Atmospheric chemistry

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Atmospheric chemistry is a branch of atmospheric science in which the chemistry of the Earth's atmosphere and that of other planets is studied. It is a multidisciplinary field of research and draws on environmental chemistry, physics, meteorology, computer modeling, oceanography, geology and volcanology and other disciplines. Research is increasingly connected with other areas of study such as climatology.

Studying the composition and chemistry of the atmosphere is important for several reasons. It includes studying the interactions between the atmosphere and living organisms. The composition of the Earth's atmosphere changes as result of natural processes such as volcano emissions, lightning and bombardment by solar particles from corona. It has also been changed by human activity. Some of these changes are harmful to human health, crops and ecosystems. Examples of problems which have been addressed by atmospheric chemistry include acid rain, ozone depletion, photochemical smog, greenhouse gases and global warming. Atmospheric chemists study the causes of these problems. Atmospheric chemists offer theories about these problems, then test the theories and possible solutions. Atmospheric chemists also evaluate the effects of changes in government policy.

Atmospheric composition[change | change source]

Visualization of composition by volume of Earth's atmosphere. Water vapor is not included as it changes a lot over time. Each tiny cube (such as the one representing krypton) has one millionth of the volume of the entire block. Data is from NASA Langley.
Schematic of chemical and transport processes related to atmospheric composition.
Average composition of dry atmosphere (mole fractions)
Gas per NASA[1]
Nitrogen, N2 78.084%
Oxygen, O2 20.946%
Argon, Ar 0.934%
Minor constituents (mole fractions in ppm)
Carbon Dioxide, CO2 383
Neon, Ne 18.18
Helium, He 5.24
Methane, CH4 1.7
Krypton, Kr 1.14
Hydrogen, H2 0.55
Water
Water vapor Highly variable;
typically makes up about 1%

Notes: the concentration of CO2 and CH4 vary by season and location. The mean molecular mass of air is 28.97 g/mol.

History[change | change source]

The ancient Greeks regarded air as one of the four elements, but the first scientific studies of atmospheric composition began in the 18th century. Chemists such as Joseph Priestley, Antoine Lavoisier and Henry Cavendish made the first measurements of the composition of the atmosphere.

In the late 19th and early 20th centuries interest shifted towards trace constituents with very small concentrations. One particularly important discovery for atmospheric chemistry was the discovery of ozone by Christian Friedrich Schönbein in 1840.

In the 20th century atmospheric science moved 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 which create and destroy compounds in the air. Two particularly important examples of this were the explanation by Sydney Chapman and Gordon Dobson of how the ozone layer is created and maintained, and the explanation of photochemical smog by Arie Jan Haagen-Smit. Further studies on ozone issues led to the 1995 Nobel Prize in Chemistry award shared between Paul Crutzen, Mario Molina and Frank Sherwood Rowland.[2]

In the 21st century the focus is now shifting again. Atmospheric chemistry is increasingly studied as one part of the Earth system. Before, scientists focused on atmospheric chemistry in isolation. Now, scientists study atmospheric chemistry as one part of a single system with the rest of the atmosphere, biosphere and geosphere. A reason for this is the links between chemistry and climate. For example, changing climate and the recovery of the ozone hole affect each other. Also, the composition of the atmosphere interact with the oceans and terrestrial ecosystems.

Methodology[change | change source]

Observations, lab measurements and modeling are the three central elements in atmospheric chemistry. All three methods are used together. For example, observations may tell that more of a chemical compound exists than previously thought possible. This will stimulate new modelling and laboratory studies which will increase scientific understanding to a point where the observations can be explained.

Observation[change | change source]

Observations of atmospheric chemistry are important. Scientist record data about the chemical composition of air over time to watch for any changes. 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 (for example, the UK's Facility for Airborne Atmospheric Measurements), ships and balloons. Observations of atmospheric composition are increasingly made by satellites giving a global picture of air pollution and chemistry.[Note 1] Surface observations have the advantage that they provide long term records at high time resolution but provide data from a limited vertical and horizontal space. Some surface based instruments such as LIDAR can provide concentration profiles of chemical compounds and aerosol but are still restricted in the horizontal region they cover. Many observations are shared on line in Atmospheric chemistry observational databases.

Lab measurements[change | change source]

Measurements made in the laboratory are essential to our understanding of the sources and sinks of pollutants and compounds found in nature. Lab studies tell which gases react with each other and how fast they react. Scientists measure reactions in the gas phase, on surfaces and in water. Scientists also study photochemistry which quantifies how quickly molecules are split apart by sunlight and what the products are. Scientists also study thermodynamic data such as Henry's law coefficients.

Modeling[change | change source]

Scientists use computer models (such as chemical transport models) to synthesize and test theoretical understanding of atmospheric chemistry. Numerical models solve the differential equations governing the concentrations of chemicals in the atmosphere. The models can be very simple or very complicated. Model designers must trade off between the number of chemical compounds and chemical reactions modelled 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. Atmospheric chemistry modules are used as one part of larger earth system models in which the links between climate, atmospheric composition and the biosphere can be studied.

Sometimes, model builders use automatic code generators (e.g. Autochem or KPP) to help write the equations used in computer programs. In this approach, the scientist choses a set of chemical compounds, 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 computer builds the ordinary differential equations (ODE) that describe their time evolution.

Notes[change | change source]

  1. Satellites include important instruments such as GOME and MOPITT.

References[change | change source]

Further reading[change | change source]

  • Brasseur, Guy P.; Orlando, John J.; Tyndall, Geoffrey S. (1999). Atmospheric Chemistry and Global Change. Oxford University Press. ISBN 0-19-510521-4.
  • Finlayson-Pitts, Barbara J.; Pitts, James N., Jr. (2000). Chemistry of the Upper and Lower Atmosphere. Academic Press. ISBN 0-12-257060-X.
  • Seinfeld, John H.; Pandis, Spyros N. (2006). Atmospheric Chemistry and Physics: From Air Pollution to Climate Change (2nd Ed.). John Wiley and Sons, Inc. ISBN 0471828572.
  • Warneck, Peter (2000). Chemistry of the Natural Atmosphere (2nd Ed.). Academic Press. ISBN 0-12-735632-0.
  • Wayne, Richard P. (2000). Chemistry of Atmospheres (3rd Ed.). Oxford University Press. ISBN 0-19-850375-X.

Other websites[change | change source]