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Lecture 15

CHEM H90 Lecture 15: The Ozone Hole and the Man Who Saved the Planet, pt. 1 (05/08/17)

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University of California - Irvine

LECTURE 15 Monday, May 8, 2017 Atmospheric Chemistry: Ozone Hole The Ozone Hole and the Man Who Saved the Planet Ozone and how it protects us • The ozone molecule is O 3 • O 3n the atmosphere absorbs lots of UB radiation • UV can be very bad for your skin, causing skin cancer • With less O 3n the upper atmosphere, there’d be more skin cancer Solar radiation • Solar flux, the light that reaches us from the sun • A little over half of it is in the infrared (IR) o We don’t see that, but it’s what makes the sunlight feel warm o We experience it as heat • About 40% is in the visible spectrum • About 10% is more energetic – shorter the wavelength, the higher the energy – UV What is the O 3ayer? • UV-A (315~400 nm) o The closest to visual • UV-B (280~315 nm) o Some gets through the ozone layer • UV-C (100~280 nm) o Nothing gets through the ozone layer O 3oncentration with height • 10-15 miles in altitude, stratospheric, there is a sudden increase in the ozone layer Measuring O 3 • The basic unit is called the Dobson Unit • 1 Dobson = 1 ppb O i3 the air • Actually measures amount of O in column of air, by measuring UV radiation 3 • Typically, about 300-400 DU’s in the northern hemisphere o This is what’s normal Types of UV • A: Closest to violet, not so bad for skin o  between 320 and 340 nm • B: Worst skin and other damage o  between 280 and 320 nm • C: Really bad, but all filtered out by upper atmosphere, so not an issue o  is less than 280 nm Effect of O3depletion on UV radiation • There’s a clear general trend so that if you go out someplace where ozone is depleted, you find a lot more ultraviolet radiation o If you lose half your ozone, you double the UV radiation arriving Chapman cycle: Natural • UV photons break an O mo2ecule into two oxygen atoms o Wavelength must be short enough to make this happen • A single atom, a radical, which is very reactive, is released o Bonds with an O and2produces ozone • The ozone bonds with an oxygen atom to form two oxygen molecules o This happens very slowly, so there’s a buildup of ozone before it is converted into oxygen molecules o There’s a steady fraction of ozone – there’s an equilibrium, a steady state Chemical equilibrium • In the presence of UV-C, some fraction of O i2 constantly turning into O a3d back again • Leads to about 300 Megatons of O in3the ozone layer naturally • If there were more UV-C, there’d be more O , 3nd vice versa o It’s at equilibrium, with the same amount of ozone produced as eaten up • Gives us our natural protective ozone layer, keeping UV-C out Why UV only? • Look up the energy of the O b2nd: 495 kJ/mol o If you were able to write things in scientific units: 4.95 x 10 J/mol • That means that for one molecule: o About 5 x 10 / (6 x 10 ) = 0.8 x 10 -18J/molecule o Avogadro’s number • If one photon breaks one bond, it
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