His research addresses the chemistry and dynamics of Earths atmosphere, with particular emphasis on observations of trace gases and aerosols and their impact on stratospheric ozone, the oxidative capacity of the atmosphere, and radiation balance.  His group studies stratospheric ozone depletion over the Arctic, the impact of rockets on stratospheric chemistry, long-range transport of pollutants, and the role of aerosols in modification of cloud properties. He has participated in over forty field campaigns, most involving research aircraft and scientific balloons. They have conducted work in Antarctica, Spitsbergen, New Zealand, Sweden, Nepal, the Virgin Islands, Alaska, Hawaii, and throughout the continental United States.  He develops instruments for fast-response in situ' measurements from the ground, balloons, and aircraft. His work helped demonstrate the link between chlorine-containing and bromine-containing compounds on the destruction of ozone over the Arctic and Antarctic.