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FIGURE 11.15 Measurement of OH using DOAS as a function of time (UT) after subtraction of the contributions of other known absorbers. The solid lines through the data are OH reference spectra (adapted from Dorn et al., 1996).

FIGURE 11.16 DOAS spectrum taken at Alert, N.W.T., on April 20, 1992, overlaid by fitted BrO reference spectrum (—) (adapted from Piatt and Hausmann, 1994).

FIGURE 11.16 DOAS spectrum taken at Alert, N.W.T., on April 20, 1992, overlaid by fitted BrO reference spectrum (—) (adapted from Piatt and Hausmann, 1994).

2. Mass Spectrometry

Mass spectrometry has the potential for being a very powerful analytical technique for atmospheric measurements, and indeed, it has been used for a number of decades in upper atmosphere measurements of ions and neutrals. Viggiano (1993) has reviewed ion chemistry and the application of mass spectrometry to tropo-spheric and stratospheric measurements through 1993. The first mass spectrometric measurements were made in the upper atmosphere from 64 to 112 km in 1963 (Narcisi and Bailey, 1965), followed by stratospheric measurements in 1977 (Arnold et al., 1977) and, finally, tropospheric measurements in 1983 (Eisele, 1983; Heitmann and Arnold, 1983). They have also been extended to measurements in jet aircraft exhaust (e.g., Arnold et al., 1998).

Table 11.4 summarizes measurements of various species in the stratosphere and troposphere by mass spectrometry through the early 1990s (Viggiano, 1993, and references therein). The altitude at which they were measured and the concentration ranges are shown, as well as whether they were detected using positive or negative ions (see later discussion).

Mass spectrometric measurements require four components: (f) an inlet to introduce the sample; (2) a means of ionizing the species of interest; (3) mass filtering/separation; (4) detection of the ions. Accomplishing this under atmospheric conditions is difficult due to the high sample pressure, which is incompatible with the high voltages used in the ion acceleration region and mass analyzers, and to the complexity of the mixtures found in air. Special considerations imposed by atmospheric conditions are discussed briefly next.

a. Sample Introduction

Because mass separation techniques use high voltages and hence require high vacuum, a means of transmitting the sample from the relatively high pressures found in the atmosphere into the low pressures in the analyzer is required. This typically involves differential pumping stages or the use of pulsed nozzles, which results in lowering of the sample pressure but also a proportionate loss of sensitivity.

TABLE 11.4 Some Species Measured by Mass Spectrometry in the Atmosphere up to about 1990"

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