Sampling of Stratospheric Trace Constituents

Abstract

The measurement of stable atmospheric trace constituents is often separated into two steps: (a) the collection of a representative sample and (b) the analysis of the trace constituents in the laboratory. The main advantage of this approach is that the most sensitive detection methods can be used. The main disadvantage is that only a relatively limited number of data points can be taken along one sampling profile.Basically two sampling techniques are used: (a) the collection of whole air samples or (b) the selective collection of one or more trace gases on a suitable trapping device. Whole air samples are taken either in the form of grab samples or by compression of air into pressure tanks or by condensing air in cryogenically cooled vessels. Stratospheric trace gases which have been measured by these methods include: CO2, CH4, CO, H2O, H2, N2O, and the noble gases. Selective sampling is accomplished by passing air through molecular sieve beds, impregnated filters, or liquid nitrogen cooled traps. Trace gases retained and measured this way include CO2, H2O, N2O, HNO3.Whole air samplers have been flown on high-flying aircraft, balloons, and rockets. The maximum altitude to which this technique can be usefully extended is 80 km. Selective trace gas samplers have been operated aboard aircraft and on balloons up to an altitude of about 35 km. Examples of the resulting trace gas profiles will be reported.