This is a Presidential Young Investigator project for atmospheric chemistry research. A major focus of this research is expected to be the development of a three.dimensional model to simulate the global distributions of chemically reactive species in the troposphere. The long.term goal is to apply the model to such complex environmental problems as increases in greenhouse gases and acid rain. In the coming years it is expected that investigation on the changes in the composition of the atmosphere associated with changes in climate will be pursued. In particular, a general circulation model will be used to study the atmospheric chemistry during the Ice Ages. Data available form cores in Antarctica, suggest very different atmospheric transport patterns during the Ice Ages than today. These transport patterns might provide a clue as to the causes of sudden cooling. Understanding the mechanisms of climate change during the Ice Ages would have important implications on our ability to comprehend current changes in climate due to the greenhouse effect.