USGCRP-sponsored research continues to advance understanding of the causes, magnitude, and consequences of changes in stratospheric ozone, UV radiation, and atmospheric chemistry. The FY96 edition of Our Changing Planet included a summary of recent key findings. Highlights of more recent findings follow:

• Atmospheric concentrations of ozone-depleting chemicals are starting to decrease. Over the past few decades, global atmospheric monitoring stations had reported the steady growth in the concentrations of CFCs and other ozone-depleting chemicals. Recent observations have not only shown a slowdown in growth, but also are revealing, for the first time, decreasing concentrations. These downward trends are the first observed atmospheric response to the international agreements to halt production of these ozone-depleting chemicals.

• Year-to-year changes in the stratospheric ozone layer have been explained. Superimposed on the observed overall downward decadal trends in the ozone layer are variations over periods of a few years. The long-term declines have been associated with the growth in the use of ozone-depleting chemicals, such as CFCs, but the shorter term changes have been unexplained. Recent theoretical modeling studies have shown that these shorter term variations can be explained by the augmented ozone depletion caused from the chemistry on surfaces of particles resulting from episodic volcanic eruptions, thereby providing a more complete picture of ozone layer changes.

• Soils may be a larger methyl bromide sink than previously thought. Because methyl bromide is involved in the destruction of ozone in the stratosphere, this finding suggests that management of soils may have implications for the stratospheric ozone budget and the amount of UV radiation that reaches the Earth's surface.

• Satellite data have confirmed, on global scales, the expected stratospheric abundance's of ozone-depleting chemicals. Surface-level stations have long monitored the lower atmospheric growth of the ozone-depleting chemicals that are released into the atmosphere by human activities. Balloon and airborne measurements have tracked the resulting ozone-related chlorine and bromine compounds in the stratosphere in particular regions. However, recent satellite data have provided the first truly global look at these species and have confirmed the picture of the human contributions to the global budget of these ozone-related species.

• Carbon monoxide concentrations were measured from latitudes from Alaska to the Antarctic using an instrument mounted in the Space Shuttle on two flights in 1994. These measurements demonstrated a capability for providing global data sets and measuring seasonal variations. The measurements showed the expected higher levels near the source regions, mainly in the Northern Hemisphere and near areas of biomass burning in the tropics, but were also able to observe the much lower concentrations over the Southern Ocean areas that are seldom measured. Carbon monoxide affects the rate of tropospheric chemical removal of other pollutants that affect the concentration of tropospheric ozone, which is both a photochemical oxidant and a greenhouse gas.