Ozone layer and CFCs sim
Open a CFC valve under a model atmosphere, let the years pass, and watch chlorine atoms thin the ozone layer and the UV index at the ground climb; then close the valve and see how slowly it recovers.
The atmosphere: ozone depletion · covers the O-Level topic
What the sim shows
Ozone (O₃) high in the atmosphere, in the stratosphere about 15 to 35 km up, absorbs most of the Sun's harmful ultraviolet (UV) light. CFCs (chlorofluorocarbons) were used in fridges and aerosol cans. They are so unreactive that they last long enough to drift up to the ozone layer, where UV light breaks them apart and frees chlorine atoms. A chlorine atom reacts with ozone, and the chlorine atom is made again at the end, so one atom can destroy many thousands of ozone molecules. With less ozone, more UV reaches the ground: more sunburn, skin cancer and eye cataracts, and harm to crops and sea life. Countries signed the Montreal Protocol in 1987 and phased CFCs out (by 1996 in richer countries, by 2010 worldwide), and the ozone layer is now slowly recovering; it is slow because CFCs last for decades (this model uses 60 years) and take about 5 years to rise. Ozone is measured in Dobson units (DU), about 300 DU normally. The UV index here is 8 at 300 DU (a clear summer midday at mid-latitudes, such as in Europe; in Singapore, with the Sun high all year, the midday UV index is often 11 or more) and rises as 1 ÷ (ozone to the power 1.1), the measured "radiation amplification factor" for sunburning UV. Illustrative: the release levels and how much ozone each ppb of chlorine destroys are chosen to show the effect clearly; real chlorine high up rose from about 0.6 to 3.6 ppb (a high release from 1960 to 1990 in this model gives about the same peak), but the real worldwide ozone loss was smaller than shown (over Antarctica each spring it was much larger: the ozone hole). The real recovery is slower than shown: the ozone layer is expected back to 1980 levels only around 2040 to 2066. The model stops at 160 DU (UV index 16): leaving the valve open for a century would otherwise give values far beyond anything real. The equations are kept simple, at O-Level standard; the syllabus asks for the idea, not these equations.
- Ozone layer: it fades as chlorine atoms destroy ozone
- CFC cylinder: drag the valve to release CFCs (0 to 3)
- UV meter: the UV index on the ground under the column
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