Methods for, and Tropospheric Levels of, Specific Gases

As we have seen in earlier chapters, NO is the major form of nitrogen oxides emitted from combustion processes, but in the atmosphere it is oxidized to NOz and other oxides of nitrogen. The term NO, is used for the sum of (NO + N02). The term NOy denotes the sum of NO, N02 (i.e., NO.), plus all other oxides of nitrogen where the nitrogen is in an oxidation state of +2 or greater  

The term NO, is also occasionally used in the literature. In these cases, it is defined by 

Operationally, NOy is defined by the measurement method used to measure it, as discussed in more detail in Section A.4.a(2). Since NO, NOy, and NO, are commonly measured simultaneously using variants of the same techniques, these are discussed together in the following sections, and in that order, for reasons that will become apparent. 

Intercomparison studies of these two measurement methods for NO generally show good agreement for levels of 25 ppt and greater (e.g., Hoell et al., 1987a Gregory et al., 1990 Crosley, 1996). For example, Fig. 11.24 shows the results of one aircraft study in which the chemiluminescence method and the TP-LIF method were compared (Crosley, 1996). The slope of the plot in Fig. 11.24 was 0.94, with an intercept of —0.1 + 0.8 ppt and r2 = 0.90. For the data 25 ppt, although the slope was 0.989, the correlation was poorer, r2 = 0.66. 

A technique that has been used in laboratory studies for oxides of nitrogen and shows promise for field measurements is resonance-enhanced multiphoton ionization (REMPI) (Guizard et al., 1989 Lemire et al., 1993 Simeonsson et al., 1994). For example, Akimoto and co-workers (Lee et al., 1997) have reported a REMPI system in which a (1 + 1) two-photon absorption of light at 226 nm by NO results in ionization (vide supra). They report a detection limit of 16 ppt in their laboratory studies. Other oxides of nitrogen such as NOz and HN03 can also photodissociate in the 

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