You bring lots of black particulate matter into the atmosphere which absorbs the solar energy that would normally reach the Earth's surface. John W. Birks, chemistry professor at the University of Colorado, Boulder, spent a sabbatical with Crutzen in Mainz on this study. The study concluded that far more people could die as a result of the climatic and other environmental consequences of a nuclear war than directly by the explosions. He was one of the first scientists to show that aerosols played a role in annual depletion of stratospheric ozone above Antarctica. His work was based on a mechanism he had proposed in the 1970s for the formation of sulfate aerosols in the stratosphere.
Chlorofluorocarbons are the source of most of the chlorine in the stratosphere.
But the main focus of Crutzen's work at the moment is on the chemistry of the marine boundary layer. Crutzen loves to discover new topics to work on. His research is curiosity driven. Most of my ideas I get by reading not only work in my own discipline but also very much in other disciplines such as biology, ecology, and agronomy. He says he spends almost all his time thinking either consciously or unconsciously about atmospheric chemistry.