Compounds

Oxides of nitrogen play a central role in essentially all facets of atmospheric chemistry. As we have seen, N02 is key to the formation of tropospheric ozone, contributing to acid deposition (some are toxic to humans and plants), and forming other atmospheric oxidants such as the nitrate radical. In addition, in the stratosphere their chemistry and that of halogens interact closely to control the chain length of ozone-destroying reactions.

As discussed in Chapter 2.A.1, most of the primary emissions of NOx (= NO + N02) are in the form of nitric oxide, NO. The overall oxidation sequence is conversion of NO to N02, which is ultimately converted to HN03 and other oxidized forms such as PAN. Even in the case of PAN, the end product is ultimately HN03 since PAN can decompose back to N02 (see Chapter 6.1). While there has been speculation that there are processes that can convert HN03 back into reactive forms, which could be important in both the troposphere and stratosphere (e.g., see Chat-field, 1994; Hauglustaine et al., 1996; and Lary et al., 1997), none have been confirmed to date to occur in the atmosphere.

Figure 7.1, for example, shows the concentrations of the major nitrogen-containing products as a function of reaction time during a typical smog chamber experiment. Curve I is the concentration of NO + N02 expected if no reaction occurred and the concentrations decreased only due to dilution during the experiment. Curve III is the sum of the measured gas-phase concentrations of (NO + N02 + PAN + HNO3). The difference between the measured concentrations and those expected from the concentrations of the initial reactants increases significantly during the run. However, in separate studies, the rate of loss of HN03 to the chamber walls was determined. Using this rate, Spicer (1983) calculated the amount of HN03 expected to be adsorbed on the chamber walls; this is shown by the shaded area between curves II and III. When this adsorbed HNO3 is taken into account, about 90% of the nitrogen can be accounted for at the end of the run.

The ratio of PAN to HN03 in such experiments depends on a number of factors, especially the initial VOC/NOx ratio. Figure 7.2 shows the ratio of the final concentrations of PAN to HN03 (sum of gaseous and adsorbed) as a function of the initial VOC/NOx ratio. The increase in PAN relative to HN03 is due to increasing concentrations of the CH3C(0)00 radicals that form PAN as the VOC concentrations increase. This makes the CH3C(0)00 + N02 reaction more competitive with the OH + N02 reaction, the major HNO3 source in this system. At VOC/NOx ratios of ~5-10 typical of urban areas, smog chamber experiments suggest that HN03 should exceed PAN by factors of ~2-5. However, the actual ratios depend on the particular conditions, e.g., chemical composition, temperature, and light. As discussed in Chapter 6, PAN can under some conditions (e.g., the Arctic) constitute the major portion of NOr (Recall NO is defined as NOx + HNO, + 2N205 + N03 + organic nitrates + particulate nitrate + ••• .)

Nitric acid undergoes both wet and dry deposition rapidly and can be neutralized by ammonia, the major gaseous base found in the atmosphere. As discussed in Section E.2, the neutralization reaction is an equilibrium reaction so that by itself, this does not result in permanent removal from the atmosphere. However, as seen in this chapter and in Chapter 9, this acid-base reaction has some important implications for visibility in the atmosphere and for the nitrate concentrations found in respirable particles.

While nitric acid is one of the major contributors to acid deposition (more colloquially "acid rain"), we treat its chemistry separately from that of sulfuric and organic acids discussed in the following chapter. The reason for treating it first is that the chemistry of

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