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BOULDER--Scientists at the National Center for Atmospheric Research (NCAR) and colleagues at universities and NASA have clarified the process by which ozone--an essential shield in the stratosphere, but a pollutant at lower levels--reaches its peak abundance across North America each spring. The new findings come from a comprehensive study that links computer models with airborne measurements of gases, particles, and ultraviolet radiation. A set of papers outlining results from the Tropospheric Ozone Production about the Spring Equinox (TOPSE) experiment appears in the February 28 issue of the Journal of Geophysical Research-Atmospheres (JGR). The principal investigators are NCAR scientists Elliot Atlas, Christopher Cantrell, and Brian Ridley. In addition to its important role in chemical reactions that determine the "cleansing capacity" of the atmosphere, tropospheric ozone "is known to have detrimental effects on human health and agricultural crop production," note the authors. Recent evidence also points to a significant relationship between tropospheric ozone chemistry and toxic trace elements, such as mercury. In the lower to middle troposphere, about one to five miles above the United States and Canada, ozone levels peak as springtime arrives. "Understanding the sources of this ozone and the processes that produce and destroy it will help us determine how human-produced emissions affect air quality on a global scale," says Atlas. It's been unclear whether the ozone peak develops due to seasonal intrusions of ozone-rich air from the stratosphere above or whether it forms in place through photochemical effects of the intensifying spring sun. The answer, the TOPSE team found, is a little of both, though photochemical effects dominate in the winter-to-spring ozone increase. From February to May 2000, scientists from NCAR and other institutions took to the skies above North America for the TOPSE field campaign. Seven round-trip flights aboard the National Science Foundation/NCAR C-130 aircraft took scientists and instruments from Broomfield, Colorado, to northernmost Canada (up to latitude 87 degree N) and back. The team then analyzed the results and compared them to the output of two computer models that simulate air chemistry and winds over the Northern Hemisphere. Together, the data and model results paint a picture that answers some key questions about springtime ozone and air chemistry above North America. For example, the flight data strongly confirmed that the amount of ozone descending from the stratosphere was too small to account for the springtime peak. By tracing chemical reactions and following stratospheric "markers" through their models, the scientists found that "stratospheric sources could only account for a small fraction of the observed ozone [during the spring increase], but stratospheric ozone is an important contributor to the observed background levels." Thus, "the seasonal ozone trend was primarily driven by in situ [in-place] ozone production." By late spring, up to five times more ozone was found to be produced locally than delivered from aloft. TOPSE also addressed a quite different puzzle: how ozone can disappear so quickly in wintertime from surface air across the Arctic Ocean and adjacent land areas. As reported in previous studies, Arctic surface ozone depletion appears to be due to natural halogen compounds, such as bromine and chlorine, that react with ozone and the Arctic snowpack as the spring sun arrives. This surface ozone depletion in the north is unrelated to the better-known ozone "hole" in the Antarctic stratosphere, which also forms in the spring. That southern ozone thinning involves a different set of reactions with chlorine derived from industrial chemicals, including chlorofluorocarbons. During TOPSE, "A virtual ozone hole was observed for the first time over much of Hudson Bay and over the Arctic Ocean," write the authors. Low-level winds, they note, "can distribute ozone-depleted air over a larger region beyond the Arctic than had been previously recognized." TOPSE was able to map episodes of surface ozone depletion over much of the Arctic Ocean, northern Canada, and Greenland. The Arctic Ocean appears to be the origin of these depletions, but winds can move these chemically processed air masses to more southerly latitudes. Even at its peak levels, Northern Hemisphere ozone is far less prevalent in the lower to middle troposphere than in the higher stratosphere. This means that the seasonal waxing and waning at lower altitudes studied in TOPSE should have little effect on the ultraviolet light that reaches people, animals, and plants. Still needed, according to TOPSE scientists, are more-extensive measurements of the halogens that drive ground-level Arctic ozone depletion, as well as a better understanding of the atmospheric exchange between stratosphere and troposphere--a process the scientists note is "far from understood." The National Science Foundation provided major funding for TOPSE. On the Web: http://topse.acd.ucar.edu