A

I HO2+O

J N02+0 t Br0+H02

f ClO+BiO^BrCl

240 260

Temperature, K

FIGURE 5.31 Rate coefficients of rate-determining reactions in 03 loss catalytic cycles.

240 260

Temperature, K

FIGURE 5.31 Rate coefficients of rate-determining reactions in 03 loss catalytic cycles.

Concentration, molecules cnr3 Concentration, molecules cnr3

FIGURE 5.32 Stratospheric concentration profiles of key species involved in 03 loss catalytic cycles (35°N, Sept.).

Concentration, molecules cnr3 Concentration, molecules cnr3

FIGURE 5.32 Stratospheric concentration profiles of key species involved in 03 loss catalytic cycles (35°N, Sept.).

The total estimated 03 loss rate is ~ 1 x 103 molecules cm 3 s ', approximately 90% of which is due to H02 + 03 (HO, cycle 2) and 10% to BrO + H02 (HO,/BrO, cycle 1).

At 30 km (T ~ 255K), a similar analysis leads to

Ro,+o = A;o3+o[03][0] ^ (8 x 10~16) x (3 x 1012) x (3 x 107) = 8 x 104 Rho2+o = &ho2+o[H02][0] (7 x 10"11) x (1 x 107) x (3 x 107) Si 2 x 104 Rno2+o = £no2+o[N02][0] * (1 x 10"n) x (2 x 109) x (3 x 107) S 7 x 105 Rao+o = Wo [CIO] [O] e* (4 x 10") x (5 x 107) x (3 x 107) = 6 x 104 ^Bro+o = Wo [BrO] [O] ^ (5 x 10"u) x (2 x 106) x (3 x 107) ^ 3 x 103

The total estimated 03 loss rate is ~ 9 x 105 molecules cm 3 s~', about 80% of which is attributable to N02 + 0 (NO, cycle 1) and 10% each to the Chapman cycle and CIO + O (CIO, cycle 1).

Below ~ 20 km, HO, cycle 2 is dominant in 03 removal. Between 20 and 40 km, NO, cycles dominate 03 loss; NO, cycle 2 in the lower portion of this range, and NO, cycle 1 in the upper portion. Above about 40 km, HO, cycles are again dominant (HO, cycle 1). Because of the rapid release of Br in the lower stratosphere, BrO, cycles are approximately comparable in importance to CIO, cycles at 20 km. The coupled BrO,/CIO, cycle is itself responsible for ~ 25% of the overall halogen-controlled loss in the lower stratosphere. The halogen cycles achieve two maxima: (1) one in the lowermost stratosphere due to H0X/C10X cycle 1, HO,/BrO, cycle 1, and BrO,/CIO, cycles 1 and 2; and (2) one in the upper stratosphere due to CIO, cycle 1.

NO, is the dominant 03-reducing catalyst between 25 and 35 km. Because of coupling among the HO„ C10„ and NO, cycles, and the role that NO, plays in that coupling, the rate of 03 consumption is very sensitive to the concentrations of NO and N02. Partitioning between OH and H02 by H02 + NO —► N02 + OH is controlled by the level of NO. This reaction short-circuits HO, cycle 4 by reducing the concentration of H02.

FIGURE 5.33 Qualitative behavior of the ozone removal rate in the lower stratosphere as a function of NO, level (Wennberg et aL 1994). SPADF. stands for Stratospheric Photochemistry, Aerosols, and Dynamics Expedition. This is a cumulative plot; the contributions of the various processes add to produce the upper curve.

FIGURE 5.33 Qualitative behavior of the ozone removal rate in the lower stratosphere as a function of NO, level (Wennberg et aL 1994). SPADF. stands for Stratospheric Photochemistry, Aerosols, and Dynamics Expedition. This is a cumulative plot; the contributions of the various processes add to produce the upper curve.

Figure 5.33 shows schematically the overall 03 loss rate for the mid latitude lower stratosphere as a function of the NO* leve!. At very low NO* concentrations, the 03 loss rate increases as NO* decreases. This is a result of the absence of NO* to tie up reactive hydrogen and chlorine in reservoir species. As NO* increases, a critical value is reached where 03 removal through the reaction, O + NOi, is equal to the sum of 03 loss by the HO* and CIO* cycles. At this point, increases in NO* result in comparable reductions in the concentrations of HOi and CIO. As the chlorine content of the stratosphere has increased over time, this crossover point has been shifted to higher NO* concentrations. As [NO*] increases further, the NO* cycle eventually becomes dominant, and 03 loss increases linearly with [NO*].

The dependence of the 03 loss rate on NO* at higher altitudes in the stratosphere is different from that in Figure 5.33. At higher altitudes, the increasing amount of atomic oxygen means that the catalytic cycies that rely on O atoms become increasingly important. As a result, substantially higher 03 loss rates occur in the upper stratosphere than in the lower stratosphere. The fractional contribution 10 03 loss by HO* becomes dominant above 45 km. As altitude increases, OH and H02 concentrations fall off slowly, while those of CIO* and NO* fall off rapidly. The ClO/HCI ratio decreases because of a shift in the Cl/CIO ratio favoring CI and production of HCi from CI + CH4. The decrease in NOx at high altitudes results because the N ■+ NO reaction {recall Figure 5,8) begins to become important, even though the NO*/NOv ratio shifts in favor of NO*.

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