Stratospheric Chemistry Part 1 Darin Toohey CU PAOS. Some resources that I borrowed from

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1 Stratospheric Chemistry Part 1 Darin Toohey CU PAS Why Do We Care? zone Discovery and History Circulation Chapman Chemistry Catalytic Destruction The Controversy The Big Surprise of 1985 Some resources that I borrowed from m

2 Why do we care? Phytoplankton Melanoma The Human Connection The EPA estimates that 60 million Americans born by the year 075 will get skin cancer because of ozone depletion. About one million of these people will die. (thinkquest.org) DNA isn t just for humans plants and animals too!

zone History In 1785, Martinus Van Marum noted the odor of

air. Note this was before it was accepted that oxygen was a

fficially named as a chemical in 1840 by Christian

similar to that of phosphorus when exposed to air It was

3 zone History In 1785, Martinus Van Marum noted the odor of electrical matter in the description of the discharge of air. Note this was before it was accepted that oxygen was a component of air! fficially named as a chemical in 1840 by Christian Schönbein, after he noted that it had a smell that was similar to that of phosphorus when exposed to air It was soon realized that ozone was a good disinfectant. tto was first to market a water purifier based on ozone Marius Paul tto

4 In 19, Gordon Dobson developed the first spectrometer to measure ozone in the atmosphere, and he characterized its latitudinal seasonal variability Brewer-Dobson circulation Brewer-Dobson circulation 4

198 Thomas Midgley develops chlorofluorcarbons for DuPont, inhaling them to prove that they

These non-flammable compounds soon replace the deadly compounds (such as ammonia and S ) in

belts, drive cars with air conditioning, and use spray cans for just about everything!

5 198 Thomas Midgley develops chlorofluorcarbons for DuPont, inhaling them to prove that they nontoxic. These non-flammable compounds soon replace the deadly compounds (such as ammonia and S ) in home refrigerators CFCs become popular in the 1960s when americans want to live in sun belts, drive cars with air conditioning, and use spray cans for just about everything! In 190, Sydney Chapman published several theoretical papers on upper-atmospheric ozone now known as the Chapman Cycle + hυ + (λ < 4 nm) (1) + + M + M () + hυ + (λ < 6 nm) () + + (4)

6 hν rate 1 = J (J s -1, 0 km) rate = k M (k cm 6 s -1 at 50 K) rate = J (J 10 - s -1 at 0 km) hν rate 4 = k 4 (k cm s -1 at 50 K) d = J dt k M + J k4 d = k M J dt k4 dd oxygen hν Rate 1 = J Rate = k M Rate = J hν Rate 4 = k 4 d( + ) d x = = J dt dt d x In Steady State: = 0 = J dt k4 k4 P-L (note, you get the same by assuming is in steady state) 6

7 7 Substituting back: ~ 4 J M k k J M k dt d = J M k = At steady state: 1 M k J = 4 k J = M J J k k =

8 Problem Chapman mechanism predicts more ozone than what is observed! What s missing? Faster + + 8

http://www.accessexcellence.org/wn/sua06/nobchem.

introducing the concept of catalytic destruction of ozone H + H + H + H + Net - + + Bates, D.R. and M.

9 Water in the stratosphere would lead to catalytic destruction of ozone by speeding up reaction (4) of Chapman s mechanism thus, introducing the concept of catalytic destruction of ozone H + H + H + H + Net Bates, D.R. and M. Nicolet, The Photochemistry of the Atmospheric Water Vapor, J. Geophys. Res., 55, 01, 1950 Catalysis H + H + rate = k a H H + H + rate = k b H Net d k4 k H b dt Replace k 4 with apparent loss k app 4 k = + b H k4 1 k4 1 1 k J = 0.1 M app k J 4 > k 4 9

10 hν hν Catalysts (N, H, Cl, etc.) Production stays the same (it s due to photolysis of ) Loss increases Paul Crutzen proposes that N x from bacterial N can participate in atmospheric chemistry as N formed in the stratosphere N + ( 1 D) N + N "Influence of Nitrogen xides on Atmospheric zone Content" Quarterly Journal of the Royal Meteorological Society 96 (1970):0. 10

11 Formation of N + hν + ( 1 D) J ( 1 D) + N / ( P) + N / ~ gas kinetic N + hν N + ( 1 D) J N N + ( 1 D) N + k a = cm s -1 N + N k b = cm s -1 Yield k ( D) 1 = b 1 { k b+ ka} ( D) + J Loss of N N Production of N ~.5-4.0% of the N becomes N y Key connections made by Crutzen mankind can increase N emissions by fertilizing crops N has a long lifetime in troposphere, so can reach the stratosphere Increase in tropospheric N will increase stratospheric N x Increase in N x will result in decrease in steady state ozone e.g. Mankind can alter stratospheric ozone without leaving the ground! 11

12 Harold Johnston N + N + N + N + d ( k + k ) H H + kn N Net dt Richard Stolarski and Ralph Cicerone Cl + Cl + Cl + Cl + Net Stratospheric Chlorine: A Possible Sink for zone, Can. J. Chem., 5, , 1974 d dt ( k + k H + k N k ) H N + clo Cl (also McElroy and Wofsy, but more on them in Part ) 1

slightly soluble in water and might therefore persist and accumulate in the atmosphere

13 Dr. James E. Lovelock, Inventor The electron capture detector are unusually stable chemically and only slightly soluble in water and might therefore persist and accumulate in the atmosphere The presence of these compounds constitutes no conceivable hazard. Distribution of CCl F in and over the North and South Atlantic cean, Nature, Vol. 41, January 19, 197 1

destruction of ozone," Nature 49 (8 June 1974):810 1614 citations even with typo!

14 Mario Molina and F. Sherwood Rowland CFCl + hν CFCl + Cl CF Cl hν CF Cl + Cl Predicted tens of percents of ozone loss The deadly weapon! M. J. Molina and F. S. Rowland "Stratospheric Sink for Chlorofluoromethanes: Chlorine atomic-atalysed destruction of ozone," Nature 49 (8 June 1974): citations even with typo! The pieces come together! Brewer-Dobson circulation CFCl + hν CFCl + Cl CF Cl hν CF Cl + Cl release spark Cl + Cl + Cl + Cl + transport Net flame un reactivity source + hυ M + M + hυ destruction 14

15 Rowland (1974): The work is going very well, but it may mean the end of the world. x10! Spray cans banned/voluntarily boycotted 15

16 A field is born! All we need to do is to observe ozone losses in conjunction with increases in CFCs Easy!? r Not? CFCs nearly double! But no ozone loss! 16

17 What really goes on? Cl + Cl + Cl + Cl + Couples Cl x to N x Cl + N Cl + N Cl + N + M ClN ClN + hν Cl + N Cl + N Cl + H HCl + Cl + CH 4 HCl + CH Couples Cl x to H x Cl + H HCl + CH Cl + H HCl + H H + Cl Cl + H HCl + HCl + hν Cl + H And N x is coupled to H x! H + H + H + + H H + C C + H H + + M H + M H + H + H + H + H + H H + Couples H x to N x H + H H + H + N N + H H + N + M HN + M HN + H H + N HN + hν H + N H + N H N + M H N + hν H + N + halogen reactions 17

18 Build a model HCl H hν ClN, H N hν N 1. Source gas. Release of radical H CH 4 H Cl Cl. Catalytic cycle 4. Rate determining step hν H H hν HCl 5. Short-lived reserviors 6. Long-lived reserviors 7. Removal Halocarbon RAINUT How the radical families contribute to ozone loss H x (H, CH 4 ) Cl x (CFCs) N x (N ) 18

Meanwhile, scientists refine their understanding But Industry is ready, and they come our firing! CFCs are heavier than air, and don t rise to the stratosphere they sink!

19 Meanwhile, scientists refine their understanding But Industry is ready, and they come our firing! CFCs are heavier than air, and don t rise to the stratosphere they sink! There is more chlorine in sea spray than from CFCs! Chlorine may get to the stratosphere, but it doesn t form Cl! The natural variability of ozone is greater than any change expected due to man-made chemicals! Even if there is ozone loss, there is no proof that more ultraviolet light will reach the surface! Which one is it? The SST story evolves SSTs make ozone (smog chemistry) SSTs destroy ozone Those darned lab chemists 19

20 The World Breathes a Sign of Relief In the form of CFCs CFC production back on the rise! The Big Surprise of 1985! 0

21 Summary of important points Stratospheric ozone is produced by photolysis of, a process that is governed by abundances of and UV output of the sun. Mankind can t easily tamper with these parameters Sir Sydney Chapman (who spent a lot of time in Boulder) nearly got it right. He was able to account for ozone in the stratosphere to within about a factor of two with just four simple reactions. You might as well memorize these they will reappear on comps and cumulative exams (and it beats what you need to know to get the other factor of two! Gases that are long-lived in the troposphere will eventually reach the stratosphere, where they nearly all break down ( oxidize ) to produce highly reactive radicals that catalytically destroy ozone. It doesn t matter where these gases originate from the troposphere is the great homogenizer. The 1995 Nobel Prizes in Chemistry were awarded to Paul Crutzen, Mario Molina, and Sherry Rowland for recognizing the importance of this concept. The radical families are highly coupled changes in abundances of one family will result in changes in the others. Thus, the system is non-linear (although reasonably well behaved). However, it means that you can t just scale ozone losses with emissions. A simple stratospheric model has dozens of chemical species and hundreds of chemical reactions. It will run on a PC (I have one written by Michael Prather on the computer that I am using for this lecture). Having a good idea isn t good enough. It takes a lot of measurements to prove your point or a global crisis stay tuned for Part! 1

Stratospheric Chemistry Part 1 (Chapter 4, p , , , )

Stratospheric Chemistry Part 1 (Chapter 4, p 155-169, 174-176, 198-222, 231-238) zone Discovery and History The Stratosphere and circulation Chapman Chemistry Catalysts The Controversy The zone Hole International