1. Environmental Chemistry

2. Stratospheric Chemistry
The Ozone Layer - Shield against dangerous incoming radiation
How does radiation interact with molecules in the stratosphere?
What processes form and maintain the ozone layer?
How much ozone is there, and how is it distributed? How do we know?
How have human activities altered ozone chemistry and the ozone layer itself?
What controls have been put in place to protect the ozone layer?
Ozone is a “success story” in environmental chemistry – what lessons can we learn from it?
The stratosphere is also a good “case study” in how atmospheric chemistry works

3. Ozone layer characteristics

4. Global ozone
Ozone column – total amount of ozone between ground and top of atmosphere
Dobson Unit – an amount of ozone equivalent to a mm thick layer of ozone when brought to ground level
300 DU = 3 mm ozone colum

5. How is ozone measured?

6. Ozone Update NASA OzoneWatch: http://ozonewatch.gsfc.nasa.gov/
NOAA ESRL Global Monitoring Division Webpage:

7. Air circulation in the stratosphere
Air enters the stratosphere through the tropopause in the tropics
T is very low (200 K), so air in strat is DRY (~ 5 ppmv H2O)
T increases with alt so air is stratified – no vertical motion
Air stays in stratosphere for ~5 yrs before returning to troposphere

8. Solar spectrum

10. O2 Absorption Spectrum Graedel and Crutzen, Atmospheric Change, 1993
Schumann-Runge continuum
ionization continuum
Herzberg continuum
Schumann-Runge bands
Graedel and Crutzen, Atmospheric Change, 1993

11. Ozone UV absorption spectrum
Graedel and Crutzen, Atmospheric Change, 1993

12. Actinic Flux O2 O3

13. Stratospheric Chemistry
The Ozone Layer - Shield against dangerous incoming radiation
How does radiation interact with molecules in the stratosphere?
What processes form and maintain the ozone layer?
How much ozone is there, and how is it distributed? How do we know?
How have human activities altered ozone chemistry and the ozone layer itself?
What controls have been put in place to protect the ozone layer?
Ozone is a “success story” in environmental chemistry – what lessons can we learn from it?
The stratosphere is also a good “case study” in how atmospheric chemistry works

14. Comparing to observations
Ozone concentrations are lower than predicted based on oxygen-only chemistry

15. Catalysis of ozone chemistry
Radicals lower the reaction barrier so reaction goes faster

16. Reservoir species
Catalytic loss of ozone is limited by formation of “reservoir species” which decrease the concentrations of reactive radicals
NO2 + OH + M  HNO3 + M
Cl + CH4  HCl + CH3
ClO + NO2 + M  ClONO2 + M
ClO + HO2  HOCl + O2

17. Calculating rates of reactions

18. Ox production and loss rates
(24-hour averages)
Jacob, Intro. to Atmospheric Chemistry, 1999.

19. O3 loss rates with heterogeneous chemistry
With heterogeneous reactions
Jacob, Intro. to Atmospheric Chemistry, 1999.

21. Global ozone
Ozone column – total amount of ozone between ground and top of atmosphere
Dobson Unit – an amount of ozone equivalent to a mm thick layer of ozone when brought to ground level
300 DU = 3 mm ozone colum

22. Ozone column trend, 60oS-60oN
% Change
Pinatubo
Year

24. Midlatitude ozone trend vs. altitude
Jacob, Intro. to Atmospheric Chemistry, 1999.

25. https://www. researchgate

27. The ozone hole
At its peak on Sept. 28, 2016, the ozone hole extended across an area nearly three times the size of the continental United States. The purple and blue colors are where there is the least ozone, and the yellows and reds are where there is more ozone. Credit: NASA's Goddard Space Flight Center

28. Projected atmospheric chlorine levels without the Montreal Protocol
WMO, Scientific Assessment of Stratospheric Ozone: 2002.

29. CFCs today

30. The ozone hole

31. NASA Ozone Update How big is it?
The lowest value (deepest hole) ever recorded was 73 Dobson Units on September 30, 1994
Broadest hole occurred on September 29, 2000, when the ozone-depleted area stretched 29.9 million square kilometers.
The record for mean size of the ozone hole— the greatest extent over a one-month window—was September 7 to October 13, 2006, when the hole reached 26.2 million square kilometers.
The mean ozone hole in 2010 was 22.2 million square kilometers.
In their 2010 report, the science advisers to the Montreal Protocol found that
Global ozone and ozone in the Arctic and Antarctic is no longer decreasing, but is not yet increasing.
The ozone layer outside the Polar Regions is projected to recover to its pre-1980 levels some time before the middle of this century. The recovery might be accelerated by greenhouse gas-induced cooling of the upper stratosphere.
The ozone hole over the Antarctic is expected to recover much later.
The impact of the Antarctic ozone hole on surface climate is becoming evident in surface temperature and wind patterns.
At mid-latitudes, surface ultraviolet radiation has been about constant over the last decade.

32. Ozone Update NASA OzoneWatch: http://ozonewatch.gsfc.nasa.gov/
NOAA ESRL Global Monitoring Division Webpage:

33. 1986 – Discovery Of the Antarctic Ozone Hole
Column ozone decreases sharply each October since 1975
HALLEY BAY
Farman et al., Nature, 1986
Southern
Hemisphere

34. Inside the ozone hole All ozone between 15 and 20 km is gone!!!!
D. Hofmann, NOAA CMDL

35. ClO in the Antarctic stratosphere
ClO (parts per billion)
0.5
1.0
 ClO
Ozone
23 August 1987
RETURN OF SUNLIGHT
2.0
3.0
LATITUDE
(degrees south)
16 September 1987
OZONE (parts per million)
FULLY SUNLIT
Anderson et al., Science, 1991

36. Probing the Ozone Layer (Photos II)
…high-altitude research aircraft…
Probing the Ozone Layer (Photos II)
Ross Salawitch

37. How does the ozone hole form?
No sunlight in polar winter. Air over pole gets very cold (185 K!!!!)

38. The polar vortex and temperature
Slow Descent
Cold Vortex Core
Strong Winds
Mixing
Tropopause
Antarctica
1978 to 2000
POLAR STRATOS. CLOUD 
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
Temperature (K)
220
180
200
Data Courtesy P. Newman, NASA/GSFC
ANTARCTIC Polar Vortex Minimum Temperature, 20 km

39. How does the ozone hole form?
No sunlight in polar winter. Air over pole gets very cold (185 K!!!!)
Polar stratospheric clouds form from frozen HNO3 and water

40. PSCs Over Kiruna, Sweden (SOLVE Mission)

41. Polar Stratospheric Clouds over Northern Sweden
Winter 2000
Photos by Ross Salawitch

42. How does the ozone hole form?
No sunlight in polar winter. Air over pole gets very cold (185 K!!!!)
Polar stratospheric clouds (PSCs) form from frozen HNO3 and water
Reactions on the surfaces of PSCs convert chlorine reservoirs back to reactive forms (Cl2 and HOCl). NO2 is removed as HNO3.
HCl + ClONO2  Cl2 + HNO3
H2O + ClONO2  HOCl + HNO3
Sun returns in spring. Cl2 and HOCl photolyze to release Cl atoms.
Cl2 + hv  2 Cl
HOCl + hv  OH + Cl

43. How does the ozone hole form?
ClO (parts per billion)
1.0
2.0
3.0
16 September 1987
0.5
OZONE (parts per million)
FULLY SUNLIT
 ClO
Ozone
LATITUDE
(degrees south)
Cl reacts with O3 to make high levels of ClO. ClO reacts with itself in very fast catalytic cycle:
2 (Cl + O3  ClO + O2)
ClO + ClO + M  ClOOCl + M
ClOOCl + hv  ClOO + Cl
ClOO + M  Cl + O2 + M
Net: 2 O3  3 O2
Anderson et al., Science, 1991

44. Chronology of the Antarctic ozone hole
Jacob, Intro. to Atmospheric Chemistry, 1999.

45. Antarctic vs Arctic Temperatures
ANTARCTIC (S.H.) POLAR VORTEX
MINIMUM TEMPERATURE, 20 km (~12 miles)
1978 to 2000
220
Antarctic vs Arctic Temperatures
TEMPERATURE (K)
200
POLAR STRATOS. CLOUD 
180
Data Courtesy
P. Newman,
NASA/GSFC
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
TEMPERATURE (K)
SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG
220
200
 POLAR STRATOSPHERIC CLOUD
180
ARCTIC (N.H.) POLAR VORTEX
MINIMUM TEMPERATURE, 20 km (~12 miles)
1978 to 2000

46. Arctic ozone
Maps of ozone concentrations over the Arctic on March 19, 2010 (left) and the same day in 2011 (right), measured by the Ozone Monitoring Instrument on NASA's Aura satellite. Credit: NASA/Goddard

47. Arctic vs. Antarctic ozone

48. Antarctic Ozone Hole – Current Status
Updated with NASA TOMS data by Ross Salawitch
D. Hofmann, NOAA CMDL

49. Recovery of Global Ozone