ABSTRACT
Air pollution concentrations vary greatly from place to place at any one time, and with time of day and from year to year at any one place. Before we can understand where and when the concentrations of hazardous air pollutants rise to unacceptable levels, we need to have an extended period of data available from a network of pollution monitors having appropriate spatial distribution and response time. In this chapter we will look at measurement networks in the UK and elsewhere that have given us our current understanding of the occurrence of air pollutants, and present some results that demonstrate key aspects of the conclusions so far. We will also look in some detail at the factors that determine both the short and long-term airborne concentrations and the rates of dry and wet deposition of gaseous pollutants. We will relate much of the discussion of gases to sulphur and nitrogen oxides and to ozone, since they have been the subjects of the longest-established and most systematic measurement programmes.
