Stratospheric Ozone Depletion: a Discussion of Our Present Understanding

J. A. PYLE

1 Introduction

Ozone is an important stratospheric constituent. It absorbs solar radiation strongly at wavelengths around 300 nm, protecting the biosphere from harmful radiation. Ozone is also an important climate gas. The absorption of solar radiation heats the atmosphere and is responsible for the increase of temperature with altitude through the stratosphere. Ozone is also a greenhouse gas, absorbing and emitting in the infrared.

Depletion of ozone was first detected in the Antarctic stratosphere in the mid-1980s. That the depletion is global has since been determined using both satellite and ground-based ozone measurements. This anthropogenic depletion is expected to have important consequences, including leading to enhanced UV at the surface.

In this chapter in Section 2 we will first consider the background to the problem of ozone depletion. The role of ozone is briefly reviewed and the theory of stratospheric ozone depletion is introduced. Ozone loss was first detected in southern polar latitudes. Intensive observational studies then confirmed that the loss was also occurring in middle latitudes and the Arctic. In Section 3 these first observations of ozone depletion in both polar and middle latitudes are discussed, along with the consequent changes in theoretical understanding. More recent measurements of ozone loss up to the end of the 1990s are considered in Section 4. Section 5 speculates about the future state of the ozone layer into the 21st century.

2 Background

The Role of Ozone

Ozone is present in the atmosphere in trace amounts. In the troposphere below around 10 km the ozone mixing ratio is about 50 ppbv (parts per billion by volume). Mixing ratios are much higher in the stratosphere and reach a peak at around 10 ppmv (parts per million by volume) in the region known as the ozone layer. The highest concentrations occur in the low stratosphere, between about 15 and 30 km, depending on latitude.

A frequently used measure of ozone is its integrated column amount, the sum of the ozone concentration between the surface and the top of the atmosphere. Values typically range from a little over 200 m atm cm (or Dobson units, DU) in the tropics to greater than 400 DU in the high latitude spring. Figure 1 shows the latitude and seasonal distribution of column ozone. This variation with space and time has been broadly know for about 70 years, following the pioneering measurements of column ozone by Dobson and his co-workers.

The importance of ozone has been recognised for a long time. Ozone plays several important roles. It is toxic and high concentrations at the Earth's surface have implications for the health of both humans and plants. In the stratosphere, ozone absorbs solar ultraviolet radiation strongly, especially at wavelengths less than about 310 nm in the UV-A and UV-B parts of the spectrum. Ozone thus acts as a filter, preventing potentially harmful UV radiation from reaching the surface.

The penetration of radiation around 300 nm depends critically on the ozone column amount and the precise wavelength of the radiation (since the efficiency of absorption by ozone varies strongly with wavelength around 300 nm). Changes in the ozone column can have a significant impact in changing the penetration of UV to the surface, and for this reason any depletion of the stratosphere ozone column is a cause for concern.

The absorption of solar radiation by ozone also plays a very important role in determining atmospheric structure (ozone is an important climate gas). For example, the rise in temperature with altitude within the stratosphere is a result of the absorption of solar energy by ozone molecules. Absorption of infrared radiation by ozone is particularly important in the lower stratosphere, where changes in ozone are predicted to have a significant impact on surface temperature.

Theory of Stratospheric Ozone

Research in the early 1970s established a good description of the chemical processes responsible for the observed distribution of stratospheric ozone. Before that it had been thought that a sequence of reactions proposed by Chapman could explain the observations of ozone. In Chapman's theory, ozone is produced following the photolysis of molecular oxygen by solar UV radiation at wavelengths less than about 240 nm (equation 1). Two reactions (equations 2 and 3) rapidly interconvert O and O3 (so that these two species can be thought of together as 'odd oxygen'). Finally, ozone (or 'odd oxygen') is destroyed by the...