• The relationship between changing atmospheric composition, caused by large scale human activities, and changes of global and regional climate;
• The impact of changes in climate and other aspects of the Earth system on the chemical composition of the atmosphere;
• The long-range atmospheric transport and deposition of potentially harmful substances; and,
• The natural cycling of chemical elements in the global atmosphere/ocean/biosphere system and the impact of human activity on these cycles.

A number of environmental conventions have been ratified in recent years as a consequence of information derived from global monitoring of the atmosphere. GAW, therefore, provides important information to the Parties implementing the Convention on Long-range Transboundary Air Pollution, the United Nations Framework Convention on Climate Change and its Kyoto Protocol, and to the Vienna Convention for the Protection of the Ozone Layer and its subsequent protocols.

• The atmospheric information provided by the GAW network of stations was pivotal in the decisions by the world's governments to develop and ratify those environmental conventions mentioned above;
• The bringing together of a number of disparate national, regional and global networks under the GAW umbrella and with an agreed core work programme;
• The establishment of six new GAW Global Observing Stations at remote clean air sites, and the instrumentation of more than a dozen sites in the Southern Cone of South America to assist countries monitor the ozone layer hole and issue public health advisories;
• The establishment of both a global mechanism for quality assurance of the measurements and centres to archive and make available the data;
• Facilitating the participation of developing countries in environmental monitoring and research; and,
• Providing training, publishing guidelines and manuals on station operation and coordinating the development of a worldwide UV Index.
  
  

• greenhouse gases (carbon dioxide, methane, CFCs, and nitrous oxide) - mounting levels of these greenhouse gases, caused largely by human activities, in the atmosphere may lead to global warming and to change the Earth's climate and weather over the next century. They act by trapping some of the long-wave radiation emitted by the Earth that would have otherwise escaped to space.
• ozone and ultraviolet radiation- the main atmospheric ozone concentrations can be found in the 'ozone layer' at about 20 km above the Earth. This layer absorbs much of the ultraviolet radiation emitted by the Sun. Levels of ozone are being reduced as a result of ozone-depleting chemicals, principally chlorofluorocarbons used in refrigeration, industrial cleaners and as foam-blowing agents. This reduction in ozone concentrations allows more ultraviolet radiation to reach the Earth with deleterious effects on the biota and the human immune system. Ozone in the lower atmosphere acts as a greenhouse gas.
• acid deposition - acid precipitation falls on many areas of the world, notably Europe, North America and the Far East. Acids are formed in the atmosphere through the oxidation of sulphur dioxide and nitrogen oxides, produced during the smelting of ores and the burning of fossil fuels. Such acids are often transported from their region of origin over hundreds or thousands of kilometres where they raise the acidity of soils, streams and lakes causing serious damage to forests and fish populations.
• aerosols - aerosols are small solid or liquid particles suspended in the atmosphere. They are produced by human activity and are also generated naturally by, for example, wind erosion and volcanic eruptions. They have a cooling effect on the climate by reflecting incoming heat from the Sun back into space and therefore somewhat offset the action of the greenhouse gases. In high concentrations they can have an impact on human health.
• long-range transport - many atmospheric pollutants, particularly those that occur in aerosol form, can be carried over very large distances. These pollutants eventually fall to Earth transformed into acid rain, while others, equally harmful to living creatures and biota, are deposited on the Earth's surface thousands of kilometres from their place of origin. GAW both monitors these forms of pollution and facilitates research and application of mathematical models of their transport.
• reactive gases and the urban environment (carbon monoxide, sulphur dioxide and nitrogen oxides) - these gases are important as they influence the chemistry of the greenhouse gases, and as pollutant gases in their own right. Both sulphur dioxide and nitrogen oxides react in sunlight and play crucial roles in the occurrence of urban smog and in determining associated surface ozone concentrations.

With regard to the calibration and quality assessment centres, some 20 of these perform the vital function of ensuring that the data derived from the measuring sites is of sufficiently high quality to be useful for scientific research and credible for governments. They do this through a combination of calibrating instruments at the stations, hosting intercomparisons between instruments from various stations, providing on-site training to operators, and by identifying suspect data.

There are currently six World Data Centres whose task it is to collect, process and make available atmospheric data from the GAW network.

WMO provides oversight to the GAW activities by coordinating and facilitating the operation of the stations and centres, and by arranging training and intercomparisons. It also is responsible for developing and implementing the GAW Strategic Plan in collaboration with the GAW community. It is assisted by a number of Scientific Advisory Groups established to provide guidance on major themes of the GAW programme.

• To acquire and distribute data of high and known quality through the use and implementation of consistent quality control procedures and through making more use of Internet;
• To improve and expand the measurement network by maintaining operations at the present stations, selectively extending measurement capabilities and adding stations in targeted biomes and regions of the world with little or no coverage;
• To enhance capacities of station operators, technicians, and scientists in developing countries through on-site training, workshops, and advanced international training;
• To expand GAW efforts into the urban environment, biosphere burning, aerosols, and Persistent Organic Pollutants/heavy metals;
• To expand the user base for GAW data into the scientific community; and,
• To evolve GAW into a 3-dimensional global observation network through integration of surface-based, aircraft, satellite and other remotely sensed observations for real-time monitoring.