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:
and Chemistry of the Atmosphere
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.
How are global ozone
levels and surface UV fluxes changing, and how are they likely to change
in the future, given expected changes in both human industrial activity
and the underlying climate in which ozone chemistry takes place?
What changes may take
place in the concentrations of ozone, aerosols, and other chemically and
radiatively active atmospheric constituents that may contribute to climate
change, and what changes may take place in the background concentrations
of trace gases that affect regional atmospheric chemistry?
How will global changes
in surface UV flux and surface-level concentrations of ozone and other
gases and particulate matter affect human health and the productivity of
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.
(See Appendix E for additional information)
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.
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.
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.
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.
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
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.
for FY 2000:
The USGCRP will examine
the chemistry of the stratosphere at high northern latitudes in winter,
to determine the potential for an Arctic ozone hole. The study will use
combined balloon and airborne measurements together with observations from
an instrument currently planned for launch in late 1999 aboard a Russian
The USGCRP will carry
out significant modeling work in support of the Intergovernmental Panel
on Climate Change (IPCC) Third Assessment Report, to be completed in 2001.
These modeling efforts will help to simulate prior evolution of atmospheric
trace constituents and aerosol composition and to forecast its future evolution.
The output from these model runs will be used by climate modeling groups
in their simulations of the future climate.
The USGCRP will examine
the atmospheric chemistry over Southern Africa using a combination of ground-based,
airborne, and satellite-based measurements. This will help establish the
influence of land-cover and land-use change on regional atmospheric composition,
and the role of trace gases and aerosols in atmospheric warming.
The USGCRP will increase
knowledge of the distribution of ozone in the troposphere and southern
sub-tropics using an enhanced network of balloon-based measurements. The
data should provide a unique capability for the validation of tropical
ozone columns derived from satellite data.
The USGCRP will have
obtained the first full year of global carbon monoxide vertical profiles.
These data, obtained by an instrument scheduled for launch in mid-1999
as part of the Earth Observing System, should provide a significantly improved
picture of carbon monoxide distributions. When analyzed together with data
on smoke and aerosols obtained from other EOS instruments, these measurements
should lead to new insights about the role of biomass burning and industrial
emissions in global pollution.
The USGCRP will have
obtained surface UV flux data from the fully-implemented USGCRP ground-based
UV monitoring network. These data, making use of some 60 instruments at
some 50 locations, will be provided to researchers investigating biological
response to ultraviolet radiation. UV flux data for other regions of the
earth will be available from satellite-based techniques.
The USGCRP will provide
extended and updated data sets on the global methane budget, using a combination
of long-term surface-based measurements showing unexplained interannual
variations in growth rate and newly-obtained total column methane observations
made from a space-based instrument launched in 1999 as part of the Earth
The USGCRP will carry
out detailed studies of new data on the distribution and composition of
aerosols in the global troposphere, based on a combination of ground-,
ship- , airborne- , and space-based data; and the program will integrate
these data into global numerical models designed to simulate aerosol formation,
transport, and interaction with surrounding meteorology.