and Bates, 1989) coupled with colorimetric methods or ion chromatography; permeation sampling (McDermott et al., 1979); diffusion scrubber (Lindgren and Dasgupta, 1989); diffusion denuder (Possanzini et al., 1983; Keuken et al., 1988; Vossler et al., 1988); thermodenuder systems (e.g., Slanina et al., 1987); and chemiluminescence (e.g., Zhang et al., 1985).

When SO2 is present at very low concentrations (< 0.1 ppb), a sample can be accumulated by cryo-trapping an airstream over a period of several minutes; the trapped SO2 in the sample is then separated by gas chromatography and detected by one of the methods named above. Commercial instrumentation is available, but the instruments often must be modified to improve their sensitivity in relatively clean air.

Oxides of Nitrogen

The oxides NO and NO2, collectively referred to as NOx, are important in the generation of nitric acid (HNO3) in the troposphere. HNO3 can react with various bases, such as NH3, to form light-scattering airborne particles, such as NH4NO3. NO is commonly analyzed through the chemiluminescence of excited NO2 molecules formed in the NO-O3 reaction (Ridley and Howlett, 1974; Ridley et al., 1988). Commercial instruments are available for NO, but they must be modified to measure NO at low levels encountered in relatively clean air (less than 0.1 ppb) (Dickerson et al., 1984).

The other common oxide of nitrogen, NO2, is also important in airborne particle formation. NO2 is a strong absorber of visible and ultra-violet light and can thereby contribute to haze. However, because of its high reactivity and relatively short lifetime, NO2 does not normally contribute significantly to haze in remote areas; it is a problem only in areas close to sources.

NO2 can be analyzed in air by several instrumental methods: photolysis and chemiluminescence (Kley and McFarland, 1980); tunable diode laser spectrometers (Reid et al., 1978a,b; Schiff et al., 1987); and chemiluminescence from the NO2 luminol reaction (Schiff et al., 1986, 1987). A field intercomparison of the three methods has shown their utility and their limitations (Fehsenfeld et al., 1990). In some commercial instruments, NO2 is estimated by first using NO-O3 chemiluminescence to measure NO in an ambient air sample; a sample of ambient air

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