Atmospheric chemistry

Definition: Atmospheric chemistry is a subfield of atmospheric science that examines the chemical development and composition of atmospheric systems, that is, the layers of gases, liquids, and solids that surround the earth and other planets. Atmospheric chemists study how atmospheres form around developing planets and change over geological time, as well as the relationship between the biota (organisms living on a planet) and the atmosphere. Atmospheric chemists also provide important data to explain phenomena such as climate change, weather patterns, and seasonal temperature variation. These scientists work to develop solutions to various atmospheric problems, including pollution and ozone depletion.

Basic Principles

Key discoveries in the eighteenth and nineteenth centuries set the stage for the development of atmospheric science as a new branch of scientific inquiry, combining elements of chemistry, geology, and physics. The discovery of the atmospheric gas ozone by German chemist Christian Schönbein, often called the father of atmospheric chemistry, in 1839 and the discovery of the relationship between atmospheric gases and surface temperature by Swedish chemist Svante Arrhenius in 1896 were among the crucial discoveries that helped to stimulate interest in atmospheric composition.

During the twentieth century, enhancements in technology, including satellite measurements and computer modeling, allowed scientists to develop a more detailed understanding of atmospheric properties and processes. Gradually, atmospheric science became a branch of earth science or environmental science, the effort to develop a thorough scientific understanding of the earth’s environment and evolution.

By the twenty-first century, atmospheric science was divided into a number of subfields, including atmospheric physics, climatology, aeronomy, and atmospheric chemistry. Atmospheric chemistry is itself a multidisciplinary field, drawing upon contributions from meteorology, inorganic chemistry, organic chemistry, environmental chemistry, oceanography, and volcanology. Unlike climatology, which studies aspects of the atmosphere involved in climate formation, or atmospheric physics, which studies the physical modeling of the atmosphere, atmospheric chemistry focuses on chemical reactions within the atmosphere. It also studies how the chemical properties of atmospheric gases, liquids, and solids contribute to atmospheric phenomena like weather and climate.

Core Concepts

In Situ Measurement Techniques. Atmospheric chemists use in situ (in position) measurements to evaluate the gas, liquid, and solid composition of the atmosphere within a localized area. In situ measurements can be collected by devices mounted to balloons or aircraft to sample atmospheric composition at high altitudes, or they can be taken at ground level. In situ measurements are limited in spatial dimensions, but they allow for the most accurate and detailed analyses of atmospheric properties. Samples collected in situ can be either analyzed immediately by the collecting device or trapped and carried to a laboratory for further analysis.

Remote Atmospheric Measurements. Remote-sample methods involve measuring gas compositions in areas relatively distant from the measuring apparatus. Atmospheric chemists can use equipment in balloons, aircraft, satellites, specialized long-distance spectrometers, and also in spacecraft to measure gas composition in distant areas. Remote measurements are necessary for studying the gas composition of other planets. They are also widely used to gain a better understanding of large-scale atmospheric patterns over the earth. Unlike in situ techniques, which are generally used to measure gas concentrations, remote-measurement techniques are more useful for modeling system-wide movements of gases. For instance, atmospheric scientists use radar measurements to track the movement of clouds and water vapor through the atmosphere, thereby producing a three-dimensional model of atmospheric movement.

Gas Chromatography. Chromatography is a common technique used in chemical analysis that involves determining the chemical composition of a sample by dissolving mixtures of gases within a liquid medium. The liquid is then passed through a device containing another liquid of known properties; this causes the various compounds within the mixture to separate at different intervals. The various constituents can then be measured, providing data on the composition of the original gas mixture. Chromatography is one of the most basic techniques used in atmospheric chemistry to determine the mixture of elements in atmospheric gases.

Mass Spectrometry. Mass spectrometry is a method used for measuring the atomic or molecular composition of a mixture or sample of material. The basic process entails converting the atoms contained within the mixture or sample to ions, or charged atoms, by bombarding the material with a stream of negatively charged electrons. The resulting ions, because of their charge, will interact with electrical fields. A spectrometer is used to separate ions according to their mass and charge; the separated ions are next filtered to a detector, where the relative composition of each type of ion is recorded. This data then allows chemists to reconstruct the chemical and atomic structures of the sample material. Spectrometry can be used to analyze gas, liquid, and solid samples of material.

Sources and Sinks. Atmospheric chemists investigate the life cycles of atmospheric components, including how these substances are produced and absorbed within the environment. A chemical source is a process or feature that produces atmospheric material. For instance, the metabolic processes within organisms produce methane, carbon dioxide, and water vapor, which then become part of the earth’s atmosphere. A sink is a process that removes a particular type of material from the atmosphere, often to be stored in another environmental compartment. Plants and certain types of bacteria, for instance, absorb atmospheric carbon dioxide during photosynthesis, which they use to fuel growth and reproduction. These organisms therefore represent a carbon sink within the environment. They also produce oxygen as a byproduct, which is incorporated into the atmosphere.

Greenhouse Effect. The greenhouse effect is a model used to explain how the buildup of gases within the atmosphere interacts with solar radiation to influence the climate and surface temperature of a planet. Greenhouse gases such as carbon dioxide, methane, and ozone trap solar energy and heat reflected from the earth’s surface, preventing this energy from escaping into space. Increases in greenhouse-gas levels in the atmosphere translate directly into increased surface temperatures; this plays a major role in determining climate variation.

Ozone Cycle. The ozone cycle is the process by which ozone is created from atmospheric oxygen and decomposes to release oxygen back into the atmosphere. Ozone is an unstable molecule containing three atoms of oxygen bonded together. Ultraviolet light originating from the sun impacts oxygen (O2) molecules within the upper layers of the atmosphere, causing them to split into individual atoms of oxygen. These individual oxygen atoms then bond with molecules of oxygen, forming a molecule of ozone (O3). The process also works in reverse, as ultraviolet radiation causes ozone to split into its constituent parts, yielding a molecule of oxygen and a free oxygen atom. By absorbing ultraviolet radiation and converting it to chemical energy through the ozone cycle, the ozone layer protects life on earth from the harmful effects of overexposure to solar radiation.

Chemical Transport Models (CTM). A chemical transport model (CTM) is a theoretical model used to emulate the creation and transport of specific species of atoms and molecules within a chemical system. Most CTMs are complex computer-aided systems that combine data taken from measurements of atmospheric concentrations with equations modeling known chemical reactions that occur in the system. There are a variety of CTM systems used to model the origin and transport of individual types of atmospheric materials. Various CTM systems, developed by both governmental and university-based research groups, measure the movement of atmospheric substances including carbon dioxide, methane, water vapor, and nitrous oxide.

The National Center for Atmospheric Research (NCAR) cooperated with researchers from a number of US universities to create a CTM known as MOZART (Model for Ozone and Related Chemical Tracers), which models various ozone concentrations within the atmosphere and can be used to create simulations of atmospheric conditions given differing concentrations of ozone. MOZART has been used in critical studies examining the role of ozone in climate change and temperature variation. Another CTM, called CAMx (Comprehensive Air Quality Model with Extensions), is an open-source system, allowing computer programmers and other specialists to participate in enriching the design of the system. The CAMx program primarily models air quality within a targeted modeling area.

Applications Past and Present

Atmospheric and Climate Modeling. Atmospheric chemistry is essential to climate modeling, which is the effort to create detailed computer models that predict the development of climate patterns. General circulation models (GCMs) use complex data involving chemical and physical properties and thermodynamic principles to model circulation patterns within either the atmosphere or the oceanic systems. These models can be combined into atmosphere-ocean general circulation models (AOGCMs), which attempt to emulate the overall movement of materials and energy between both the ocean and the atmosphere.

Atmospheric and climate modeling is used to predict the future development of the earth’s climate given current conditions and likely future changes to composition, temperature, and other factors. To complete this task, scientists collect detailed data regarding atmospheric composition and then link different atmospheric states with corresponding conditions elsewhere in the environment. For instance, changes in atmospheric oxygen composition may be linked to reductions in tree cover in certain areas. These changes in oxygen levels will affect levels of carbon dioxide and ozone, thereby leading to alterations of temperature and climatic patterns on a planet-wide scale.

By creating detailed models that represent the state of the earth’s current atmosphere, atmospheric chemists can perform experiments to study how atmospheric composition and chemical behavior would change under conditions involving increases in greenhouse gases, reduction or increases in ambient temperature, and a variety of other theoretical scenarios that might have global repercussions.

Climate Change. Climate-change science is an interdisciplinary effort to predict and understand the way that the earth’s climate will change, given both current conditions and expected future influences on climate dynamics. One of the major debates of the twentieth and twenty-first centuries concerns whether the earth is currently experiencing a period of global warming due to the accumulation of pollutants and greenhouse gases in the atmosphere, and whether this trend is related to human activities, such as the burning of fossil fuels.

Atmospheric chemists use both in situ and remote detection and sampling equipment to measure atmospheric material around the globe and to compare this information with temperature measurements taken at the same locations. In this way, atmospheric chemists can make correlations between concentrations of atmospheric materials and relative temperatures both in the atmosphere and on the earth’s surface. This data can then be used to test theories about future climate development and to predict how increases or reductions in atmospheric substances, like carbon dioxide and methane, will affect future climate development.

The Intergovernmental Panel on Climate Change (IPCC) is a group of scientists and researchers from more than sixty countries that conduct research on climate change and publish annual reports on the state of the world’s climate. Atmospheric chemistry is a major part of IPCC research, because greenhouse gas concentrations are one of the most important factors precipitating climate change. Atmospheric chemists working for the IPCC were responsible for research indicating that, in addition to the direct release of greenhouse gases, chemical reactions in the atmosphere produce additional greenhouse gases that can contribute to the building greenhouse effect.

Ozone Depletion. The stratosphere is a layer of atmospheric gases located between ten to thirty miles from the earth’s surface. The upper stratosphere contains the ozone layer, which is a thin membrane of ozone gas created by the interaction between solar radiation and atmospheric oxygen. In the 1980s, atmospheric scientists realized that ozone levels were dropping and that a hole had formed in the ozone layer over Antarctica. In 1987, the Antarctic Airborne Ozone Experiment sampled the atmosphere in the stratosphere and found compelling evidence that the ozone hole was related to the release of chlorine and bromine through human activity. The use of aerosol sprays and refrigerants containing chlorofluorocarbons (CFCs) was the key factor, as these compounds release gases that rise into the stratosphere and catalyze reactions that destroy the ozone layer.

Over the next decade, the use of CFCs and other ozone-destroying bromine and chlorine aerosols were restricted and banned in many countries in an effort to stem the destruction of the ozone layer. Without sufficient ozone coverage, increased solar radiation impacting the planet would have deleterious effects on life and would cause temperatures to rise. The loss of ozone over Antarctica could eventually lead to the melting of polar ice, which would destroy the ecosystem of Antarctica and flood other continents as ocean levels rise. Atmospheric chemists monitor ozone levels and sample atmospheric gases to test for the presence of ozone-destroying pollutants.

Acidic Deposition. Acidic deposition, also known as acid rain, occurs when atmospheric chemical reactions result in an increase of acidic chemicals within atmospheric discharge, such as rain, fog, and snow. Acid rain results from the buildup of gases like sulfur dioxide (SO2) and nitrogen oxides, which are produced by a variety of industrial processes, including the burning of fossil fuels. These compounds react with water, carbon dioxide, and oxygen in the atmosphere. Fueled by energy from solar radiation, the reactions create sulfuric acid (H2SO4) and nitric acid (HNO3), which fall to the earth along with normal precipitation. Acidic precipitation can damage trees and root systems, kill fish and other aquatic organisms, and poison water supplies. Large-scale environmental damage has been recorded in areas where pollution is severe.

Atmospheric chemists study the factors that lead to the development of acidic deposition and monitor levels of chemicals in the atmosphere to predict potential incidents of acid rain. In addition, experimental chemistry is involved in developing methods used to purify the atmosphere and to reduce levels of pollutants that cause acid deposition. Research in atmospheric chemistry has also be used in creating state and federal regulations to control pollutants released from industrial processes in an effort to reduce the severity and frequency of acid rain incidents.

Public Safety. Atmospheric chemistry is used to monitor conditions in the atmosphere that pose a risk to public safety. The air quality index (AQI) used by the Environmental Protection Agency (EPA) to monitor air quality in the United States was developed with the aid of atmospheric chemistry research. The AQI is set by measuring levels of gases such as carbon dioxide, methane, sulfur dioxide, ozone, and nitrogen dioxide, based on previously measured concentrations known to cause medical issues among the populace. Federal, state, and private organizations make daily or hourly AQI measurements available to the public in an effort to help the populace avoid dangerous atmospheric conditions.

The 1990 amendments to the United States’ Clean Air Act states that the EPA must reevaluate the AQI every five years to adjust for new discoveries and research regarding air purity and the dangers of pollutants. The creation of AQI guidelines and changes to the AQI system are largely due to research conducted by atmospheric chemists. When pollutant levels rise above a certain level, federal and state regulations require certain measures to be taken, including the broadcasting of health advisories and the shutting down of traffic and industrial processes to reduce known sources of pollutants.

Paleoclimatology. As atmospheric materials cycle through the earth’s systems, they leave traces of molecules in the terrestrial and oceanic environments. Atmospheric chemists can investigate the remnants of organisms and physical structures from the distant past. They can then use chemical signatures and concentrations of certain atoms fixed within these ancient materials to reconstruct a model of past climate systems. Information about past climatological systems can also be used to understand current climate-change patterns and the potential effects of changing atmospheric conditions.

In ice cores taken from deep within permanent glaciers, chemists have found pockets of atmospheric gases that were trapped when the liquid water became frozen in the distant past. Paleoclimatologists use chemical analyses of these gases to reconstruct the composition of the atmosphere during the period. In addition, different atmospheric conditions lead to the deposition of different types of chemical residues that can be incorporated into fossils. The proportions of certain types of isotopes and characteristic chemicals therefore provide clues to the nature of the earth’s ancient atmosphere.

Social Context and Future Prospects

Atmospheric chemistry has frequently been at the forefront of environmental research. The field was essential for efforts to address ozone depletion and other atmospheric-pollution issues in the 1980s and 1990s, leading to government bans on a variety of substances known to cause atmospheric damage. In the twenty-first century, atmospheric chemistry and physics research has played a major role in the debate over climate change.

One of the most controversial issues in the global-warming debate concerns the degree to which current climate-change patterns are related to anthropogenic factors. This debate has numerous consequences for the future of governmental and industrial regulations and will have a consequent impact on national and international economic prospects. For this reason, atmospheric chemistry research is important in determining the degree to which greenhouse-gas emissions, climate change, and warming or cooling trends are related to human industry.

In addition, the potential for new legislative and regulatory measures to reduce industrial pollution has created opportunities for atmospheric scientists, with numerous corporations seeking to market environmentally friendly products in the future. As the petroleum industry faces reductions in supply and profitability, atmospheric scientists will likely have increasing opportunities for employment in the automotive and energy industries, helping to design the next generation of technology to meet evolving standards in pollution control.

Bibliography

Dessler, Andrew E., and Edward A. Parson. The Science and Politics of Global Climate Change: A Guide to the Debate. Cambridge UP, 2010.

Frederick, John E. Principles of Atmospheric Science. Jones, 2008.

Hoffmann, Matthew J. Ozone Depletion and Climate Change: Constructing a Global Response. State U of New York P, 2005.

Jacob, Daniel B. Introduction to Atmospheric Chemistry. Princeton UP, 1999.

MIT Atmospheric Chemistry, 2023, atmoschem.mit.edu/. Accessed 28 Aug. 2024.

NASA Goddard Institute for Space Studies, Natl. Aeronautics and Space Admin., 2024, www.giss.nasa.gov/. Accessed 28 Aug. 2024.

National Oceanic and Atmospheric Administration, US Dept. of Commerce, www.noaa.gov/. Accessed 28 Aug. 2024.