10 10 Lifetime (s)

Figure 4.3 Lifetime of the components of Clv. Calculated for 45°N equinoctial conditions based on daytime-average constituent abundances and photolysis rates: tn<) ~ 1/(irlo<.N()|NO| + ^Vio+olO]), tcl ~ l/(£ct.«),[Oi]). ^ciono, ~ '/CAiono2 + £o+ciono,|0] + £nHrf.K|M),[OH|), T1)n ~

¡/(¿hcm™[OHD, ti(<x:i ~ 1/(./„„;,), t(.r ~ 1/(A*(,o;noJNO;i] + kvnrH [CH4|(iCll/|C10]) + k, ¡■).!l.;JHO | 1.

Figure 4.4 shows in more detail the chemistry between these species. CIO is converted to CI in the reactions

while CI is converted to CIO in the reaction

Because the lifetimes of both CIO and CI are short, the ClO-Cl system reaches equilibrium rapidly compared to the rate that conditions in the atmosphere change. It is therefore a standard assumption that CIO and CI are always in the photochemical steady state.

Using the assumption that CIO and CI are in the photochemical steady state, we can calculate the ratio

During the day, this ratio is -10 ' in the lower stratosphere, rising to -0.1 in the upper stratosphere. In other words, [CIO] > [CI], so that to a good approximation [CI J [CIO] in the stratosphere. In the lower stratosphere, the term A( i0iNO[NO] WolO], Above -35 km, the atomic O term becomes important.

At night, the abundances of NO and O are very small. Thus, after sunset, CI is converted to CIO through reaction with C);, but there is no competing reaction to reform CI. As a result, at night Clx is composed entirely of CIO. This is predicted by Equation (4.9), which predicts a [C1]/[C10[ ratio of 0 when [NO] and [O] are 0.


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