Other Peroxynitrates, and Alkyl Nitrates

The formation and fate of peroxyacyl nitrates, RC(0)00N02, were discussed in Chapter 6.1. These compounds are almost universally measured using gas chromatography with electron capture detection (GC-ECD), although a luminol chemiluminescence detector has also been used in which PAN is thermally decomposed to N02 at the end of the column and the N02 measured (Burkhardt et al., 1988 Blanchard et al., 1990 Gaffney et al., 1998). In polluted atmospheres where the concentrations are higher, FTIR has also been used (Table 11.2). For a summary of methods, see reviews by Gaffney et al. (1989) and Kleindienst (1994). 

FIGURE 11.41 Diurnal variation of median concentrations of PAN and Os at a monitoring site in Athens, Greece, during periods of high photochemical activity (adapted from Suppan et al., f998). 

Simple alkyl nitrates are also commonly measured using GC-ECD, usually with preconcentration either by cryotrapping or using a solid sorbent (e.g., Atlas and Schauffler, 1991 Ridley et al., 1997). Another approach is GC with an NO detector as described earlier (e.g., Flocke et al., 1998). In this approach, the compounds are converted to NO over a catalyst as they emerge from the GC column, and the NO measured by its chemiluminescence reaction with 03. 

A number of alkyl nitrates have been observed in the troposphere, including methyl nitrate and ethyl nitrate, as well as all of the isomers of the higher alkyl nitrates up to C5 (e.g., see Buhr et al., 1990 Ridley et al., 1990a O Brien et al., 1995 and Flocke et al., 1998). Although the specific isomers were not identified, the Cf)-C8 alkyl nitrates have also been measured (O Brien et al., 1995 Flocke et al., 1998). A summary of the measurements through about 1998 is found in Flocke et al. (1998). 

Of the simple alkyl nitrates, methyl nitrate is present in the highest concentration. For example, in measurements made in Schauinsland, a rural area in Germany, concentrations of CH30N02 up to 216 ppt were measured. The median value, however, was only 19 ppt (Flocke et al., 1998). In the same studies, the median concentrations for ethyl nitrate, n-propyl nitrate, 2-pro-pyl nitrate, and 1-butyl nitrate were 9, 3, 12, and 2 ppt, respectively. The sum of the C,-C8 alkyl nitrates averaged 120 ppt, which is only 3% of the NO. Similarly, in rural Ontario, Canada, 17 different organic nitrates were identified in air, but their sum was only 0.5-3% of NOj. (O Brien et al., 1995). In aircraft measurements over the Pacific Ocean near Hawaii, average values for methyl nitrate near the surface were 6 ppt and the sum of C,-C5 alkyl nitrates was 5% of the total NO, (Ridley et al., 1997). 

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