The study looked at how ozone in the air changed from 1979 to 2010. They used a special model to see how ozone levels were affected by different factors. They found that the decrease in a type of chemical called chlorofluorocarbons in the air led to less ozone in the upper atmosphere. This caused a decrease in ozone coming down to the lower atmosphere from 1979 to 1994. But from 1994 to 2010, even though emissions stayed the same, more ozone from the upper atmosphere came down, leading to a small increase in lower atmosphere ozone. This could affect air quality and how much heat is trapped in the atmosphere in the future.