During the winter of 1992-1993, scientists studied ozone and chlorine monoxide in the Arctic lower stratosphere. They compared these observations with data from other winters, looking at long-lasting tracers and how the polar vortex changed over time. In February 1993, they found that chlorine in the lower stratospheric vortex was mainly in forms that react with chemicals. This chemical reaction caused a reduction in ozone in the lower stratosphere during that winter.