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The following are some of
the USGCRP's major accomplishments related to the Composition and Chemistry
of the Atmosphere during Fiscal Year 2001:
Observations from the Upper Atmosphere Research Satellite (UARS)
show a decline in the total abundance of chlorine compounds in the stratosphere.
This result adds credence to the model calculations used to project future changes
in atmospheric chemistry and validates the basic strategy that was embarked
on with the Montreal Protocol. This evidence of a stratospheric peaking of chlorine
compounds follows a similar pattern observed five years ago in the troposphere,
demonstrating consistency in our understanding of the transport of chemicals
between the troposphere and the stratosphere and of atmospheric chlorine chemistry.
Recent data from the Total Ozone Mapping Spectrometer (TOMS) demonstrate
how short-term variability in global climate can combine with fires originating
in connection with forest-clearing activity to produce massive air pollution
over a wide area. These results describe a buildup of pollution in Southeast
Asia during the last El Niño event, in September 1997. Ozone column measurements
document an intensely polluted air mass covering densely populated areas around
Singapore and Indonesia. These high pollution levels resulted from an unprecedented
incidence of fires occurring on the island of Borneo (Kalimantan). This analysis
built on a decade of research that has led to an increasingly precise description
of tropospheric ozone.
A synthesis of results from meteorological data and satellite
measurements shows a strong historical relationship between late-wintertime
minimum temperatures and stratospheric ozone depletion in the Arctic region -- colder
temperatures are associated with greater ozone depletion.
Recent measurements add support to the hypothesis that the Arctic
springtime will show individual years of substantial depletion of the ozone
layer, even as the trend of increasing atmospheric concentrations of chlorine
is reversed. Concentrations of reactive nitrogen and chlorine during the
coldest Arctic winters of the 1990s are similar to the levels observed in the
Antarctic. This finding illustrates the importance of the linkage between atmospheric
chemistry and temperature, and suggests that the future health of the ozone
layer in the Northern hemisphere will be linked to future changes in stratospheric
temperature caused by rising concentrations of greenhouse gases.
With multiagency support, the Aerosol Characterization Experiment-Asia
(ACE-Asia) was completed successfully in spring 2001. Newly developed instrumentation,
including improved airborne sampling techniques, was used to characterize the
distribution of aerosols in the region of outflow of air masses in Northeast
Asia. NSF, Navy, and NOAA aircraft-based observing instruments were complemented
by ground-based and satellite observations to measure a complex mixture of pollution-derived
and natural aerosols, including mineral dust.
Improved atmospheric transport and chemistry models have been
developed that assimilate satellite observations in real time and include descriptions
of atmospheric aerosols and their transport. Information on distributions and
chemical nature of aerosols is needed in order to determine their radiative
impact. Aerosol models with predictive capabilities have been developed and
used successfully in several field campaigns to guide the deployment of research
aircraft.
The relative importance of seasonally and geographically varying
processes that affect the production and fate of oxidants has been evaluated
quantitatively for several metropolitan areas in the United States, providing
vital information on the likely success of various possible control strategies
for tropospheric ozone.
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