Measurement of trace constituents

Trace atmospheric constituents exist in the atmosphere in one of three phases: gases, liquids (where minor constituents dissolve inwater droplets) and solids (as components of aerosol particles). These different forms have a wide range of physical and chemical properties, leading to a range of atmospheric lifetimes ranging from milliseconds to millennia.

The main aspects of trace-gas measurement that need to be considered are

1. (i) the sampling technique;

2. (ii) the physico-chemical basis of the measurement technique; and

3. (iii) the laboratory requirements for achieving the requisite accuracy and precision.

Sampling

Some of the main divisions in the type of sampling are the following.

Flask sampling. One of the simplest strategies is that in which air is collected in containers and returned to a central laboratory for analysis. This has the advantage of avoiding differences in the measurement calibration between different laboratories and the additional advantage of requiring only simple technology in the field. The disadvantages of flask sampling are the low frequency of data and, for many compounds, the possibility of changes in composition either during the trapping process or during the period in transit from the sampling site to the measurement laboratory. The primary requirement is for flasks with inert surfaces, generally glass or stainless steel. However, other flask characteristics such as the valves and their lubrication have also proved to be important for some compounds.