In previous chapters, many examples are presented in which emission estimates have been derived from flux measurements at specific sites and then extrapolated to larger areas by, for example, knowledge of the spatial extent of the conditions represented by the measurement site. This can be thought of as a "bottom up" approach. In the present and in the following chapter, "top-down" approaches will be discussed, in which measured atmospheric spatial and temporal distributions of a trace gas are used to determine the global budget chemical species and their emissions.
Over the past decades, measurements at a number of globally-distributed surface locations have been used to provide information on emissions for a range of atmospheric trace gases. This chapter describes several examples of global datasets of atmospheric observations, as well as a few examples of the use of these observations. A number of surface measurement networks are described in section 2 in terms of how the networks were developed, which trace compounds are measured and by which techniques. Section 3 relates to remote sensing and specifically discusses measurement of trace gases and aerosols by instruments on board satellites. Section 4 describes the methodology of using observations to analyze atmospheric budgets and determine emissions, with examples presented in section 5. This chapter does not extend to inverse modelling using complex chemical transport models (CTMs), which is covered in chapter 12, but shows how observations, often combined with models, can provide constraints on emissions estimates.