[0ss2[02ss VCiiM

We see now why the ratio of oxygen atoms to diatomic molecules increases as we go higher and higher above the strate sphere: it is because the air pressure drops, and therefore so does [M], so the 02 re-formation rate decreases.

Steady-State Analysis of the Chapman Mechanism

After this introduction, we now are ready to apply the steady-state analysis to the Chapman mechanism. The four reactions of concern are shown again below. Notice that the recombination of O atoms, i.e., reaction (ii) above, is not included because its rate in the mid- and low stratosphere is not competitive with other reactions, since the oxygen atom concentration is small there.

*03 + M (2) ■*02 + 0 (3) * 2 02 (4)

Noting that O is produced or consumed in all four reactions, we obtain four terms in its overall rate expression and assume it is in a steady state:

rate of change of [O] = 2 ratet — rate2 + rate3 — rate4 = 0 (A)

Other useful information about concentrations can be obtained by considering the steady-state expression for the ozone concentration:

rate of change of [03] = rate2 — rate3 - rate4

If we add together the expressions for the rates of change in [O] and in [03], i.e., equations (A) and (B) above, we find that the rates for reactions 2 and 3 cancel, and we obtain

Using the expressions for these two rates in terms of reactant concentrations, we find

Continue reading here: [03[0 k[02lc4C

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