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FIGURE 11.3 Typical FT-IR spectra in ambient air as a function of time in (a) the HN03 region and (b) the NH3 region on September 14 and 16, 1985, respectively, in Claremont, California. NH3 peak is marked by the arrow. Concentrations of each are shown on the right-hand side (adapted from Biermann et al., 1988).

Figure 11.3 shows typical ambient air spectra in two regions in which HN03 (Fig. 11.3a) and NH3 (Fig. 11.3b), respectively, have characteristic absorption bands (Biermann et al., 1988). Figure 11.4 shows, for comparison, some typical reference spectra for HN03 and NH3 taken at much higher concentrations in a 25-cm-long cell (see Problem 6). It can be seen that the absorption bands in air even in a polluted urban area are relatively weak. However, FTIR has also proven particularly useful as a standard for intercomparison studies in polluted urban atmospheres (e.g., see Hering et al., 1988).

Table If.2 summarizes the detection limits for FTIR measurements in the atmosphere for some gases of interest. Typical concentrations of each in remote to polluted atmospheres are discussed below with respect to the individual species; however, in general, it can be stated that FTIR is most suitable for measuring

Wavenumbers (cm"1)

FIGURE 11.4 Reference spectra of gaseous HNO, and NH3, respectively, at L = 25 cm and Ptot = 740 Torr in N2. Asterisks denote peaks used in analysis of ambient air (adapted from Biermann et al., 1988).

Wavenumbers (cm"1)

FIGURE 11.4 Reference spectra of gaseous HNO, and NH3, respectively, at L = 25 cm and Ptot = 740 Torr in N2. Asterisks denote peaks used in analysis of ambient air (adapted from Biermann et al., 1988).

atmospheric trace gases in polluted urban areas or close to sources where they are found at the highest concentrations.

For example, Yokelson et al. (1996, 1997a, 1997b) have used FTIR to measure species emitted from combustion processes; this has permitted the simultaneous measurement of such species as HCHO, CH3OH,

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