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helyett Turányi Tamás

The chemistry of the troposphere and stratosphere Prof. M. J. Pilling The University of Leeds (UK). helyett Turányi Tamás. Eredeti Pilling előadások: 2009. február 23-27 A PowerPoint file-ok és a videofelvételek letölthetők: http://garfield.chem.elte.hu/Turanyi/oktatas/Pilling.html.

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helyett Turányi Tamás

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  1. The chemistry of the troposphere and stratosphereProf. M. J. PillingThe University of Leeds (UK) helyett Turányi Tamás Eredeti Pilling előadások: 2009. február 23-27 A PowerPoint file-ok és a videofelvételek letölthetők: http://garfield.chem.elte.hu/Turanyi/oktatas/Pilling.html

  2. Structure of the atmosphere

  3. Temperature and pressure variations in the atmosphere Heating by exothermic photochemical reactions Convective heating from surface. Absorption of IR (and some VIS-UV) radiation z Barometric equation p = p0exp(-z/Hs)

  4. Atmospheric transport • Random motion – mixing • Molecular diffusion is slow, diffusion coefficient D ~ 2x10-5 m2 s-1 • Average distance travelled in one dimension in time t is ~(2Dt). • In the troposphere, eddy diffusion is more important: • Kz ~ 20 m2 s-1. Molecular diffusion more important at v high altitudes, low p. Takes ~ month for vertical mixing (~10 km). Implications for short and long-lived species. • Directed motion • Advection – winds, e.g. plume from power station. • Occurs on • Local (e.g. offshore winds) • Regional (weather events) • Global (Hadley circulation)

  5. Winds due to weather patterns As air moves from high to low pressure on the surface of the rotating Earth, it is deflected by the Coriolis force.

  6. Global circulation – Hadley Cells Intertropical conversion zone (ITCZ) – rapid vertical transport near the equator.

  7. Horizontal transport timescales

  8. Stratospheric chemistry

  9. O2 O(3P) + O(3P) Threshold  = 242 nm O2 O(3P) + O(1D) Threshold  = 176 nm

  10. UV absorption spectrum of O3 at 298 K Hartley bands Small but significant absorption out to 350 nm (Huggins bands) Very strong absorption Photolysis mainly yields O(1D) + O2, but as the stratosphere is very dry (H2O ~ 5 ppm), almost all of the O(1D) is collisionally relaxed to O(3P)

  11. Integrated column - Dobson unit

  12. Timescale Slow (J is small) Fast < 100 secs Fast ~ 1000 s Slow (activation barrier)

  13. Altitude/km z J1 • J1 = rate of O2 photolysis (s-1) • J3 = rate of O3 photolysis (s-1) • Graph shows the altitude dependence of the rate of photolysis of O3 and O2. Note how J1 is very small until higher altitudes • The ratio J1/J3increases rapidly with altitude, z • As pressure  exp (-z) then [O2]2 [M] decreases rapidly with z J3 J1 This balance results in a layer of O3 J3

  14. HOW GOOD IS THE CHAPMAN MECHANSIM? The Chapman mechanism overpredicts O3 by a factor of 2. Something else must be removing O3 (Or the production is too high, but this is very unlikely) Altitude / km

  15. Catalytic ozone destruction The loss of odd oxygen can be accelerated through catalytic cycles whose net result is the same as the (slow) 4th step in the Chapman cycle Uncatalysed: O + O3  O2 + O2 k4 Catalysed: X + O3 XO + O2 k5 XO + O  X + O2 k6 Net rxn: O + O3  O2 + O2 X is a catalyst and is reformed X = OH, Cl, NO, Br (and H at higher altitudes) Reaction (4) has a significant barrier and so is slow at stratospheric temperatures Reactions (5) and (6) are fast, and hence the conversion of O and O3 to 2 molecules of O2 is much faster, and more ozone is destroyed.

  16. The sources of X

  17. CFC’s are not destroyed in the troposphere. They are only removed by photolysis once they reach the stratosphere.

  18. Data from NOAA CMDL Ozone depleting gases measured using a gas chromatograph with an electron capture detector (invented by Jim Lovelock) These are ground-based measurements. The maximum in the stratosphere is reached about 5 years later 45 years 100 years Why are values in the N hemisphere slightly higher?

  19. Removal of the catalyst X. Reservoir is unreactive and relatively stable to photolysis. X can be regenerated from the reservoir, but only slowly. [X] is reduced by these cycles. For Cl atom, destroys 100,000 molecules of O3 before being removed to form HCl “Do nothing” cycles Ox is not destroyed Reduces efficiency of O3 destruction

  20. Interactions between different catalytic cycles Reservoir species limit the destruction of ozone ClONO2 stores two catalytic agents – ClO and NO2

  21. Effects of catalytic cycles are not additive due to coupling Mechanism Ozone Column (Dobson units) Chapman only (C) 644 C + NOx 332 C + HOx 392 C + ClOx 300 C + NO x+ HOx + ClOx 376 Coupling to NO leads to null cycles for HOx and ClOx cycles Increase of Cl and NO concentrations in the atmosphere has less effect than if Cl or NO concentrations were increased separately (because ClOx and NOx cycles couple, hence lowering [X])

  22. Bromine cycle Br + O3 BrO + O2 Cl + O3 ClO + O2 BrO + ClO  Br + ClOO ClOO  Cl + O2 Net 2O3 3 O2 Br and Cl are regenerated, and cycle does not require O atoms, so can occur at lower altitude Source of bromine : CH3Br (natural emissions from soil and used as a soil fumigant) Halons (fire retardants) Catalytic cycles are more efficient as HBr and BrONO2 (reservoirs for active Br) are more easily photolysed than HCl or ClONO2 But, there is less bromine than chlorine Bromine is very important for O3 destruction in the Antarctic stratosphere where [O] is low

  23. Total Ozone Mapping Spectrometer (TOMS) Monthly October averages for ozone, 1979, 1982, 1984, 1989, 1997, 2001 Dobson units (total O3 column)

  24. October 2000 “For the Second time in less than a week dangerous levels of UV rays bombard Chile and Argentina, The public should avoid going outside during the peak hours of 11:00 a.m. and 3:00 p.m. to avoid exposure to the UV rays” Ushaia, Argentina The most southerly city in the world

  25. At 15 km, all the ozone disappears in less than 2 months This cannot be explained using gas-phase chemistry alone US Base in Antarctica

  26. Steps leading to ozone depletion within the Antarctic vortex ClO+BrOCl+Br+O2

  27. Simultaneous measurements of ClO and O3 on the ER-2 Late August 1987 September 16th 1987 Still dark over Antarctica Daylight returns The “smoking gun” experiment – proved the theory was OK

  28. Ozone loss does appear in the Arctic, but not as dramatic Some years see significant depletion, some years not, and always much less than over Antarctica Above Spitzbergen

  29. Tropospheric chemistry

  30. Global tropospheric chemistry Questions to be addressed: • Many organic compounds emitted to the atmosphere are oxidised, eventually forming CO2 and H2O. What determines the oxidising capacity of the atmosphere? • Methane is a greenhouse gas, whose atmospheric concentration has more than doubled since the industrial revolution. What governs it concentration? • Tropospheric oxidation is strongly influenced by NOx, whose lifetime is ~ 1 day. How is NOx transported to regions with no NOx emissions? • Ozone is a secondary pollutant. In the boundary layer it affects human health, growth of vegetation and materials. It is also a greenhouse gas. What governs its concentration?

  31. Methaneoxidation CH4 + OH (+O2)  CH3O2 + H2O CH3O2 + NO  CH3O+ NO2 CH3O + O2  HO2 + HCHO HO2 + NO  OH + NO2 HCHO + OH (+O2)  HO2 + CO + H2O HCHO + hn  H2 + CO HCHO + hn (+2O2)  2HO2 + CO Note: 2 x(NO  NO2) conversions HCHO formation provides a route to radical formation.

  32. General oxidation scheme for VOCs O3 + h  O1D + O2 O1D + H2O 2OH OH+ RH (+O2)  RO2+ H2O RO2+ NO  NO2+ RO RO  HO2(+R’CHO) HO2+ NO  OH +NO2 NO2+h NO + O; O + O2  O3 OVERALL NOx + VOC + sunlight  ozone The same reactions can also lead to formation of secondary organic aerosol (SOA)

  33. THE OH RADICAL: MAIN TROPOSPHERIC OXIDANT Primary source: O3 + hn g O2 + O(1D) (1) O(1D) + M g O + M (2) O(1D) + H2O g 2OH(3) Sink: oxidation of reduced species CO + OH g CO2 + H CH4 + OH g CH3 + H2O HCFC + OH Major OH sinks g H2O + … GLOBAL MEAN [OH] ~ 1.0x106 molecules cm-3

  34. Other oxidising species NO3 NO2 + O3 NO3 + O2 NO2 + NO3 + M  N2O5 + M NO3 is rapidly lost in the day by photolysis and reaction with NO ( NO2), so that its daytime concentration is low. It is an important night time oxidant. It adds to alkenes to form nitroalkyl radicals which form peroxy radicals in the usual way. O3 Ozone reactswith alkenes to forma carbonyl + an energised Criegee biradical. The latter can be stabilised or decompose. One important reaction product is OH: O3 reactions with alkenes can act as a source of OH, even at night.

  35. Sources: Natural 160 Anthropogenic 375 Total 535 Natural Sources: wetlands, termites, oceans… Anthropogenic Sources: natural gas, coal mines, enteric fermentation, rice paddies, Sinks: Trop. oxidation 445 by OH Transfer to 40 stratosphere Uptake by soils 30 Total 515 Global budget for methane (Tg CH4 yr-1) • Notes: • The rate of oxidation is k5[CH4][OH], where the concentrations • are averaged over the troposphere • 2. Concentrations of CH4 have increased from 800 to 1700 ppb since pre-industrial times • 3. Methane is a greenhouse gas.

  36. HISTORICAL TRENDS IN METHANE Historical methane trend Recent methane trend Recent measurements at Mace Head in W Ireland. 1mg m-3 = 0.65 ppb NB – seasonal variation – higher in winter

  37. GLOBAL DISTRIBUTION OF METHANENOAA/CMDL surface air measurements • Seasonal dependence – higher in winter than summer (maximum in NH correlates with minimum in SH). • NH concentrations > SH – main sources are in NH; slow transport across ITCZ.

  38. GLOBAL BUDGET OF CO

  39. GLOBAL DISTRIBUTION OF CONOAA/CMDL surface air measurements • Compare CH4. What are the differences and why? (Rate coefficients at 298 K/10-12 cm3 molecule-1 s-1: CH4: 7x10-3; CO: 0.24)

  40. Global VOC emissions (Tg yr-1) Anthropogenic: fuelproduction and distribution 17; fuel consumption 49; road transport 36; chemical industry 2; solvents 20; waste burning 8, other 10. Total 142 Tg yr-1 Biogenic: isoprene 503; monoterpenes 127; other reactive VOCs 260, unreactive VOCs 260; Total 1150 Tg yr-1 Typical atmospheric lifetimes (for [OH] = 1x106 molecule cm-3) t = 1/k[OH] CH4 6 yrisoprene 2.7 h CO 48 days ethane 46 days benzene 6 days ethene 30 h

  41. Global budget for NOx • Global sources (Tg N yr-1): Fossil fuel combustion 21; Biomass burning: 12 Soils 6 Lightning 3 Ammonia oxidation 3 Aircraft 0.5 Transport from strat 0.1 • Coupling (rapid - ~ 1 minute in the day) NO + O3→ NO2 + O2 NO2 + Light → NO + O; O + O2 + M → O3 + M Also HO2 + NO → NO2 + OH • Loss OH + NO2 + M → HNO3 + M Rainout of HNO3 • Lifetime of NOx is about 1 day. NOx is a key component in ozone formation. Can it be transported to regions where it is not strongly emitted?

  42. PEROXYACETYLNITRATE (PAN) AS RESERVOIR FOR LONG-RANGE TRANSPORT OF NOx

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