Atmospheric chemistry
Atmospheric chemistry involves study of the chemistry of the Earth's atmosphere of Earth and other planets. It is a branch of atmospheric science and is a multidisciplinary field of research, drawing on environmental chemistry, meteorology, physics, computer modeling, oceanoraphy, geology, volcanology, and other disciplines. In addition, it is being increasingly connected with the field known as climatology.
Earth's atmosphere is composed of about 78 percent nitrogen, 21 percent oxygen, and small amounts of water vapor, carbon dioxide, argon, and other gases. This mixture of gases, commonly called air, protects and sustains life on Earth in a variety of ways. It provides oxygen for respiration, carbon dioxide for photosynthesis, nitrogen for nitrogen fixation, and water vapor for precipitation that nourishes the land. Carbon dioxide and water vapor are greenhouse gases that keep the planet warm enough for living organisms, reducing temperature extremes between day and night. Ozone in the upper atmosphere (stratosphere) absorbs ultraviolet solar radiation that could damage living tissue. In addition, higher layers of the atmosphere protect the Earth from bombardment by meteorites and charged particles in the solar wind.
The composition of Earth's atmosphere has been altered by human activities such as fuel burning and industrial production, and a number of these changes are harmful to human health, crops, and ecosystems. Examples of problems that have been addressed by atmospheric chemistry include acid rain, photochemical smog, and global warming. Atmospheric chemistry seeks to understand the causes of these problems and to look for possible solutions. It informs and evaluates government policies that are related to the environment.
Atmospheric composition
| Atmospheric sciences [cat.] |
|---|
Meteorology [cat.]
|
Climatology [cat.]
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| Average composition of dry atmosphere, by volume | ||
|---|---|---|
| Gas | per NASA | |
| Nitrogen, N2 | 78.084% | |
| Oxygen, O2 | 20.946% | |
| Argon, Ar | 0.934% | |
| Water vapour | Highly variable; typically makes up about 1% | |
| Minor constituents in ppmv. | ||
| Carbon Dioxide, CO2 | 383 | |
| Neon, Ne | 18.18 | |
| Helium, He | 5.24 | |
| Methane, CH4 | 1.7 | |
| Krypton, Kr | 1.14 | |
| Hydrogen, H2 | 0.55 | |
Notes: the concentration of CO2 and CH4 vary by season and location. ppmv represents parts per million by volume.
The mean molecular mass of air is 28.97 g/mol.
History
The ancient Greeks regarded air as one of the four elements, but the first scientific studies of atmospheric composition began in the eighteenth century. Chemists such as Joseph Priestley, Antoine Lavoisier, and Henry Cavendish made the first measurements of the composition of the atmosphere.
In the late nineteenth and early twentieth centuries, interest shifted towards trace constituents at very low concentrations. One particularly important discovery for atmospheric chemistry was the discovery of ozone by Christian Friedrich Schoenbein in 1840.
In the twentieth century, atmospheric science moved on from studying the composition of air to a consideration of how the concentrations of trace gases in the atmosphere have changed over time and the chemical processes that create and destroy compounds in the air. Two particularly important examples of this were the explanation of how the ozone layer is created and maintained by Sydney Chapman and Gordon Dobson, and the explanation of Photochemical smog by Haagen-Smit.
In the twenty-first century, the focus is shifting once again. Atmospheric chemistry is increasingly studied as an important component of the Earth system. Instead of concentrating on atmospheric chemistry in isolation, the focus is now on seeing it as one part of a single system with the rest of the atmosphere, biosphere, and geosphere. An especially important driver for this is the links between chemistry and climate, such as the effects of changing climate on the recovery of the ozone hole and vice versa but also interaction of the composition of the atmosphere with the oceans and terrestrial ecosystems.
Methodology
Observations, lab measurements and modeling are the three central elements in atmospheric chemistry. Progress in atmospheric chemistry is often driven by the interactions between these components and they form an integrated whole. For example observations may tell us that more of a chemical compound exists than previously thought possible. This will stimulate new modeling and laboratory studies which will increase our scientific understanding to a point where the observations can be explained.
Observation
Observations of atmospheric chemistry are essential to our understanding. Routine observations of chemical composition tell us about changes in atmospheric composition over time. One important example of this is the Keeling Curve - a series of measurements from 1958 to today which show a steady rise in of the concentration of carbon dioxide. Observations of atmospheric chemistry are made in observatories such as that on Mauna Loa and on mobile platforms such as aircraft (e.g. the UK's Facility for Airborne Atmospheric Measurements), ships and balloons. Observations of atmospheric composition are increasing made by satellites with important instruments such as GOME and MOPITT giving a global picture of air pollution and chemistry. Surface observations have the advantage that they provide long term records at high time resolution but are limited in the vertical and horizontal space they provide observations from. Some surface based instruments e.g. LIDAR can provide concentration profiles of chemical compounds and aerosol but are still restricted in the horizontal region they can cover. Many observations are available on line in Atmospheric Chemistry Observational Databases.
Lab measurements
Measurements made in the laboratory are essential to our understanding of the sources and sinks of pollutants and naturally occurring compounds. Lab studies tell us which gases react with each other and how fast they react. Measurements of interest include reactions in the gas phase, on surfaces and in water. Also of high importance is photochemistry which quantifies how quickly molecules are split apart by sunlight and what the products are plus thermodynamic data such as Henry's law coefficients.
Modeling
In order to synthesize and test theoretical understanding of atmospheric chemistry, computer models are used. Numerical models solve the differential equations governing the concentrations of chemicals in the atmosphere. They can be very simple or very complicated. One common trade off in numerical models is between the number of chemical compounds and chemical reactions modeled versus the representation of transport and mixing in the atmosphere. For example, a box model might include hundreds or even thousands of chemical reactions but will only have a very crude representation of mixing in the atmosphere. In contrast, 3D models represent many of the physical processes of the atmosphere but due to constraints on computer resources will have far fewer chemical reactions and compounds. Models can be used to interpret observations, test understanding of chemical reactions and predict future concentrations of chemical compounds in the atmosphere. One important current trend is for atmospheric chemistry modules to become one part of earth system models in which the links between climate, atmospheric composition and the biosphere can be studied.
Some models are constructed by automatic code generators. In this approach a set of constituents are chosen and the automatic code generator will then select the reactions involving those constituents from a set of reaction databases. Once the reactions have been chosen the ordinary differential equations (ODE) that describe their time evolution can be automatically constructed.
See Also
ReferencesISBN links support NWE through referral fees
- Wayne, Richard P. 2000. Chemistry of Atmospheres. 3rd ed. Oxford University Press. ISBN 0-19-850375-X
- Seinfeld, John H., and Spyros N. Pandis. 2006. Atmospheric Chemistry and Physics - From Air Pollution to Climate Change. 2nd Ed. John Wiley and Sons, Inc. ISBN 0471828572.
- Finlayson-Pitts, Barbara J., and James N. Pitts, Jr. Chemistry of the Upper and Lower Atmosphere. Academic Press. ISBN 0-12-257060-X.
- Warneck, Peter. 2000. Chemistry of the Natural Atmosphere. 2nd ed. Academic Press. ISBN 0-12-735632-0.
- Brasseur, Guy P., John J. Orlando, and Geoffrey S. Tyndall. 1999. Atmospheric Chemistry and Global Change. Oxford University Press. ISBN 0-19-510521-4.
External links
All links retrieved October 3, 2007.
- IGAC The International Global Atmospheric Chemistry Project - IGAC
- Paul Crutzen Interview - Vega Science Trust.
- The Cambridge Atmospheric Chemistry Database by David Lary.
- Environmental Science Published for Everybody Round the Earth
- Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies - NASA-JPL
- Kinetic and photochemical data - IUPAC Subcommittee for Gas Kinetic Data Evaluation
- Atmospheric Chemistry Glossary - Sam Houston State University
- Tropospheric chemistry - atmosp.physics.utoronto.ca
- Calculators for use in atmospheric chemistry by Theodore S. Dibble
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