C

o re

FIGURE 6.30 Calculated rates of processes leading to the formation and loss of 03 for conditions of primarily free tropospheric air (see also Fig. 6.29) (adapted from Cantrell et al., 1996).

technique described in Chapter lf.A.4, peroxides, 03, and the photolysis rate of 03, /(O'D), in remote marine air. Assuming that reaction (109) is sufficiently slow compared to reaction (108) that it can be ignored, steady-state analysis can be applied to OH, HOz, and R02 to give the following expressions (Penkett et al., 1997):

+ &i«5[H02][03] - &103[OH][CO] - fclo4[OH][CH4] = 0. (B)

In Eq. (B), / is the fraction of O('D) that reacts with water vapor to form OH,

/= ^i02b[H2O]/{&H)2a[H2O] + kw2h [M]}, and is typically about 0.1.

-¿i08[H02][CH302] = 0, (C) rf[CH302]/df = ¿104[OH][CH4]

The net change in the free radical concentration (i.e., OH + H02 + CH302) is given by the sum of Eqs. (B)-(D):

Under steady-state conditions, the net change is 0.

If the [H02]/{[H02] + [R02]} ratio is a constant, a, then a rate constant representing both the H02 + H02 and H02 + CH302 reactions, ksum, can be formulated:

Setting the right-hand side of Eq. (E) to 0 and using the composite rate constant defined by Eq. (F), one can show (see Problem 11) that

That is, the peroxy radical concentrations in the absence of NO should vary with the square root of /(01D) under conditions of relatively constant 03 concentrations.

0 0

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