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Physik der Atmosphäre II

U. Platt SS 2009, Tuesday, 16:15-17:45, INF 229 , SR 108/110. Physik der Atmosphäre II. Physics of the Atmosphere Physik der Atmosphäre. WS 2010 Ulrich Platt Institut f. Umweltphysik R. 424 Ulrich.Platt@iup.uni-heidelberg.de. Last Week.

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Physik der Atmosphäre II

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  1. U. Platt SS 2009, Tuesday, 16:15-17:45, INF 229 , SR 108/110 Physik der Atmosphäre II Physics of the Atmosphere Physik der Atmosphäre WS 2010 Ulrich Platt Institut f. Umweltphysik R. 424 Ulrich.Platt@iup.uni-heidelberg.de

  2. Last Week • The carbon cycle has a strong influence on our climate(both, in natural changes and anthropogenic changes) • Important carbon reservoisrs are the sediments, the ocean, the land biomass, and the atmosphere • Time constants in the carbon cycle range from years to 105 years • Oceans get more acidic  less carbon uptake • The role of the biomass is unclear

  3. Contents

  4. 1st Definition: Radicals are a combination of atoms that have one or several unpaired electrons. (i.e. S  0). This definition, however, includes O2, NO or NO2. 2nd Definition: Radicals are (short lived) combinations of atoms, which play the role of elements and can combine like these with elements and with themselfes. (Justus von Liebig) Ozone, Hydrogen Radicals (HXOY ), and the Oxidation Capacity First question: What are „Free Radicals“? Examples: OH, HO2, as well as the "stable" Molecules H2O2 (hydrogen peroxyde) and CH3OOH (methyl hydroperoxide).

  5. Why are Radicals important? Atmospheric mixing ratios of radicals are exceedingly low e.g. about 1 OH-radical for every 1013 air molecules But: In chemical reactions with molecules the radical status is „hereditary“ e.g.: OH + CO  CO2 + H Example: oxyhydrogen gas (Knallgas): H2 + O2 HO2 + H „Start“ H2 + HO2 OH + H2O Propagation H2 + OH H + H2O (inherit radical status) H + O2 OH + O Branching O + H2 OH + H(from one radical we get two) OH + HO2 H2O + O2 Termination Net: 2 H2 + O2 2 H2O Consequence: The radikal concentration and thus the speed of the reaction can increase exponentially. Does that also happen in the atmosphere?

  6. First suggestion of OH reactions in the atmosphere by B. Weinstock 1969.Formation of OH by:O3 + h O(1D) + O2(1) O(1D) + H2O OH + OHRole of OH in the formation of CO and formaldehyde suggested [H. Levy 1971].Role of OH in tropospheric ozone formation (P. Crutzen 1974) Other Radicals (by either definition)? O2(1), Cl-atoms, NO3 ... Free Radicals - Which ones are Important in Atmospheric Chemistry? HOX - Cycle OH  Degradation of most VOC  Key intermediate in O3 formation  NOX NOY conversion HO2  Intermediate in O3 formation  Intermediate in H2O2 formation RO2  Intermediate in ROOR´ formation  Aldehyd – precursor  PAN – precursor  Intermediate in O3 formation NO3  Degradation of certain VOC  NOX NOY conversion (via N2O5)  RO2 – precursor XO  Catalytic O3 destruction (X = Cl, Br, I)  Degradation of DMS (BrO)  Particle formation (IO)  Change of the Leighton – Ratio X  Degradation of most (some) VOC: Cl (Br)  Initiates O3 formation  RO2 – precursor

  7. Degaradation of Trace Species by Reaction with OH- or other Free Radicals. Mean Lifetime in Hours against Radical Reaktions

  8. OH Formation in the Atmosphere The most important source of OH-radicals is the photolysis of ozone by UV-radiation with wavelengths < ca. 320 nm (R1a) (UV-B region) or  411nm (R1b): O3 + h  O(1D) + O2(1) (R1a) O3 + h O(1D) + O2 (3) (R1b) O3 + h O(3P) + O2 (3) (R1c) Reaction R1b ist spin forbidden, thus unlikely. O (1D) + M  O (3P) + M (R2) O (1D) + H2O  OH + OH (R3) OH - yield:

  9. Photochemical OH - Formation Photolysis Frequency:

  10. OH Rate of Formation cm-3s-1 Month Annual Variation of the Photochemical OH – Formation Rate at Mid-Latitudes

  11. The HXOY - Cycle

  12. Diurnal Variation of the Various OH – Sources in an Urban Region (Berlin)(Alicke 2000)

  13. Nitrous Acid (HONO) H2O(g) H2O(ads) (a) NO2(g) NO2(ads) (b) H2O(ads) + NO2(ads) H2O.NO2(ads) (c) NO2(ads) + H2O.NO2(ads) HNO3(ads) + HONO(g) (d) [ HNO3(ads) + HONO(g) 2 NO2(g) + H2O(g) ]

  14. 82% + O3 + NO + NO The OH – Cycle as Function of the NOX - Concentration

  15. Model Calculations of the OH - Concentration as Function of the NOX (Oxides of Nitrogen) - Level(D. Poppe)

  16. Concentrations of OH and HO2 as a function of the NOX level Conditions: 37.6 ppb ozone, 88.7 ppb CO, 0.92 ppb CH2O, J(O3)=9.110‑6 s‑1, J(NO2)= 9.110‑6 s‑1. The HO2 concentration and therefore H2O2 production rate are highest in unpolluted air at NOX levels below a few 100ppt. Figure from Ehhalt [1999]

  17. Observed Diurnal Variation of the OH - Concentration Red Circles: Measurements by Laser-Induced Fluorescence, LIF by A. Hofzumahaus et al., Jülich) Blue, drawn Line: Ozone – Photolysis Frequency

  18. BERLIOZ81% of OH dependent on J(O3)

  19. NOX Dependence of OH -Observation Forschungszentrum Jülich

  20. OH (LIF) Summer & Winter D.E.Heard, J.D.Lee, D.J. Creasey, University of LeedsL.J.Carpenter (University of York) c.f. George et al., 1999

  21. Noontime RO2 Peak Radical Gap: no J(O3) but still J(NO3) Nighttime RO2 Peak: NO3 + VOC Average ROX diurnal profile (Chemical Amplifier) during BERLIOZ(July 14 to 15, 17 to 18, 24 to 26, Aug. 3, total of 8 days in 1998) NO3 Data from Dieter Perner, in Platt et al. 2002

  22. HOX in the Upper Troposphere Too much HOX ? --> No, have to include other sources Photolysis of acetone, aldehydes, peroxides, ... However, do these additional sources always explain observed HOX? OH in the UT controlled by P(HOx) and NOx Jaeglé et al., Atmos. Env. 2001

  23. HOX Source Gases in the Free Troposphere(Pacific Ocean, spring 1999, Singh et al., Nature 2001) SH,0-30oS, 165oE-100oW NH,0-30oN, 170o-120oW

  24. Nighttime Peroxy Radicals Salisbury et al. 2001 NO3 + olefins O3 + olefins NO3 Data from Dieter Perner, Platt et al. 2002

  25. hohes NOX niedriges NOX Observed Diurnal Variations of OH-, HO2-, RO2- and NOX- Concentrations During BERLIOZ 1998 (Platt et al. 2002)

  26. What is the Evidence for OH in theAtmosphere ?

  27. What is the Evidence for HO2 (RO2) in the Atmosphere ?

  28. Simplified Outline of the NOX- (=NO + NO2) and NOY- Cycles Temperature dependent!

  29. Photolysis of the Nitrate Radical The NO3 Spectrum Product Yield of NO3 Photolysis

  30. NO3 Field Measurements Cavity Ringdown(Boulder, CO)Brown et al. 2001 - Ravishankara DOAS(Edwards AFB)Platt et al. 1984

  31. NO3 Vertikal Profile - Schematic z NO2 1 km Low NO3 production(little NO3) NO, olefins NO3 high NO3 destruction(lot of NO, terpenes) 0

  32. SZA Direction of Observation Terminator Odenwald Mountains Z Institut für Umweltphysik Distance NO3 - Vertical Profile Measurements

  33. Relative Importance of the Different NO3 - Loss Processes During BERLIOZ 1998 Geyer et al. 2001

  34. Contribution of NO3 to the Atmospheric Oxidation Capacity in Pabstthum (BERLIOZ 1998) Geyer et al. 2001

  35. The Oxidation Capacity by Radical X

  36. Initial Idea: Stratosphere O3 – Flux 5-81010Molec.cm-2s-1 Troposphere No Chemistry, since ( 242 nm) = 0 O3 – Depos. 3-61010Molec.cm-2s-1 Today: Stratosphere O3 – Flux 5-81010Molec.cm-2s-1 Troposphere CO, HC O3 O3 + h + H2O  OH 30-501010Molec.cm-2s-1 O3 – Depos. 3-61010Molec.cm-2s-1 Ozone Formation in the Troposphere

  37. Penetration-Depth of UV - Radiation Intensity at 0, 20, 30, 40, 50 km altitude) DeMore et al., 1997

  38. NOX- and HOX - Katalysis of the Photochemical Ozone Formation in the Troposphere Ozone formation rate P(O3) : P(O3) = [NO]·(k1[HO2]+ki·[RO2]i)

  39. Disturbance of the Leighton Ratio and the rate of O3 Formation There is no net ozone production. However, if other reactions, in particular by Peroxy radicals oxidise NO to NO2: RO2 + NO  NO2 + RO (R4) Ozone will be formed at the rate PO3. Also the Leighton Ratio will be reduced: The rate of ozone formation, PO3 in a given airmass can be calculated from measurements of the Leighton Ratio[NO]/ [NO2]=L1 together with [O3], and J: Since PO3 relates to the concentration of peroxy radicals (RO2), there concentration can also be inferred from these measurements.

  40. Lines of constant O3 mixing ratios (ppb) NOX-limited Hydrocarbon-limited Smog Chamber Experiments  Ozone Isoplets

  41. Ozone Formation as a Function of NOX Level Ehhalt 1998, Science 279. No. 5353, 1002 – 1003, DOI: 10.1126/science.279.5353.1002

  42. Observed O3 Production Rates 'Southern Oxidant Study‘ June 15 to July 15, 1999 at Cornelia Fort Airpark, Nashville, Tennessee [Thornton et al. 2002] Averaged O3 production rates PO3 calculated from simultaneous observations of NO, NO2, O3, OH, HO2, H2CO, actinic flux, and T. Data were placed into three PHOx bins: high (0.5 < PHOx < 0.7 ppt/s, circles), moderate (0.2 < PHOx < 0.3 ppt/s, squares), and low (0.03 < PHOx < 0.07 ppt/s, triangles), and then averaged as a function of NO. All three PHOx regimes demonstrate the expected dependence on NO: PO3 increases linearly with NO for low NO (<600 ppt NO), PO3 becomes independent of NO for high NO (>600 ppt NO). Crossover between NOX - limited and NOX - saturated PO3 occurs at different levels of NO in the three PHOx regimes.

  43. Diurnal variation of O3 levels in different air masses during August 11-14, 2000: Forrest (Weltzheimer Wald) and city of Heilbronn (southern Germany). Landesanstalt für Umweltschutz Baden-Württemberg and UMEG)

  44. Vertical profiles of the concentrations of ozone, NO, H2O2, and humidity (dew point). The maximum H2O2 concentration occurs in an altitude range layer (ca. 1.1 - 1.4 Km) with high humidity but low NO [Tremmel et al. 1993].

  45. The Evolution of Tropospheric Ozone Monks et al. 2009

  46. Ozon – Annual Variation at Different Altitudes Isobars 200, 300, 500, 800 hPa Logan, J. Geophys. Res., 16115-16149, 1999.

  47. Ozon – Annual Variation at Sea Level Logan, J. Geophys. Res., 16115-16149, 1999.

  48. Summary • OH is the most important free radical in the Atmosphere, since it inititates the degradation of most oxidisable air pollutants it is sometimes called the „cleansing agent of the atmosphere“ • However, there are several other relevant Radical in the Atmosphere • Ozone is a central species, in the troposphere it is mostly formed by reactions catalysed by OH

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