Stratospheric Chemistry

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Egbert Stanley
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1 Stratospheric Chemistry

2 Stratospheric Chemistry Why Ozone? Part of global energy balance determines T and hence u. Protection of the biosphere from UV. Sensitive to human pollution. Search for extraterrestrial life What makes it challenging Itself controlled by complicated chemistry and dynamics. Useful Texts A. Dessler, The Chemistry & Physics of Stratospheric Ozone D.G. Andrews, An Introduction to Atmospheric Physics R.M. Goody, Principles of Atmospheric Physics & Chemistry (Chapter 6) R.P. Wayne, Chemistry of Atmospheres (Chapter 4)

3 Ozone Morphology Volume mixing ratio (vmr) in ppmv Note that this is the bottom of the stratosphere not the ground! O 3 number density (10 12 molecules cm -3 ) O 3 reaches a maximum value of both number density and VMR over the equator. Moving towards the poles the VMR peak moves to higher altitudes while the number density peak moves to lower altitudes.

4 Basic Gas Phase Chemistry: Introduction Ignoring transport, chemistry is responsible for creation and destruction of constituents. There are two basic types of process: Photolysis Collisions

6 Photolysis in the Atmosphere dz θ Zenith angle Superterrestrial solar irradiance Mass absorption coefficient Mass mixing ratio Atmospheric density Solar zenith angle

7 Penetration of Radiation into the Atmosphere The figure on the left shows typical average daytime photolysis frequencies. (from Dessler, 2000) The photon flux is a function of many variables e.g. total ozone above the point, the albedo of the surface and the solar zenith angle. Because the atmosphere is essentially transparent to visible radiation the radiation field is fairly constant with height, hence the photolysis frequency for species that absorb at these wavelengths (e.g. NO 2 ) is fairly constant with height. For species that absorb shortwave radiation there is a significant dependence on altitude. O 3 efficiently absorbs uv radiation so there is more uv above the ozone layer.

8 Solar Irradiance 1180 O 2 < 242 nm

9 Oxygen Photodissociation I The state of excitation of the products of photodissociation can be important and this depends upon the frequency of the absorbed photon. It is convenient to divide the spectrum into regions having common properties, e.g. for oxygen: The Hertzberg continuum The Schumann-Runge bands and continuum Lymann-α 242

10 Oxygen Photodissociation II The region of greatest absorption changes with altitude At low altitudes the weakest band (Herzberg) dominates

11 Ozone Photodissociation I Hartley Band

12 Ozone Photodissociation II

14 Basic Gas Phase Chemistry: Collisions II The functional form of the second order rate coefficient is often expressed Where A is related to the fraction of collisions between reactants that results in a successful reaction. A ~ cm 3 molecule -1 s -1 where every collision results in a reaction A ~ cm 3 molecule -1 s -1 where the reactants must have a specific orientation E a denotes the activation energy, effectively the energy the reactants must have overcome the repulsive force between reactants. The term exp(-e a /RT) is proportional to the number of molecules whose average translational kinetic energy exceeds the threshold E a.

15 Basic Gas Phase Chemistry: Collisions III Consider the three body reaction The M is required so that both energy and momentum are conserved. The rate is expressed However the rate coefficient is fitted more empirically

18 Slow reactions Fast reactions

20 VII

21 Basic Ozone Chemistry: Odd Oxygen Profiles Measured values (from Hudson 1981). Modelled values (from Dessler, 2000). Nearly all the odd oxygen in the stratosphere is in the form of O 3. At night the photolytic creation of O ceases and all of the O atoms are converted to O 3 through Reaction 2.

22 dz secθ dz θ

26 Basic Ozone Chemistry: Ozone Loss Rates

27 How long does it take to reach photochemical equilibrium? Where and when does transport matter? How do the fast and slow time constants compare?

28 Basic Ozone Chemistry: Catalytic Destruction

31 After subtracting

32 From [VI]

winds blow from day to night so that they smooth out day-night differences but don t effect meridional

34 Ozone Transport I z > 40 km shortest lifetime is photochemical so the atmosphere approaches photochemical equilibrium 20 < z< 40 km zonal motion is the only competition for photochemistry zonal winds blow from day to night so that they smooth out day-night differences but don t effect meridional gradients z < 20 km Every class of motion will perturb the photochemistry and photochemical equilibrium is irrelevant.

35 Ozone Transport II From, Dessler 2000 Chemistry Observed

36 Stratospheric Chemistry

37 The story so far Ozone profile is described qualitatively by the Chapman scheme Improvement is gained by Inclusion of trace gas catalytic cycles Cl/ClO, OH/H 2 O Inclusion of dynamics

38 Observational Evidence Time series of globally averaged changes in column ozone (top) and lower stratospheric temperatures (bottom) for the period The ozone observations show results derived from a number of ground-based and satellite data sets, from Fioletov et al. (2002); the light lines show results derived from several radiosonde data sets (described in Seidel et al., 2004). Both the ozone and temperature time series have been deseasonalized and changes are expressed with respect to a reference period of

39 Ozone Record: Midlatitudes

40 Ozone Record: Antarctica I Halley Bay

41 Ozone Record: Antarctica III Vertical Profiles McMurdo

42 Summary of Ozone Trends Dobson record shows no global long term decline between 1957 and 1980 Latitude/month cross section data shows Antarctic ozone hole as the main feature Decline in column ozone since 1975 in Antarctica spring, from 300 DU to 150 DU Decrease is in altitude region km where ozone now disappears completely Steady decline in global (60 S 60 N) since about Around 3 % per decade Decline is visible in individual station data under the annual cycle Episodes visible in the record of rapid decreases recovering over a few years Altitude variation As predicted in the mid 1980s there is a significant loss of ozone in the upper stratosphere, around, 5-10 % decline at all latitudes. However only a few percent of O 3 is found there so that changes in this region cause a relatively small change in column ozone. Around km slightly greater decrease at all latitudes, not understood

43 Ozone Depletion We need to explain Observed episodic global changes in O 3 Large local changes during the Antarctic spring You can t underestimate how much of a scientific & political shock this caused State of the art models predicted that CFCs would decrease O 3 in the upper stratosphere (near 40 km) with the effect only becoming noticeable in the 21 st century. Little or no effect of CFCs where expected in the lower atmosphere. Possible reasons Source gas increases catalytic gases? Incomplete understanding of O 3 chemistry?

44 Observational Evidence What causes these anomalies?

45 Volcanic Stratospheric Aerosol The background loading of stratospheric aerosols is quite low, <0.5 Mt, but can increase by several orders of magnitude following the direct injection of sulphur dioxide, SO 2, into the stratosphere during a large volcanic eruption.* * Mt = megatonne = 10 6 tonnes = 10 9 kg

46 Ozone Record: Pinatubo Effect

47 Aerosol Effects: Heterogeneous Chemistry I (diagram from Hanson et al. 1994) Aerosol provide a surface or volume that allows reactions to proceed many orders of magnitude faster than the equivalent gas phase reactions net flux of gas into the condensed phase gas concentration J nc 4 mean molecular speed T uptake coefficient

Aerosol Effects: Heterogeneous Chemistry II The heterogeneous reaction rate depends upon reactant concentrations, temperature, drop composition, phase and depending on the ratio between the

48 Aerosol Effects: Heterogeneous Chemistry II The heterogeneous reaction rate depends upon reactant concentrations, temperature, drop composition, phase and depending on the ratio between the reaction rate and the diffusion length into the drop either the aerosol surface area density or the aerosol volume density of the aerosol. Reaction in sulphuric acid aerosols (W is weight % of H 2 SO 4 )

49 Midlatitude Ozone Depletion I

50 Aerosol Effects: Midlatitude Ozone Depletion II For the NO X catalytic cycle the rate of O X loss is proportional to the abundance of NO X The increase in aerosol surface area leads to a reduction of in NO X and therefore a decrease in the loss rate of O X due to the NO X cycle The same reduction in NO X leads to an increase in the rate of O X loss due to the Cl X and HO X catalytic cycles Reservoirs

51 Aerosol Effects: Midlatitude Ozone Depletion III Because the abundance of NO X is very sensitive to the amount of aerosol surface area even small changes in the surface area density can lead to important changes in the chemistry of the stratosphere. An eruption like Mt Pinatubo can perturb the chemistry of the stratosphere over the entire globe and it will remain perturbed until the aerosol decays from the stratosphere which takes several years If the chlorine in the stratosphere was consistent with natural sources about 0.6 ppbv then the eruption of Mt Pinatubo would have resulted in an O X increase.

52 Antarctic Ozone Hole Decrease in column O 3 at high southern latitudes in late winter and early spring.

Antarctic Ozone Hole Observations By the mid 1990s the ozone hole covered ~25

each year, i.e. Starting in mid-august (corresponding to the return of sunlight)

depth of the low ozone increases reaching a maximum in mid-october Thereafter

53 Antarctic Ozone Hole Observations By the mid 1990s the ozone hole covered ~25 million km 2 or about 10 % of the area of the Southern Hemisphere Same pattern each year, i.e. Starting in mid-august (corresponding to the return of sunlight) a region of low column O 3 develops centred over the South Pole The area and depth of the low ozone increases reaching a maximum in mid-october Thereafter the column ozone starts to increase until the O 3 levels are back to normal by early December

54 Polar Vortex During Antarctic winter No heating of the atmosphere by solar radiation Cooling by thermal emissions Cold polar temperatures + warm temperatures at mid latitude cause a strong pressure gradient to form Creates a zonal wind (due to the coriolis force) Gives two important properties Very cold vortex temperatures The edge of the polar vortex acts as a mixing barrier and so isolates the vortex from midlatitude air

55 Ozone Loss Catalytic Cycle The most important catalytic cycle destroying ozone in the Antarctic polar vortex is The rate-limiting step is the photolysis of the ClO dimer. The O X loss per day is small in the middle of August increasing rapidly through the end of August and the first half of September. During this period the daytime abundance of active chlorine is about constant it is the rapidly rising number of sunlit hours per day that is causing the increase.

PSC Formation I As the polar stratospheric temperature goes below about 195 K water vapour and nitric acid condense to form Type I polar stratospheric clouds (PSCs).

56 PSC Formation I As the polar stratospheric temperature goes below about 195 K water vapour and nitric acid condense to form Type I polar stratospheric clouds (PSCs). At colder temperatures the air is saturated with respect to water ice so that much larger ice particles form. The exact composition and formation pathways of PSCs is the subject of ongoing research.

PSC Formation II PSCs are often associated with local cooling due to gravity or lee-waves This photo is taken at the Kiruna airport on 25 January 2000.

57 PSC Formation II PSCs are often associated with local cooling due to gravity or lee-waves This photo is taken at the Kiruna airport on 25 January (Lee waves form when there is a strong wind from the west over the Scandinavian mountains. The wave structure can be clearly seen.) (from NILU/THESO 2000).

58 Chlorine Activation Normally 95 % of total inorganic chlorine, Cl y, in the lower stratosphere is in the form of HCl and ClONO 2 (reservoir forms). In the Antarctic winter stratosphere nearly all the Cl y is in the form of ClO and ClOOCl (active forms) This repartioning is occurs because of heterogeneous reactions on the surface of PSC particles i.e. These reactions convert long-lived Cl reservoir species into unstable forms of chlorine Cl 2 and HOCl. Upon exposure to sunlight Cl 2 and HOCl are easily photolysed to Cl atoms which are rapidly converted to ClO or ClOOCl molecules. The relative abundance is set by temperature, solar zenith angle (SZA) and the total abundance of active chlorine.

59 Denitrification and Dehydration As PSC particles grow they remove HNO 3 and H 2 O from the gas phase. As H 2 O is about 10 3 times more abundant than HNO 3 the growth of type I PSCs can denitrify the polar stratosphere while leaving the amount of water vapour essentially unchanged. As Type II particles are mostly composed of water ice the existence of these particles is associated with both denitrification and dehydration Type II clouds particles have radii of 5-20 µm so can fall a kilometre a day. This transports water and HNO 3 to lower altitudes leading to irreversible dehydration and denitrification.

60 Effect of Denitrification Denitrification is important as HNO 3 is able to reverse chlorine activation through The removal of HNO 3 from the air mass through sedimentation of HNO 3 rich PSC particles shuts off this deactivation pathway and allows active chlorine abundances to remain high.

61 Summary of Chlorine Activation and Denitrification

62 Ozone Loss Summary Darkness PSCs Sunlight Return of sunlight at the end of winter begins heating the polar vortex. This reduces the latitudinal pressure gradient which in turn reduces the wind speeds around the vortex. The stability and isolation of the vortex is reduced. With the final stratospheric warming the Antarctic vortex breaks into smaller air masses which mix with midlatitude air.

63 2011 Ozone Hole 13 October

68 Appendix I: Brief History of Ozone 1878: Cornu suggested UV cut-off of solar radiation was due to the atmosphere 1880: Hartley suggested ozone (Hartley band nm) might exist in the atmosphere 1890: Huggins found bands in the spectrum of Sirius ( nm) thought they were stellar. 1912: Fabry & Buisson made laboratory and atmospheric absorption measurements and concluded there was about 0.5 atm-cm of O : Fowler & Strutt identified that the Huggins bands in the spectrum of Sirius where due to atmospheric O : Fabry & Buisson measured more accurately and found 0.3 atm-cm of O : Lindemann & Dobson measured air density at high altitudes from meteor trails Above 50 km densities were 100 to 1000 times higher than expected Explained by solar heating due to O 3 warm region larger scale height Confirmed by sound propagation (Whipple in 1932) 1924: Dobson spectrophotometer 1926: European Network 1930: Chapman scheme the maintenance of an unstable species must be the photolysis of O 2 by solar photons in the UV.

69 Appendix I: Brief History of Ozone Post WW II Bates & Nicolet showed that catalytic recombination involving hydroxyl radicals, derived from water vapour, played a part. 1970s Johnston show that the exhausts from a fleet of supersonic transports could strongly impact the ozone layer 1973: Rowland and Molina first discover that chlorofluorocarbons (CFCs) can destroy stratospheric ozone. They establish the importance of chlorine from their study of the fate chlorofluorocarbons from refrigerants and spray-can propellants 1974: First US government hearings are held on the CFC-ozone theory. 1975: Natural Resources Defense Council sues Consumer Product Safety Commission for a ban on CFCs used in aerosol spray cans. Lawsuit is rejected due to insufficient evidence that CFCs harm the ozone layer. 1976: National Academy of Sciences releases report verifying Rowland- Molina hypothesis, but recommends postponing government action. 1977: The United Nations Environmental Programme holds first international meeting to discuss ozone depletion. 1978: United States bans CFCs used in aerosols. Total Ozone Mapping Spectrometer (TOMS) is launched. 1984: Joe Farman observes 40 % ozone loss over Antarctica during austral spring.

70 Appendix I: Brief History of Ozone 1985: Vienna Convention, calling for additional research, is signed. Satellite images confirm existence of an Antarctic ozone hole. 1986: International negotiations resume in Geneva. United States requests global CFC reductions of 95 % over the next 10 years. 1987: Montreal Protocol, for CFC reductions of 50% by 1999, is signed. Antarctic studies find chlorine to be primary cause of ozone depletion. 1990: London Agreement to strengthen the Montreal Protocol and to complete phase-out of CFCs by : Upper Atmospheric Research Satellite (UARS) is launched.. Mt. Pinatubo erupts, increasing natural levels of atmospheric chlorine : Record levels of ClO, 1.5 parts per billion, are measured over Bangor, Maine. Ozone depletion rates of up to 20% are found in the Northern Hemisphere. Maximum losses of 40 to 45 % discovered over Russia. Parties to the Montreal Protocol meet in Geneva and agree to a 75 % reduction in CFCs by 1994 and overall phase-out by January of Production grace period, to supply CFCs for essential purposes and the needs of developing countries, is extended to : Professor Paul Crutzen, Professor Mario Molina, and Professor F. Sherwood Rowland receive the Nobel Prize in Chemistry

71 Appendix II: Dobson Unit A widely used quantity is column O 3 i.e. the total number of molecules in a column of unit crosssectional area running from the Earth s surface to the top of the atmosphere. A Dobson Unit (DU) is the height of a column of ozone compressed to STP then expressed as milli-atm-cm. Two reasons why this measure is useful It is relatively easy to measure from ground and from space There is an extensive record of column O 3 measurements stretching back to the 1920s

Measurements of Ozone. Why is Ozone Important?

Measurements of Ozone. Why is Ozone Important? Anthropogenic Climate Changes CO 2 CFC CH 4 Human production of freons (CFCs) Ozone Hole Depletion Human production of CO2 and CH4 Global Warming Human change of land use Deforestation (from Earth s Climate: