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The ozone layer as a UV shield

Compare solar UV at the top of the atmosphere and at the ground, then read the ozone absorption cross-section.

Goal

Quantify how much UV-C and UV-B the ozone layer removes, and connect the dip to the Hartley and Huggins bands.

Apparatus and reagents

Two spectrum panels: solar irradiance 200–400 nm with and without ozone, and the $\ce{O3}$ absorption cross-section on a log scale.

Procedure

In the first view, compare the two curves below 280 nm: the ground-level curve is essentially zero — UV-C never reaches us.
Follow the ground-level curve through 280–315 nm (UV-B): it rises steeply but stays far below the top-of-atmosphere curve.
Switch to the cross-section view: locate the Hartley maximum near 255 nm and the weaker Huggins tail toward 350 nm.

What to observe

Essentially 100 % of UV-C (< 280 nm) and ≈ 90 % of UV-B are removed before the surface; UV-A passes almost untouched.
The cross-section peaks at $\sigma \approx 10^{-17}$ cm²/molecule near 255 nm — huge enough that a layer only ≈ 3 mm thick at STP shields the whole planet.

Explanation

Absorbed UV photolyses ozone: $\ce{O3 + h\nu -> O2 + O}$ (Hartley band, λ < 310 nm) then $\ce{O + O2 + M -> O3 + M}$ re-forms it — the Chapman cycle. Each photon’s energy becomes heat, which is why the stratosphere is warmest at its top. If $\ce{O3}$ drops (e.g. under CFC-driven chlorine catalysis), the transmitted UV-B grows exponentially because absorption follows the Beer–Lambert law $I = I_0 e^{-\sigma N l}$ : a 10 % ozone loss raises surface UV-B by roughly 12 %.

Chemists behind it

Paul J. Crutzen Mario J. Molina F. Sherwood Rowland