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FIGURE 6.18 Concentration-time profiles in an environmental chamber for (a) propene and (b) 03 as a function of increasing initial concentrations of PAN. Temperature ~30°C, relative humidity -60%, [NO] = [N02] = 0.26 ppm, [C3H,J = 0.5 ppm. ( a) No added PAN; (•) 0.06 ppm PAN added; (■) 0.13 ppm PAN added; (♦) 0.26 ppm added PAN (adapted from Carter et al., 1981a).

followed by the reaction of CH3 with 02, etc., to form HCHO and HOz, or HCHO, CH3OH, and CH3OOH under low-NOx conditions. As a result of this chemistry, it was suggested by Hendry and Kenley (1979) that PAN should act as an accelerator for photochemical smog formation, and indeed this is the case. Figure 6.18, for example, shows that in smog chamber studies, the loss of propene and the formation of 03 are accelerated by the addition of PAN (Carter et al., 1981a).

At lower temperatures such as those found in the Arctic, the majority of NOy is often present in the form of PAN because of its thermal stability. For example, 50-90% of NOy may exist in the form of PAN in the Arctic spring (Bottenheim et al., 1986; Barrie and Bot-tenheim, 1991; Jaffe, 1993). It is also interesting that, in relatively "clean," low-NOx regions at higher tempera-

PAN loss rate (s"1)

FIGURE 6.19 Calculated first-order loss rates of PAN due to thermal decomposition, OH reaction, and photolysis as a function of altitude (assuming diurnally averaged actinic fluxes for 30°N, July 4) (adapted from Talukdar et al, 1995).

tures, PANs may also tie up significant amounts of NOt (Singh et al., 1985; Madronich and Calvert, 1990).

The reaction of OH with PAN and its homologs appears to be sufficiently slow, k < 3 X 10"14 cm3 molecule"1 s" 1 at 298 K (Talukdar et al., 1995), that it cannot compete with thermal decomposition. For example, Figure 6.19 shows the calculated loss rates for PAN in the atmosphere as a function of altitude (Talukdar et al., 1995). Photolysis (see Chapter 4.J) only becomes important above 5 km, and the reaction with OH does not compete at any altitude.

For a detailed treatment of peroxyacyl nitrates, see the recent reviews by Gaffney et al., f989; Roberts, 1990; Altshuller, 1993; and Kleindienst, 1994.

2. Alkyl Nitrates and Nitrites a. Alkyl Nitrates

As discussed earlier in this chapter, alkyl nitrates (R0N02) are expected to be formed as minor products of the R02 + NO reaction, especially in the case of larger alkylperoxy radicals where as much as ~ 30-35% of the reaction may give stabilized alkyl nitrates rather than RO + N02. It is noteworthy that while NOx levels tend to be low in "clean" areas, modeling studies by Madronich and Calvert (1990) suggest that a significant fraction (~ 30-70%) of NOy may be tied up as various alkyl nitrates (including multifunctional nitrates) in these regions.

The absorption cross sections and photochemistry of alkyl nitrates are discussed in Chapter 4.J, where the

0 60 120 180 240 300 360 Time (min)

FIGURE 6.18 Concentration-time profiles in an environmental chamber for (a) propene and (b) 03 as a function of increasing initial concentrations of PAN. Temperature ~30°C, relative humidity -60%, [NO] = [N02] = 0.26 ppm, [C3H,J = 0.5 ppm. ( a) No added PAN; (•) 0.06 ppm PAN added; (■) 0.13 ppm PAN added; (♦) 0.26 ppm added PAN (adapted from Carter et al., 1981a).

TABLE 6.22 Room Temperature Rate Constants for the Reactions of OH with Some Simple Alkyl Nitrates at 298 Ka

Alkyl nitrate

k2wK cm3 molecule"

ch3ono2 c2h5ono2

n-c3h70n02 /-C,n7ONO, ch3ch(0n02)ch2ch3 ch3ch(0n02)ch2ch2ch3

ch3ch2ch(ono2)ch2ch3

0.35, 0.30,6 0.24r 4.9, 2.0/', 1.8' 7.3 4.9, 2.9'' 9.2 18.5 11.2 38.8

" From Atkinson et al. (1997a) and Atkinson (1994). b Kakesu et al. (1997), from 304 to 310 K for ch30n02 from 298 to 310 Kfor c2h50n02. ' Talukdar et al. (1997).

and photolysis appears to occur via breaking the 0-N02 bond. However, the absorption cross sections are not large, and hence the lifetimes with respect to photolysis are quite long, of the order of a week or more at the earth's surface and several days at higher altitudes where the solar flux is larger (e.g., see Clemitshaw et al., 1997).

Reaction with OH is, however, reasonably rapid as might be expected and is of the same order of magnitude as the OH-alkane reactions. Table 6.22, for example, shows the room temperature rate constants for the reactions of OH at 298 K with some alkyl nitrates. With 2-butyl nitrate as an example, the lifetime with respect to the OH reaction with OH at f X 106 radicals cm"3 is about 13 days, comparable to the photolysis rate. As with the alkanes, abstraction of a hydrogen atom occurs to form an alkyl radical, whose fate is the same as discussed in Section 6.D earlier.

b. Alkyl Nitrites

Alkyl nitrites (RONO) absorb light strongly in the actinic region, dissociating to form RO + NO. Because of this rapid photolysis, other reactions such as that with OH cannot compete, and alkyl nitrites have not been generally observed in the troposphere at significant concentrations.

3. Amines, Nitrosamines, and Hydrazines a. Amines

Aliphatic amines are emitted from a variety of sources, including feedlots, sewage treatment, waste incineration, and industrial activities (e.g., Schade and Crutzen, 1995). They have also been measured in nonurban areas (e.g., see Van Neste et al., 1987; Gorzelska and Galloway, 1990; and Eisele and Tanner, 1990). Because they do not absorb light in the actinic region (Calvert and Pitts, 1966), they are not removed by photolysis. Hence reactions with atmospheric oxidants such as OH and 03 are the major removal processes for these organic nitrogen compounds. Reaction with HN03 may also occur in polluted urban areas.

Table 6.23 gives the rate constants for the reactions of OH and 03 with some simple alkyl amines at room

TABLE 6.23 Room Temperature Rate Constants and Estimated Atmospheric Lifetimes for the Gas-Phase Reactions of Some Alkyl Amines and Amides with OHu and 03

Amine or amide molecule-' s~') (h) molecule~' s ~') Trf

Amine or amide molecule-' s~') (h) molecule~' s ~') Trf

ch,nh2

2.2," 1.7''

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