The stratosphere extends from the tropopause to a height of ~50km above the Earth's surface (see Fig. 3.1). The height of the tropopause increases from ~10km over the poles to ~17km over the equator, but it varies with the seasons and meteorological conditions. In passing across the tropopause, from the troposphere to the stratosphere, there is generally an abrupt change in concentrations of many of the trace constituents in the air. For example, water vapor decreases, and ozone (O3) concentrations often increase by an order of magnitude, within the first few kilometers above the tropopause. The strong vertical gradients across the tropopause reflect the fact that there is very little vertical mixing between the relatively moist, ozone-poor troposphere and the dry, ozone-rich stratosphere. Within the stratosphere the air is generally neutral or stable with respect to vertical motions. Also, the removal of aerosols and trace gases by precipitation, which is a powerful cleansing mechanism in the troposphere, is generally absent in the stratosphere. Consequently, materials that enter the stratosphere (e.g., volcanic effluents, anthropogenic chemicals that diffuse across the tropopause or are carried across the tropopause by strong updrafts in deep thunderstorms, and effluents from aircraft) can remain there for long periods of time, often as stratified layers.

In this chapter, three topics of particular interest in stratospheric chemistry are considered: unperturbed (i.e., natural) stratospheric O3, anthropogenic perturbations to stratospheric O3, and sulfur in the stratosphere. Our emphasis is on chemical processes. However, it should be kept in mind that in the atmosphere chemical, physical, and dynamical processes are often intimately entwined.