10. Stratospheric chemistry. Daniel J. Jacob, Atmospheric Chemistry, Harvard University, Spring 2017

Size: px

Start display at page:

Download "10. Stratospheric chemistry. Daniel J. Jacob, Atmospheric Chemistry, Harvard University, Spring 2017"

Alexander French
6 years ago
Views:

1 10. Stratospheric chemistry Daniel J. Jacob, Atmospheric Chemistry, Harvard University, Spring 2017

2 The ozone layer Dobson unit: physical thickness (0.01 mm) of ozone layer if compressed to 1 atm, 0 o C 1 DU = 2.69x10 16 molecules cm -2 Latest satellite ozone data (March 12):

3 Structure of the natural ozone layer Ozone number density, molecules cm -3 Based on ozonesonde observations in the 1970s

The natural ozone layer: chemical production (P) and loss (L) Ozone number density, 10

4 The natural ozone layer: chemical production (P) and loss (L) Ozone number density, molecules cm -3 L > P P > L P > L L > P L > P Brewer-Dobson stratospheric circulation

5 Chapman mechanism for stratospheric ozone (1930) (R1) O2 + hν O + O ( λ < 240 nm) (R2) O + O2 + M O3+ M (R3) O3+ hν O2 + O ( λ < 320 nm) (R4) O + O 2O O 2 slow R1 3 2 R2 O fast O 3 R3 R4 slow Odd oxygen family [O x ] = [O 3 ] + [O]

6 Calculation of photolysis rates dx [ ] X+ hν... = kx [ ] dt k is the photolysis rate constant (also called photolysis frequency) k = q ( λσ ) ( λ) I( λ) dλ X X 0 quantum yield absorption x-section actinic flux (omnidirectional) Probability of absorption for incoming photons = σ/a photon is not absorbed Absorption cross-section σ photon is absorbed Molecular cross-section A

7 Solar spectrum and absorption cross-sections O 2 +hv O 3 +hv

8 Calculation of 3-body reaction rates A + B AB * (1) AB* A + B (2) AB * + M AB + M * (3) M* M + heat (4) Net: A+ B+ M AB+ M A and B are reactants; AB* is the activated product; AB is the stable product; M is the third body (N 2, O 2 ) General solution: d[ AB] kk 1 3[ A][ B][ M] = dt k + k [ M ] 2 3 Low-pressure limit (Rate(2) >> Rate (3)): High-pressure limit (Rate(2) << Rate (3)): d[ AB] = dt d[ AB] = dt kk k 1 3 k [ A][ B] 1 2 [ A][ B][ M]

9 Energy states of the O atom (1s 2 2s 2 2p 4 ) determined by the arrangement of the four electrons in the 2p orbitals multiplicity total electronic orbital angular momentum number Multiplicity = 2S+1, where S is the spin. The spin of an electron is ±1/2. Hund s Rule: lowest-lying energy state is the one of maximum multiplicity Energy O( 1 S) O( 1 D) O( 3 P) : :.. O O( 3 P) is a diradical

10 Steady-state analysis of Chapman mechanism = [O] 1 k k kc n a Lifetime of O atoms: τ O 2 2[O][O 2][M]+ 4[O 3][O] 2 O2 is sufficiently short to assume steady state for O: x 3 ~ 1 s [O] k τ R2 = R3 k [O][O ][M]= k [O ] = = << 1 [O ] [O ] Lifetime of O x : 3 O [O 3] kc 2 O2na τ O3 x τ Ox = k4 3 k4 so the budget of O 3 is controlled by the budget of O x. [O ] 1 2 [O ][O] 2 [O] τ Steady state for O x : Ox 1 = 2 3 kk = k4 3 ][ [O3 = CO2na kk 3 4 2R1 2R4 k [O ] [O O] ]

11 Photolysis rate constants: dependence on altitude X+ h... k q ( ) ( ) Id ν λσ λ λ = I( z + dz) 0 X quantum yield X absorption X-section λ photon flux optical depth dδ = ( σ n ( z) + σ n ( z)) dz O2 O2 O3 O3 I( z) I( z) = I( )e z δ δ = ( σo2no2( z ') + σo3no3( z ')) dz '

12 Chapman mechanism vs. observations shape determined by k 1 n O2-3 Chapman mechanism reproduces shape, but is too high by factor 2-3 missing sink!

14 Radical reaction chains in the atmosphere Initiation: non-radical radical + radical photolysis thermolysis oxidation by O( 1 D) Propagation: radical + non-radical non-radical + radical bimolecular redox reactions Termination: radical + radical radical + radical + M non-radical + non-radical radical redox reaction non-radical + M 3-body recombination

15 Water vapor in stratosphere H 2 O mixing ratio Source: transport from troposphere, oxidation of methane (CH 4 )

16 Initiation: 1 H2O + O( D) 2OH Propagation: Termination: Ozone loss catalyzed by hydrogen oxide (HO x H + OH + HO 2 ) radicals Net: OH + O HO + O HO O OH + 2O 3 3O 2 2O OH + HO 2 H2O + O2 2 slow H 2 O OH fast HO 2 HO x radical family slow

17 Questions 1. In the upper stratosphere, OH reacts with O atoms: OH + O H + O 2 followed by addition of O 2 to H: H + O 2 + M HO 2 + M What is the effect on ozone? 1. A termination step for the HO x radical chain is HO 2 + HO 2 H 2 O 2 (hydrogen peroxide) Hydrogen peroxide can go on to either photolyze or react with OH: H 2 O 2 + hν 2OH H 2 O 2 + OH H 2 O + HO 2 Whether H 2 O 2 photolyzes or reacts with OH has a large effect on HO x -catalyzed ozone loss, explain why.

18 Supersonic aircraft (Concorde) cruising at 60,000

19 WHAT IS A RATE-LIMITING STEP? From IUPAC: A rate-controlling (rate-determining or rate-limiting) step in a reaction occurring by a composite reaction sequence is an elementary reaction the rate constant for which exerts a strong effect stronger than that of any other rate constant on the overall rate. It is not necessarily the slowest reaction in the sequence!

20 NITROUS OXIDE IN THE STRATOSPHERE H 2 O mixing ratio

21 N 2 O: LOW-YIELD PRODUCT OF BACTERIAL NITRIFICATION AND DENITRIFICATION Important as source of NO x radicals in stratosphere greenhouse gas Main anthropogenic source: agriculture IPCC [2014]

22 ATMOSPHERIC CYCLING OF NOx AND NOy

23 STRATOSPHERIC OZONE BUDGET FOR MIDLATITUDES CONSTRAINED FROM 1980s SPACE SHUTTLE OBSERVATIONS Salawitch et al. [1989]

24 Chlorofluorocarbons (CFCs) in the atmosphere (ppb) Chlorofluorocarbons (CFCs)

25 ATMOSPHERIC CYCLING OF ClOx AND Cly

26 Chlorine partitioning in stratosphere WMO [2014]

27 Source gas contributions to stratospheric chlorine WMO [2014]

28 Decrease of Cl-containing gases following Montreal protocol τ = 45 years τ = 100 years τ = 5 years τ = 26 years Original Montreal protocol (1987): cap production rates at 1980s levels London (1990), Copenhagen (1992) amendments: phase-out in developed world Beijing (1999): worldwide ban on production

29 WMO [2014] Loss of HNO 3 by PSC sedimentation suppresses conversion of ClO to ClNO 3 in Antarctic spring

30 WMO [2014] Loss of HNO 3 by PSC sedimentation suppresses conversion of ClO to ClNO 3 in Antarctic spring

31 CHRONOLOGY OF ANTARCTIC OZONE HOLE

32 What about the Arctic? PSCs in Kiruna, northern Sweden

33 PSC FORMATION AT COLD TEMPERATURES PSC formation Frost point of water MO [2014]

34 Ozone depletion can take place in Arctic stratosphere in spring Movie of Arctic ozone season WMO [2014]

1971-present trend in Arctic ozone column (March) March 1979-2016 Arctic

35 1971-present trend in Arctic ozone column (March) March Arctic ozone movie Arctic ozone watch, WMO [2014]

36 Embryo of ozone hole is occasionally seen in Arctic following cold winters

37 Correlation of Arctic ozone loss with temperature

38 Rising CO 2 warms the surface but cools the stratosphere stratosphere COLD CO 2 troposphere WARM ground IPCC [2014]

39 Global ozone trend WMO [2014]

reversed the stratospheric chlorine trend Antarctic ozone hole

40 Montreal Protocol as example of successful global environmental policy Montreal Protocol and its amendments have reversed the stratospheric chlorine trend Antarctic ozone hole is on its way to recovery, Arctic ozone hole appears to have been avoided

41 Model projections for future ozone recovery WMO [2014]

2. Sketch a plot of R vs. z. Comment on the shape. Explain physically why R(z) has a maximum in the atmospheric column.

2. Sketch a plot of R vs. z. Comment on the shape. Explain physically why R(z) has a maximum in the atmospheric column. 190 PROBLEMS 10. 1 Shape of the ozone layer Consider a beam of solar radiation of wavelength λ propagating downward in the vertical direction with an actinic flux I at the top of the atmosphere. Assume