A combination of human and natural processes can affect the chemical composition of the global atmosphere. These changes can have important implications for life on Earth, including such factors as biologically damaging ultraviolet (UV) radiation, radiative forcing of the Earth/atmosphere system (which in turn affects climate), and the global composition of the atmosphere, which can affect air quality in regions. Human activity that can affect atmospheric composition on a global scale includes the use of chlorofluorocarbons and other halogenated hydrocarbons, fossil fuel combustion and the associated release of air pollutants, and changes in agricultural practices that affect the concentration of gases such as nitrous oxide and methane, as well as that of smoke. Changes in climate driven largely by increases in greenhouse gases can also be expected to affect atmospheric chemistry in complex ways that are difficult to predict. Natural processes affecting global atmospheric composition include volcanic eruptions, variations in solar radiation, and normal weather. Particular questions addressed by this element of USGCRP include:     Current USGCRP activity in these areas builds on the accomplishments of previous research. For example, significant reductions in the total amount of stratospheric ozone over most of the Earth have been demonstrated over the past 20 years. A combination of airborne-, ground-, balloon-, and space-based instruments have all shown that industrially-produced chlorine- and bromine-containing chemical species contribute significantly to the observed ozone depletion. Observations have shown that the surface concentrations of several of the compounds regulated under the Montreal Protocol on Substances that Deplete the Ozone Layer have been reduced significantly, while those of the longer-lived chlorofluorocarbons have essentially reached a maximum and will soon begin to decline. It is expected that maximum levels of stratospheric chlorine will be reached around the turn of the century. The stratosphere should be most susceptible to ozone depletion at that time; recovery of the ozone layer could, in principle, begin shortly thereafter. It is possible, however, that global climate change (which is projected to cool the stratosphere as the lower atmosphere warms), or a large volcanic eruption, could delay the projected recovery.

    (See Appendix E for additional information)

    Key research challenges include:

  1. Stratospheric Ozone and UV Radiation: Defining and predicting trends in the intensity of ultraviolet exposure the Earth receives by documenting the distribution of stratospheric ozone and surface UV flux, the chemical species that control the destruction of ozone, and the meteorological variables that define the physical environment of the stratosphere; and describing the coupling between chemistry, dynamics, and radiation in the stratosphere and upper troposphere.
  2. Photochemical Oxidants: Defining the global processes that control ozone precursor species, tropospheric ozone, and the oxidizing capacity of the global atmosphere; and developing better understanding of what determines the ability of the atmosphere to cleanse itself of pollutants, both now and in the coming decades.
  3. Atmospheric Modeling: Improving atmospheric models to better represent the trace gas and aerosol composition of the global atmosphere, as well as its transport properties, and predicting the atmosphere’s response to future levels of pollutants and to changes in climate at both global and regional scales.
  4. Atmospheric Aerosols and Radiation: Documenting the chemical and physical properties of aerosols; and elucidating the chemical, microphysical, and transport processes that determine their size, concentration, and chemical characteristics.
  5. Toxics and Nutrients: Documenting the rates of chemical exchange between the global atmosphere and ecosystems; and elucidating the extent to which interactions between the atmosphere and biosphere are influenced
  6. Clouds: Documenting the role of clouds in the partitioning of trace gases in the global atmosphere between different chemical forms and in their removal from the atmosphere, as well as their contribution to surface deposition.
    The USGCRP work in several of these areas, notably photochemical oxidants and toxics and nutrients, will be carried out in close collaboration with the more regionally focused work on air pollution, acid deposition, and airborne toxics carried out through other Federal research programs organized under the auspices of the Air Quality Research Subcommittee of the Committee on Environment and Natural Resources.