The atmosphere is a chemically complex and dynamic system that interacts significantly with the land, oceans, and ecosystems. Most trace gases emitted into the atmosphere are removed by oxidizing chemical reactions involving ozone and the hydroxyl free radical. The rate of this self-cleansing process is often referred to as the oxidation capacity of the atmosphere. Without this process, atmospheric composition and climate would be very different from what we observe today. The fundamental chemistry involved and the influence of human activity on oxidation capacity are reviewed. Both the current measurements designed to determine rates of oxidation and evidence for changes in oxidizing capacity over recent decades are critically discussed.

© 2003 Annual Reviews