M

HONO,

Thus the steady-state concentration of OH is given by

where &ll3 is the effective second-order rate constant. However, the OH concentration is also determined by its interconversion with H02 and CH302. Treating the OH + CO and HOz + NO as one cycle and OH + CH4 and CH302 + NO as another,

*103[OH][CO] =*110[HO2][NO], (I) *104[OH][CH4] = /cm[CH302][N0]

since the rate constants for reactions (Iff) and (lfO) are similar. Adding (I) and (J), one obtains

┬┐noLNOJ

and using Eq. (H) for [OH], this becomes: {[H02] + [CH302]}

That is, in the presence of sufficient quantities of NO that HOz and CH302 react primarily with NO rather than with each other, the total concentration of peroxy radicals should vary directly with the photolysis rate constant for 03 rather than with its square root as was the case at low NO.

Figure 6.33 shows a plot of the total peroxy radical concentrations measured at Mace Head, Ireland, as a function of either the square root or the first power of /(O'D) (Carpenter et al., 1997). Consistent with Eq. (L), the concentrations vary with the first power under these conditions.

From such studies, Carpenter et al. (1997) conclude that the crossover point between 03 destruction and formation occurs at NO concentrations of ~55 + 30

8 12 16 Time (hours)

FIGURE 6.34 Measured diurnal variation of NO, and H02 + R02, respectively, at Weybourne, U.K. (adapted from Carslaw et al., 1997).

8 12 16 Time (hours)

FIGURE 6.34 Measured diurnal variation of NO, and H02 + R02, respectively, at Weybourne, U.K. (adapted from Carslaw et al., 1997).

ppt at Mace Head, Ireland, in the late spring and 23 + 20 ppt at Cape Grim, Tasmania, during the summer.

As discussed earlier in this chapter, N03 drives nighttime chemistry. Through its reactions with organ-ics, it would be expected to generate HOz and ROz at night and hence N03 and peroxy radical concentrations should be related. Figure 6.34 shows one set of measurements of these species in a coastal marine boundary layer at Weybourne, U.K. (Carslaw et al., 1997). The temporal profile of peroxy radicals at night follows that of N03; during the day, there are additional sources, of course, through OH and 03 reactions. Carslaw et al. (1997) suggest that the reactions of HOz, and perhaps CH3SCH202 (from DMS oxidation), with N03 at night may also be important.

3. Upper Troposphere

While a great deal is known about the chemistry of the lower troposphere, particularly the boundary layer, as well as the stratosphere (see Chapters 12 and 13), much less is known about the upper troposphere, the region between the two. This region has attracted increasing attention for a number of reasons, including the potential impact of commercial aircraft. Of particular concern is understanding the formation and fate of 03, whose concentration in this region is important for its role as a greenhouse gas (see Chapter 14.B), in addition to its role in the photochemical reactions.

Measurements made in this region have raised questions regarding our understanding of the chemistry involved, as well as the transport processes that can affect ozone in this region (e.g., Suhre et al., 1991). Figure 6.35, for example, shows one set of measurements of OH as a function of solar zenith angle at an altitude of 11.8 km near Hawaii (Wennberg et al., 1998). Also shown are model predictions based on the m

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