It was little more than a decade ago that the first attempts were made to compile comprehensive budgets for the trace constituents of the atmosphere. The realization that material flows through the atmosphere—that atmospheric gases have sources and sinks—was still strange, exciting, and new. The budgets were grossly in error, of course, because the photochemical activity of the troposphere, associated with the hydroxyl radical, was not appreciated. The biogeochemical cycle has become a household name in the world of science in a very short period of time.