This chapter is not as detailed as the previous ones. In part this is because most of the instruments used are largely of the industrial type, pressed into the service of environmental monitoring and, being very specialised, would require too much space to do them justice; nor is such a description really necessary. Also in the sections on total ozone and ozone profiles, the techniques have already been covered in Chapters 16 and 19 and it would be tedious to repeat it all. Readers should refer to these chapters for the detail of the methods.

The variables and their measurement

Earth's atmosphere is composed of 78% nitrogen, 21% oxygen and 0.9% argon. The remaining 0.1% is made up of carbon dioxide (0.036%), hydrogen, ozone, carbon monoxide, helium, neon, krypton and xenon in more or less fixed quantities, and water vapour in amounts that vary greatly. In addition, there are very small amounts of other, trace, gases measured in parts per million (ppmv, etc., where v indicates that the ratio is by volume) and parts per billion or trillion. It is these trace gases, and atmospheric particulate matter, that are the topic of this chapter, more specifically those that are generated by, or are affected by, human activities.