B o3

Detection techniques. Detection techniques for surface-based measurements of ozone include (f) UV absorption at 254 nm; (2) chemiluminescence on reaction with NO (or ethene); (3) DOAS; (4) TDLS; and (5) wet chemical methods, mainly those involving the oxidation of r to 12 and measurement of the I2 colori-metrically or coulometrically. The wet chemical method and the principles behind DOAS and TDLS were discussed earlier and are not treated further here.

UV absorption relies on the strong peak absorption of 03 at 254 nm (see Chapter 4.B), which coinciden-tally overlaps the strong 254-nm emission from low-pressure mercury lamps. Commercial instruments based on this absorption are in widespread use. Typically, air is drawn into a cell and the absorption of the 254-nm line from a Hg lamp is measured. The airstream is then switched to pass through a catalyst that destroys 03 and the absorption is again measured to provide I{).

Alternatively, two parallel cells are used, one of which has air from which 03 has been scrubbed and the other the air containing ozone. These instruments appear to be relatively artifact-free, although interference from high concentrations of photooxidation products in laboratory studies and from contamination of the cell windows has been reported (Meyer et al., 1991; Kleindienst et al., 1993).

The principles behind the chemiluminescence methods using NO and ethene were discussed earlier. The instruments using NO also are widely used, and results obtained are in good agreement with those using UV absorption at 254 nm. However, there appears to be a water interference in the NO chemiluminescence method that gives a positive artifact of ~3.7% per percent water (e.g., see Kleindienst et al., 1993). This could be significant under some conditions. For example, Kleindienst et al. (1993) estimate that an error of 13 ppb 03 could result at high relative humidities and temperatures (30°C and 60% RH).

Another approach is to obtain tropospheric ozone levels using satellite data as described in Chapter 13.C (e.g., see Fishman et al., 1990; and Munro et al., 1998). Vertical tropospheric ozone profiles can be extracted using satellite measurements of backscattered solar radiation at wavelengths where ozone has strong absorption bands.

Finally, passive samplers have also been developed for ozone, primarily for use in epidemiological studies. For example, Brauer and Brook (1995) describe the application of a passive sampler in which air containing ozone diffuses through a Teflon membrane and reacts with nitrite. The sampler is then extracted and the nitrate product measured using ion chromatography.

Typical ambient levels. As discussed in Chapter f 4.B.2d, levels of ozone worldwide before the industrial revolution appear to have been ~ 10-15 ppb. However, at the present time, levels of 30-40 ppb are found in even the most remote regions. This increase has been attributed to increased anthropogenic emissions of oxides of nitrogen, since photolysis of N02 is the sole known significant source of anthropogenically derived ozone.

Peak levels in rural-suburban areas are typically in the 80- to 150-ppb range, reaching as high as 500 ppb or more in the most highly polluted urban areas that have few controls on emission sources.

Detection techniques. Detection techniques commonly used for CO include two infrared techniques, TDLS and NDIR (also known as gas filter correlation, GFC), and gas chromatography with various detectors. The principles behind TDLS and NDIR have been discussed earlier. Sachse et al. (1987), for example, applied TDLS to measure CO using the P(5) line at 4.7 fim (2128 cm-1). Measurements could be made in 1 s with an accuracy of ±1.4 ppb.

The application of a commercial NDIR instrument to ambient CO measurements is described by Parrish et al. (1994); precision (lcr) of ~2 ppb with 1-h averaging times could be obtained. A similar detection principle has been used to measure middle-tropospheric CO from the space shuttle (Reichle et al., 1990).

Finally, gas chromatography can be used to separate CO from the other constituents in air. Various detection methods have been used, including conversion of CO to CH4 and measurement of CH4 by flame ionization detection (e.g., Porter and Volman, 1962). A unique method is also used for CO in which it reacts with hot HgO, releasing Hg vapor, which is measured by atomic absorption of light from a mercury lamp, known as the GC-HgO method (e.g., see Greenberg et al., 1996). Intercomparisons of chromatographic measurements and TDLS have shown that the two approaches are in good agreement (e.g., see Hoell et al., 1985, 1987b). Intercomparisons of the GC-HgO methods with NDIR have also been carried out using a round-robin approach on prepared CO standards (Novelli et al., 1998a); while agreement was good in many cases, the uncertainties associated with the NDIR method were larger by a factor of about 5 at low CO levels, ~50 ppb. Differences in the accuracy between laboratories were also noted, even among those using the same method. These were traced in some cases to inaccurate calibrations, but other factors such as nonlinearity in the GC-HgO detectors over the full range of atmospheric concentrations were also suggested as possible contributing factors.

Typical ambient levels. Typical levels of CO range from ~ 50-150 ppb in remote areas (e.g., see Parrish et al., f 991, 1994; Novelli et al., 1992, 1998a, 1998b; and Derwent et al., 1998) to ~1000 ppb in rural-suburban areas up to ~10 ppm in very polluted areas such as Mexico City (e.g., Riveros et al., 1998). It is interesting that the values that appear to be representative of clean, remote areas are about the same as those found using ice cores for the preindustrial era; for example, Haan et al. (1996) report preindustrial values of about 92 ppb for a Greenland ice core and ~ 55-60 ppb for an Antarctic ice core.

Figure 11.36, for example, shows the zonally averaged CO concentrations in the Northern and Southern Hemispheres, respectively, from f990 to 1995 (Novelli et al., 1998b). Concentrations are higher in the Northern than in the Southern Hemisphere, but both show a decreasing trend with time, -2.6 ± 0.3 ppb yr-1 in the Northern Hemisphere and —1.9 ± 0.1 ppb yr-1 in the

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