The atmospheric environment is integral to the major chemical cycles of Earth's ecosystems, and plays an essential role in controlling the planet's climate. Furthermore, the maintenance of adequate air quality is indispensable to protect human health. In this chapter, the transport and chemical processes of the atmosphere are considered at the global scale, at which climate is controlled; at intermediate scales, at which detailed knowledge of both weather formation and the transport of pollutants is often essential; and at smaller scales that are often important in determining human exposure to chemicals in urban and indoor air. The chapter builds on concepts developed in Chapters 1-3 Chapter 1 Chapter 2 Chapter 3, highlighting similarities and pointing out important differences between the atmosphere and both the surface water and subsurface environments. The photochemical processes that often govern chemical fate in the atmosphere are described in detail, as are the interactions among Earth's natural chemical cycles, the release of anthropogenic gases, and ongoing global climate change driven by the enhanced greenhouse effect.