Box 4A The structure of the atmosphere

The Earth's atmosphere consists of several layers (see Figure 4.1) extending from the Earth's surface to outer space. However, the part of the atmosphere that affects weather and climate, and in which most air pollutants are emitted and dispersed, is just a thin layer, from the surface up to a height of 10 to 15 km, called the troposphere. (If the Earth were the size of an apple this layer would be less than the thickness of its skin.) Above the troposphere lies the stratosphere, between 15 and 50 km above the Earth's surface. The stratosphere is rather isolated from the weather in the troposphere because vertical transfers between these two layers are very slow. The stratosphere holds the ozone layer (at a height of 20 to 40 km), which protects the Earth from the sun's ultraviolet radiation, an essential feature of the Earth's atmosphere for the continuing sustenance of life.

The so-called 'mixing layer' of the atmosphere is generally turbulent, varying in depth with meteorological conditions, ranging from a few hundred metres (the height of the Eiffel tower) to about 2 km. Pollutants released into this layer are thus effectively dispersed into a larger volume of air within a few hours, including some transfer to higher levels of the troposphere. If turbulence in the mixing layer is impeded, pollutants accumulate in an even smaller volume, giving rise to high concentrations ('smog'). This occurs when wind speeds are low and when the Earth's surface temperature is lower than the air above (a temperature inversion). Contrary to the troposphere, where on average the temperature decreases with height, the inversion layer (typically 100 to 300 m in depth) is characterised by a temporary increase of temperature with height, with weak wind and slow mixing, and practically no exchange with the overlying background troposphere.