Stratospheric Ozone Depletion At ground level ozone is a toxic gas and can cause respiratory problems. In the Stratosphere O3 filters harmful ultra-violet light (uv) from penetrating to the lower atmosphere O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 Short wave uv is removed and only 285 nm and greater hit the earth
O3 conc. with height Temp. with height Altitude, km O3 conc. X1012molecules/cm3 What is a ppm?? How many ppm equal 0.8x 10-12 molecules/cm3 ??
·Stratospheric ozone depletion is of great concern because the ozone layer in thestratosphere prevents 95-99% of the suns ultraviolet radiation from striking the earth. ·If this additional uv light were to strike the earth it wouldbe potentially damaging to life on earth. ·Every 1% decrease in the earth’s ozone shield is projected to increases the amount of UV light exposure to the lower atmosphere by 2%. ·Recent UV measurements from around the northern hemisphere indicate small UV increases in rural areas and almost no increase in areas near large cities.
Looking at shorter wavelength incoming ground Radiation hitting the earth vs wavelength
Solar radiation wave length vs. Intensity UV-A: 315 to 400 nm – (near ultra-violet ranging into the visible~ 7% of the solar flux (which is not particularly harmful to living species) UV-B: 280 to 315 nm (~1.5% of the total flux) which can be harmful to both plant and animals species, especially after prolonged exposure UV-C: < 280 nm ~5% of the total flux which rapidly damages all types of biota
A number of consequences can result from increased levels of UV(ultraviolet radiation) striking the earth, including: 1.genetic damage 2.eye damage 3.damage to marine life. 4.increased amounts of photochemical smog.
Eye damage: UV-B (280-320 nm) can damage the lens, cornea, retina, and conjunctive membrane; A sustained thinning of the ozone layer may cause ~200,000 new cases of cataracts (clouding of the eye’s lens) per year. Immunosuppression: Exposure to UV-B has been shown to increase the incidence and severity of infectious disease and some cancers. Those infected with HIV tend to experience the onset of AIDs symptoms when exposed to increased UV-B Green plants: Excessive exposure to UV-B inhibits the growth process of green plants. This is very important for oceanic algae, which support the food chain in the oceans and consequently land based life which depend on the oceans.
How is O3 formed in the Stratosphere? The Stratosphere begins about 10k above the surface of the earth and goes up to 50k The main gases in the stratosphere, as at the surface, are oxygen and nitrogen uv light of low wave lengths ( high energy) split molecular oxygen (O2 ) to split oxygen O2 O. + O. requires 495 kJ mole-1 of heat (enthalpy) What wave length of light can do this?? Let’s start with hn = E, where h is Planck’s constant and n is the frequency of light and E is the energy associated with one photon.
And, nl = c where c is the speed of light and l is the wave length of light Combining we can solve for the wave length that will break apart oxygen at an enthalpy of 495,000 J mole-1 l= h c/ E If the value of Planck’s constant is 6.62 10-34 joules sec c = 2.9979 x108 m sec-1 l= h c/ E = 241 nm can you verify this calculation? Hint energy E is for one photon??
Some reactions O2 + uv-light O. + O. (<240nm, slow) O. + O2 M O3 +M(Ozone)(fast) O. + O3 2O2 O3 + uv-light O2+O(1D). (230-320 nm)(fast) During the late 1960s it became clear that the above cycle would make too much O3 compared to observations, so another loss mechanisms were proposed O(1D).+ H2O 2OH. (loss of O3) O3 +OH O2 + HO2. (loss of O3) O. + HO2 O2 + OH. (loss of O3) HO2 + O3 OH + 2O2 (loss of O3)
Let’s develop some steady state relationships so that we can predict a general Steady State relationship between O. and O2 and and O3 and O2 (see pages 22-25 in Baird) O2 + uv-light O. + O. (<240nm, slow) O. + O. +M O2 For the rate expression O2 + uv-light O. + O. the rate of formation of O. is dO./dt = 2k1 [O2] The rate expression for the loss of O. is: dO./dt = 2kii[O.][O.] x M The total rate = formation + loss processes dO./dt = 2k1[O2] -2kii[O.][O.] x M at ss dO./dt = zero, and solving for [O.]2ss/[O2]ss = ki/(kiixM)
[O.]2ss/[O2]ss = ki/(kiixM) What this says is that the ratio of O atoms to oxygen molecules (O2) varies indirectly with M. M represents both oxygen and nitrogen molecules. So as we increase in altitude the ratio of [O.]2ss/[O2]ss increases. If we go back to the basic reactions that form O3 in the stratosphere O2 + uv-light O. + O. k1 O. + O2 M O3 +M(Ozone)k2 O. + O3 2O2 k3 O3 + uv-light O2+O(1D).) k4 And proceed through similar arguments to that we used for dO./dt (page 23 Baird) for dO3/dt [O3]ss/[O2]ss = M1/2 (k1 k2/k3k4)1/2 what does this say happens to O3 with z?
Paul Crutzenin 1970 showed that NO and NO2 react catalytically with O3 and can potentially remove it from the stratosphere.(he get’s a nobel prize for this in 1995) NO + O3 NO2 + O2 NO2 + O. -> NO + 2O2 So where would NO come from?? SST’s
Chloro-fluro carbons (CFCs like CCl3F) can also destroy Ozone 1. Freons are inert compounds that are very stable in the lower atmosphere. Light of high enough energy does not penetrate into the troposphere. 2. Freons do not react with OH. Radicals which remove most other organic gases in the atmosphere. OH. + H2C=CH2 products CCl3F + OH. does not happen 3. Given a very long life in the troposphere, because nothing removes them, they can ultimately mix up into the stratosphere 4. In the stratosphere, the uv light is not filtered even at low wavelengths 5. Some of this low wave length radiation can photolyze freons (Molina and Rowland)
At UC Irvine in the early 1970s a young graduate student named Mario Molina was working in Dr. Rowlan’s in the lab on freons and was looking at their absorption spectra
CCl3F + uv Cl. + .CCl2F but the free chlorine atom can react with O3 Cl. + O3 ClO. (chlorine oxides) + O2 what is really bad is that ClO. + O. Cl. + O2 Remember that: O.+ O2 O3 (Ozone) It is estimated that one molecule of chlorine can degrade over 100,000 molecules of ozone before it is removed from the stratosphere or becomes part of an inactive compound.
ClO.molecules can also react with NO2 to form chlorine nitrate. This compound was originally thought to be a sink, but it is really a “reservoir compound”. ClO. + NO2 ClO-NO2 it was known that ClO-NO2 could react with HCl to release free chlorinebut the probability of this in the gas phase was small because they are both at very low concentrations. ClO-NO2 HCl Cl2
Molina found in 1985 that HCl could be stored on the surface of small nitric acid particles in polar stratospheric clouds (PSC). The HCl then just had to wait for a ClO-NO2 to hit the particle particle Cl2 Cl2 + uv Cl. + Cl. These nitric acid particles form under extremely low temperatures in polar stratospheric clouds ClO-NO2 HCl Cl2
Your book also suggests that: That gas phase ClO-NO2 reacts on a thin liquid surface of the PSC particles with H2O ClO-NO2(g)+ H2O HOCl(aq) + HNO3(aq) HCl(g) also dissolves in PSC liquid water H HCl H+(aq) + Cl-(aq) HOCl(aq) Cl- + uv Cl2(g) + OH-(aq) ClO-NO2 HCl Cl2
Fluorine and Bromine are also potentially important, but their potency depends on the stability of the receiver compounds. Hydrogen fluoride, HF, is so very stable that fluorocarbons have relatively no known impact on ozone. Bromine reservoirs, such as HBr and BrONO2, are much more easily broken up by sunlight, causing bromine to be from 10 to 100 times more effective than chlorine at destroying ozone. From 30-60% of bromocarbons released to the atmosphere are man-made (methyl bromide fumigants and halon fire extinguishers) and both compounds will soon be restricted by international agreement.
In 1985 the world was shockedwhen the first decrease in stratospheric O3 was reported. Joseph Farmon and his colleagues found that over an 18 year study that the stratospheric O3 over Antarctica (Halley’s bay) had decreased by 35% in 1984 compared to 1957-1960 It went from 300 to less than 200 Dobson units (DU) A Dobson unit (DU) translates the O3 aloft into an amount that would cover the entire surface of the earth in pure ozone. The units are:matm cm If 100 DU were brought to the earth’s surface it would form a layer 1 millimeter thick.
1.AS a consequence of these observations The nations of the world under United Nations Environmental Program (UNEP) agreed to the Montreal Protocol (1987) calling for a 50% cutback in yearly CFC production by the end of the century. 2.It was strengthened in 1990 in London calling for a complete ban by the year 2000. 3 In Copenhagen in 1992 to a complete ban on production on CFC production and use by 1996. So has this made a difference??
HCFCs + OH. reactacts with life times shorter than CFCs These area now used in auto-air conditioners
In 1998 the ozone hole in the south and north poles have increased!!! This has been associated with the lowest stratospheric temperatures in two decades and raise concerns that the O3 hole may not “heal” as fast as predicted. It is possible that “green house” gases that heat the troposphere, may be cooling the stratosphere. Depletion of O3 also adds to cooling. According to NASA satellite measurements the O3 hole grew to 27.3 million km2, up from a previous high of 26 million km2. Also the O3 concentration in the worst section of the hole “bottomed out at 90 Dobson units, or 1/3 of what it should be.
The loss of O3 and greenhouse gases can not explain all of the observed stratospheric cooling. Typically natural disturbances rippling up to the stratosphere send pressure disturbances rippling up to the stratosphere. These planetary waves warm the polar stratosphere and slow O3 destruction. In recent years, few planetary waves have buffeted the Arctic and Antarctica during the critical season of springtime O3 loss. Current computer models are at odds on the relationship between planetary wave frequency and greenhouse warming.