= Solution
The lost <ozone-column photolysis shielding> is $\Delta N=20(2.69\times10^{16})=5.38\times10^{17}\ \mathrm{molecules\,cm^{-2}}$, so its optical-depth decrement is $\sigma\Delta N=0.0538$. Under the stipulated effective-cross-section Beer-Lambert model, the transmitted <photodissociation> flux increases by $\exp(0.0538)$.
The short-lived <oxygen singlet D atom> satisfies $[\mathrm O({}^1D)]\simeq J_{\mathrm O_3}[\mathrm O_3]/(k_q[M])$ when quenching dominates. At fixed local <ozone>, <water> and air density, <hydroxyl radical> production is proportional to $J_{\mathrm O_3}$. Consequently
$$
\boxed{\frac{P'_{\rm OH}-P_{\rm OH}}{P_{\rm OH}}
=e^{0.0538}-1=0.0553,\quad\text{a }5.53\%\text{ increase}.}
$$
A linear optical-depth approximation would give $5.38\%$. A realistic calculation integrates the wavelength-dependent <absorption cross section>, <quantum yield> and <actinic flux>; <clouds> and multiple scattering modify the simple transmitted-beam model.
<Ozone> change can also alter the radiative <temperature> structure, <water vapor> and circulation, thereby changing the <water> reaction channel and the supply of <ozone>/precursors to the <troposphere>. If the held-fixed <tropospheric ozone> assumption is relaxed, altered <ozone> itself changes the photochemical source of <oxygen singlet D atom>. These are additional feedbacks, distinct from the isolated UV-transmission calculation above.
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