Purpose of Atmospheric Chemistry Measurements

The intricate role that chemical constituents play in the processes and events that affect the climate system requires, as part of GCOS, a component observing system that exhibits long-term stability and low bias.

Chemical Constituents and the Atmosphere

The Effect of Chemical Constituents of the Atmosphere

The chemical composition of the atmosphere, in combination with cloud-related effects, exerts a dominant influence on the radiation balance of the atmosphere through scattering and absorption processes. Atmospheric composition in particular is responsible for the principal observed temperature features of the atmosphere including the tropopause itself. The atmosphere contains many gases, most in only trace quantities, which are critical in maintaining control of the earth's climate. Many of these trace components are involved in key reactions which produce the chemical constituent balance. Clearly, atmospheric composition is a crucial factor, influencing both the radiation which reaches the earth from the sun, and the long wave radiation from the earth which is so closely linked to the "greenhouse effect".

Anthropogenic Effects

Emissions caused by human activities are increasing the concentrations of the major greenhouse gases in the atmosphere, particularly carbon dioxide, methane, tropospheric ozone, chlorofluorocarbons, and nitrous oxide. Modeling studies of the greenhouse effect suggest that these increased concentrations will induce global temperature increases of up to several degrees during the next century in worst case scenarios. Evidence today indicates that the global mean temperature has already risen by 0.3 to 0.6 C over the last 100 years, which is consistent with predictions of global climate models that suggest temperature increases of between 1.5 and 4.5 C with an effective doubling of atmospheric CO2 relative to the pre-industrial level.

The Ozone Hole and Volcanoes

In recent years two major events have illustrated the importance of atmospheric constituents to climate. The first is the depletion of ozone in the lower stratosphere at mid- and high latitudes, a phenomenon linked to the chemistry of chlorofluorocarbons (CFCs) in the atmosphere. Ozone depletion not only allows an increase in the ultraviolet (UV) radiation reaching the earth's surface, but also directly affects the radiative balance of the earth and the atmosphere. The second event was the 1991 eruption of Mount Pinatubo in the Philippines. The eruption injected large amounts of sulphur dioxide into the stratosphere. Through gas-to-particle chemical conversion and subsequent optical scattering, these emissions induced negative radiative forcing, stratospheric warming and tropospheric cooling, with associated masking of climate warming for a period of a few years.

Systematic observations contribute to:

As with physical observations of the atmosphere, chemical observations must provide adequate coverage, reliability, continuity and accuracy, and the data must be readily accessible in a form which can be easily used.

GCOS is designing a network of stations with appropriate global coverage and including the necessary variables to provide data for:

User Communities for Atmospheric Chemistry Data and Products

Researchers: The process research community, represented for example by core projects of the IGBP (e.g., IGAC) and by the WCRP (e.g., SPARC), make use of systematic atmospheric constituent observations to provide baseline conditions for process studies, for separating natural and anthropogenic variability, and to detect and attribute climate change, and to study long-term phenomena.

Modellers: The climate modeling community uses dynamical/chemical general circulation models (GCMs), chemical transport models (CTMs), and biogeochemical cycle models.

Information required includes data for model initialization, boundary conditions and evaluation. A particular requirement of the modeling community is for measurements of chemical species which can be used as tracers for model development and evaluation.

Industry: While involvement of the industrial sector has been rather limited to date (a notable exception is the CFC/ozone issue), it is likely that the energy, chemical and insurance industries will, before long, develop a keen interest in changes in the climate system which have the potential to impact the conditions under which they operate.

Assessment Groups: The primary users in this category are the IPCC Working Group I, the group responsible for assessing the status of the climate system and advances in scientific knowledge of climate variability and change, and the WMO/UNEP Panel responsible for assessing the state of the ozone layer.

Policy Community: This group includes policy and regulatory officials responsible for monitoring and assessment activities specified by international agreements, such as the Framework Convention on Climate Change, the Montreal Protocol and its Amendments, Agenda 21, and the ECE Sulphur and Nitrogen Protocols. Uses here would include both monitoring compliance to emission limitations and developing international policy.

GCOS Panels: Within the GCOS community, groups other than the AOPC may have requirements for atmospheric constituent observations (e.g., OOPC, TOPC, and GOSSP). Types of observations required for the first two are likely to involve the flux of constituents between the atmosphere and the oceanic and terrestrial reservoirs, while those for space observations are more likely to required to provide ground-truth data.