No2 o3

The rate constant for this reaction at 298 K is relatively small, = 3.2 x 10"17 cm3 molecule"1 s"1 (DeMore et al., 1997). However, at an 03 concentration of fOO ppb, still near or below the air quality standards or guidelines of most countries (see Table 2.7), the lifetime of N02 with respect to this reaction is only 3.5 h.

ft was not until the late 1970s that the importance of the nitrate radical was recognized when it was first reported by Noxon and co-workers (1978) in terms of its total column abundance, i.e., the concentration integrated through a column extending through the atmosphere from the earth's surface (see Chapter fl.A.4a). N03 was subsequently confirmed to be in the troposphere by Noxon et al. (1980) and by Piatt and coworkers (1980, 1984) in polluted atmospheres and rural continental air.

As discussed in detail in Chapter 6, N03 is now recognized to be a major contributor to the chemistry of organics in the troposphere at night. Because it absorbs in the red region of the spectrum (Chapter 4.G), it photolyzes rapidly during the day so that its chemistry other than photolysis is essentially restricted to the dark hours.

In addition to reacting with organics, N03 also reacts with NOz, forming dinitrogen pentoxide in a reversible, equilibrium reaction:

The forward reaction is a three-body reaction with values of kn = 2.2 x fO"30 cm6 molecule"2 s"1 and

cm- molecule"

at 300 K with

Fc = 0.6 (see Chapter 5.A.2 for discussion of this factor) recommended by DeMore et al. (1997). Atkinson et al. (1997b) recommend ku = 2.8 x 10"3° cm6 molecule"2 s"1 and k^ = 2.0 x fO"12 cm3 molecule"1 s"1 with Fc = 0.45. The recommended value of the equilibrium constant Ky y is 2.9 x fO"" cm3 molecule"' at 298 K, with an uncertainty of ±30% (DeMore et al., 1997). This equilibrium constant has been the subject of numerous experimental studies, which have yielded results that ranged over a factor of two at room temperature. For example, a study by Wangberg et al. (1997) subsequent to the NASA and tUPAC recommendations reports a value of 2.34 x 10"" cm3 molecule"1, about 20% smaller but within the relatively large uncertainty of the recommended value.

In any case, because of this equilibrium, sinks for N2Os such as hydrolysis (see later) are, in essence, also sinks for NO-, as well.

The N02-N03 reaction may also have a small contribution from a two-body channel:

However, if it occurs, it appears to be minor. Thus, based on a review of the relevant studies reported in the literature, DeMore et al. (1997) suggest that kw = 4.5 X io-'V1261'/'- = 6.6 X 10"16 cm3 molecule"1 s"1 at 298 K. This can be compared to an effective second-order rate constant for reaction (9) at f atm of 1.3 X 10"12 cm3 molecule-1 s_l. In short, the two-body reaction is more than three orders of magnitude slower than the termolecular process at 1 atm pressure.

3. Reactions of NO and NOz with Water and Alcohols a. Uptake Into and Reaction with Liquid Water

It is known from studies carried out over many decades that oxides of nitrogen at high concentrations dissolve in aqueous solution and react to form species such as nitrate and nitrite. With the focus on acid deposition and the chemistry leading to the formation of nitric and sulfuric acids during the 1970s and 1980s, a great deal of research was carried out on these reactions at much lower concentrations relevant to atmospheric conditions (for reviews, see Schwartz and White, 1981, 1983; and Schwartz, 1984).

Through these studies, it was concluded that absorption of NO and NOz into the aqueous phase in the form of clouds and fogs in the atmosphere and their subsequent oxidation are not significant under typical atmospheric conditions. The major reasons for this are that NO and N02 are not highly soluble and, in addition, the reactions are kinetically rather slow due to the dependence of the rates on the square of the reactant concentration. As a result, like the oxidation of NO by 02, the reactions slow down dramatically when the no, no2

Transport to surface

Gas no, no2

Transport to surface

Liquid

Transport of products

Transport across air-water interface

Hydration to NO(aq), N02(aq)

Transport into bulk phase Reaction in bulk phase

Liquid

Transport of products

Transport across air-water interface

Hydration to NO(aq), N02(aq)

Transport into bulk phase Reaction in bulk phase

FIGURE 7.4 Processes involved in uptake of NO and N02 into the aqueous phase in fogs and clouds and subsequent oxidation.

reactant concentrations are lowered to atmospheric levels. Let us take a brief look at these issues.

Figure 7.4 illustrates the processes that must be taken into account when gaseous oxides of nitrogen interact with liquid water. The gas must first be transported to the liquid surface. It must then be taken up into the liquid and then diffuse away from the interface. Oxidation may occur in the bulk solution or, as the evidence increasingly suggests, at the interface itself.

Table 7.1 shows the major reactions of interest in this system. As discussed in Chapter 5, the Henry's law constant for a species X, Hx, is in effect the equilibrium constant for the gas-solution equilibrium:

That is,

To put the values in Table 7.1 in perspective, highly soluble gases have Henry's law constants of the order of fO5 and relatively insoluble gases have values of

TABLE 7.1 Some Rate and Equilibrium Constants of Aqueous-Phase Reactions of NO and N02a

Rate or equilibrium Value of rate or equilibrium

Reaction expression constant

TABLE 7.1 Some Rate and Equilibrium Constants of Aqueous-Phase Reactions of NO and N02a

Rate or equilibrium Value of rate or equilibrium

Reaction expression constant

NO(s) « NO(aq)

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