Mr. Brown's Science Labs

The Thin Shield

Electromagnetic radiation arriving from the Sun spans an enormous range of wavelengths, but the biological consequences of that radiation are not distributed evenly across the spectrum. Because photon energy varies inversely with wavelength, the ultraviolet portion of the spectrum — roughly 100 to 400 nanometers — delivers photons energetic enough to break covalent bonds in organic molecules. Visible light, at longer wavelengths, generally cannot. That distinction explains why sunlight of one wavelength merely warms your skin while sunlight of another damages the DNA inside it.

Atmospheric scientists divide ultraviolet radiation into three bands. UV-A (315–400 nm) passes almost entirely through the atmosphere. UV-C (100–280 nm) is absorbed completely by molecular oxygen and ozone high in the atmosphere and never reaches the ground. UV-B (280–315 nm) occupies the consequential middle: energetic enough to cause sunburn, cataracts, immune suppression, and skin cancer, yet only partially absorbed on the way down. The fraction of UV-B that reaches the surface is controlled almost entirely by how much ozone sits above you.

Ozone (O₃) is concentrated in a layer between roughly 15 and 35 kilometers in altitude, where it forms continuously as ultraviolet photons split O₂ molecules and the freed oxygen atoms bond to intact O₂. The same layer destroys ozone nearly as fast as it forms it, and the energy absorbed in both reactions is converted to heat — the reason stratospheric temperature increases with altitude. Total column ozone is reported in Dobson Units: 100 DU corresponds to a layer one millimeter thick if all the ozone above a point were compressed to sea-level pressure. A typical column of 300 DU is therefore about three millimeters of ozone. The shield is astonishingly thin.

It also proved vulnerable. Chlorofluorocarbons (CFCs), synthesized in the 1930s as refrigerants, propellants, and solvents, are chemically inert in the lower atmosphere and drift upward intact over decades. Ultraviolet photons in the stratosphere finally break them apart, releasing chlorine atoms that destroy ozone catalytically: a single chlorine atom can remove tens of thousands of ozone molecules before it is eventually locked into a stable compound. Losses were most severe over Antarctica, where the winter polar vortex and polar stratospheric clouds concentrate reactive chlorine that is unleashed when sunlight returns each spring.

The response was the Montreal Protocol, adopted in 1987 and strengthened repeatedly since. It phased out global production of CFCs and related compounds. Because the CFCs already airborne persist for 50 to 100 years, ozone kept declining for roughly a decade after the treaty took effect. Measurements since 2000 show the decline has stopped and column ozone is slowly rising. Full recovery to 1980 values is projected near the middle of this century — a delay that is itself a lesson in how long the atmosphere holds onto what we put into it.