21%), Nitrogen (78%), and Argon (0.93%), with minute amounts of trace gases also present. This part of the atmosphere also contains water vapour, the amount of which depends on local environmental conditions. Water content is an important influence on local weather since it can exist in the atmosphere as a gas (humidity), a liquid (clouds and rain) and a solid (hail), and is one of the most important mediums for the transport of energy between different parts of the Earth.
Atmospheric temperature and chemistry are believed to be controlled by the trace gases. There is increasing evidence that the percentages of environmentally significant trace gases are changing because of both natural and human factors. Examples of man-made gases are the chlorofluorocarbons CFC-11 and CFC-12 and Halons. Carbon dioxide, nitrous oxide, and methane (CH4) are produced by the burning of fossil fuels, expelled from both living and decaying biomass, and released by the metabolic processes of micro-organisms.
The atmosphere is divided into several distinct concentric spherical bands separated by narrow transition zones. The upper boundary at which gases disperse into space lies at an altitude of about 1000km above sea level, however more than 99% of the total atmospheric mass is concentrated in the first 40km. Each atmospheric band is characterised by differences in chemical composition that produce variations in temperature.
Troposphere
The troposphere is the bottom layer at the Earth's surface and, thus, is where the atmosphere is at its highest density. Its actual height varies with latitude, being around 18km at the equator and 8km at the poles. It also varies with the seasons, being higher in summer when the air within it is warm and lower in winter when it is cooler. All weather phenomena occur within the troposphere, although some turbulence effects may extend into the lower portion of the stratosphere.
Temperature and water vapour content in the troposphere decrease rapidly with altitude. Vertical temperature drops by an average of 6 °C for each kilometre in height. 99% of the water vapour in the atmosphere is contained within the troposphere, with concentrations being greatest above the tropics, where they may be as high as 4-5% by volume, and lowest toward the poles. Water vapour content plays a major role in regulating air temperatures because it absorbs short-wave radiation from the sun as well as long-wave radiation from the Earth's surface.
A narrow zone called the tropopause separates the troposphere from the stratosphere. Air temperatures within the tropopause remain relatively constant with increasing altitude.
Stratosphere
The stratosphere sits above the tropopause and ranges in height from 20-50km. The air temperature in the stratosphere remains relatively constant up to an altitude of 25km. Then it increases gradually to 200-220 K at the lower boundary of the stratopause (~50 km), which is marked by a decrease in temperature. Because the air temperature in the stratosphere increases with altitude, it does not cause convection and has a stabilising effect on atmospheric conditions in the region. Ozone plays the major role in regulating the thermal regime of the stratosphere, as water vapour content within the layer is very low. Temperature increases with ozone concentration.
Solar energy is converted to kinetic energy when ozone molecules absorb ultraviolet radiation, resulting in heating of the stratosphere.
The ozone layer is located at an altitude between 20-30km. Approximately 90% of the ozone in the atmosphere resides in the stratosphere. Ozone concentration in the this region is about 10 parts per million by volume as compared to approximately 0.04 parts per million by volume in the troposphere.
Ozone absorbs the bulk of solar ultraviolet radiation in wavelengths from 290 nm - 320 nm. These wavelengths are harmful to life because they can be absorbed by the nucleic acid in cells. Increased penetration of ultraviolet radiation to the planet's surface would damage plant life and have harmful environmental consequences. Appreciably large amounts of solar ultraviolet radiation would result in a host of biological effects, such as a dramatic increase in cancers.
Meteorological conditions strongly affect the distribution of ozone. Most ozone production and destruction occurs in the tropical upper stratosphere, where the largest amounts of ultraviolet radiation are present. Dissociation takes place in lower regions of the stratosphere and occurs at higher latitudes than does production.
Mesosphere
The mesosphere, a layer extending from approximately 50 to 80km, is characterised by decreasing temperatures, which reach 190-180 K at an altitude of 80km. In this region, concentrations of ozone and water vapour are negligible. Hence the temperature is lower than that of the troposphere or stratosphere.
With increasing distance from Earth's surface the chemical composition of air becomes strongly dependent on altitude and the atmosphere becomes enriched with lighter gases. At very high altitudes, the residual gases begin to stratify according to molecular mass, because of gravitational separation.
Thermosphere
The thermosphere is located above the mesosphere and is separated from it by the mesopause transition layer. The temperature in the thermosphere generally increases with altitude up to 1000-1500 K. This increase in temperature is due to the absorption of intense solar radiation by the limited amount of remaining molecular oxygen. At an altitude of 100-200km, the major atmospheric components are still nitrogen and oxygen. At this extreme altitude gas molecules are widely separated.
Exosphere
The exosphere is the most distant atmospheric region from Earth's surface. The upper boundary of the layer extends to heights of perhaps 960 to 1000 km and is relatively undefined. The exosphere is a transitional zone between Earth's atmosphere and interplanetary space.
About Ozone
Ozone is a pale blue, relatively unstable molecule made up of three oxygen atoms. It is usually formed when high energy electromagnetic radiation (usually in the ultra-voliet range but even a large electrical current can do it) passes through oxygenated gas, breaking apart O2 molecules - some of which recombine into O3. It is characterised by a unique odour that is often noticed during electrical storms and in the vicinity of electrical equipment. It is a powerful oxidising agent, being a significant source of excess oxygen atoms, which are also known as free radicals.
Bad Ozone
Depending on where ozone occurs, it can either protect or harm life on Earth. When it is close to the planet's surface, in breathable air, ozone is a harmful pollutant that causes damage to lung tissue and plants, and is considered to be "bad ozone." It is a powerful photochemical oxidant that damages rubber, plastic, and all plant and animal life. It also reacts with hydrocarbons from automobile exhaust and evaporated gasoline to form secondary organic pollutants such as aldehydes, ketones and peroxyacyl nitrates. The peroxyacyl nitrates are especially damaging photochemical oxidants that are very irritating to the eyes and throat.
Ozone is produced in cities and anywhere there are large electrical currents. Concentrations up to 8 times natural background levels have been recorded in many large urban areas. Photochemical oxidants are a significant cause of agricultural loss in many western countries. Their damaging effects on vegetation and crops have been confirmed in the eastern United States, adjacent areas in Canada and much of Europe. Ozone alone, or in combination with sulphur dioxide (SO2) and nitrogen dioxide (NO2), accounts for 90% of the annual crop losses in the U.S. that are caused by air pollution.
Good Ozone
Most ozone is concentrated in the stratosphere, at about 25 km in altitude, and is considered to be "good ozone." In this region, ozone acts as a shield to protect Earth's surface by absorbing harmful ultraviolet radiation. Without this shield, we would be more susceptible to skin cancer, cataracts, and impaired immune systems. A 1% decrease in total column ozone causes the amount of transmitted UV radiation, in the spectral region that damages deoxyribonucleic acid (DNA), to increase by about 2%. Although good ozone only represents a tiny fraction of the atmosphere, it is crucial for life on Earth.
The proportion of good and bad ozone in the atmosphere depends on the balance between processes that create ozone and those that destroy it. An upset in this balance can have serious consequences for life on Earth, and scientists are finding evidence that the balance has changed. Concentrations within the protective ozone shield are decreasing, while levels in the air we breathe are increasing.
Related Links
- Guide to the science of the atmosphere - USA Today
- http://www.usatoday.com/weather/wworks0.htm
- Atmosphere & Climate Links
- http://www.igc.org/igc/issues/ac/or.html
- Getting Around The Coriolis Force
- http://www.physics.ohio-state.edu/~dvandom/Edu/newcor.html
- Ozone and the Atmosphere - Goddard Space Flight Center Earth Sciences
- http://daac.gsfc.nasa.gov/CAMPAIGN_DOCS/ATM_CHEM/ozone_atmosphere.html
- MODIS (Moderate Resolution Imaging Spectroradiometer) Images
- http://modis-atmos.gsfc.nasa.gov/IMAGES/index.html
- Aviation and the Global Atmosphere
- http://www.grida.no/climate/ipcc/aviation/
- Calculate Your CO2 Footprint
- http://www.SafeClimate.net/