P

(total production rate)

Note that the replacement lifetime with respect to loss in Equation (2.27) gives the same l/L lifetime as was derived earlier in this chapter. At photochemical steady state the lifetimes of X with respect to production and loss are equal. It is also possible to define a replacement lifetime with respect to transport Tx, but it is rarely used. Throughout this book and most of the literature, the lifetime of a constituent generally refers to the reciprocal of the loss frequency, l/L.

2.3.1 Multiple-loss processes

If a constituent X has several loss pathways, then the total loss frequency is the sum of the loss frequencies of the individual pathways: L„„= L, + L2 + ... + L„. For each individual loss process, we can define a loss lifetime for that process t„ = 1/L„. The lifetime with respect to the combined loss pathways is

Note that lifetimes add in the same way that resistors in parallel add.

It should be clear from Equation (2.29) that one can ignore loss pathways whose loss frequency is much smaller than other loss frequencies for the constituent. Stated another way, one can ignore any loss process if the loss lifetime for that process is much longer than the loss lifetimes of other loss processes.

2.3.2 Global lifetimes

The l/L lifetime that we have discussed up to this point is a "local" lifetime: it is the length of time that a molecule will survive at a given point in space before being chemically destroyed. For long-lived species, meaning species whose local lifetime in the lower stratosphere is years or longer (l/L > 10s s), transport of the species is an important determinant of its stratospheric distribution. For these species the "global lifetime" is often of interest. The global lifetime can be thought of as the average length of time between the emission or formation of a molecule and its

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