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Tropospheric Aerosol

Tropospheric Aerosol. Description of tropospheric aerosol Chemical composition Solubility, aqueous-phase chemistry. Ein Beispiel aus der Praxis …. The „Asian Brown Cloud“. A 3km dust layer over Asia causing health problems and climate change effects.

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Tropospheric Aerosol

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  1. Tropospheric Aerosol Description of tropospheric aerosol Chemical composition Solubility, aqueous-phase chemistry

  2. Ein Beispiel aus der Praxis …

  3. The „Asian Brown Cloud“ A 3km dust layer over Asia causing health problems and climate change effects

  4. Direkte Beobachtung von Aerosolen aus dem Weltraum Rauchwolken von Waldbränden nahe Sydney (Dez. 2001)

  5. Sources of aerosol • windborne dust • sea spray • volcanoes • fossil fuel combustion • pollen and plant fragments

  6. Definitions Aerosol: suspension of fine solid or liquid particles in a gas primary aerosol: emitted directly as particles secondary aerosol: formed in the atmosphere by gas-to-particle conversion fine aerosol: particles < 2.5 µm coarse aerosol: particles > 2.5 µm

  7. Definitions (2) dust: solid particles produced by mechanical disintegration of material (D > 1 µm) smoke: small gas-borne particles from incomplete combustion (D > 0.01 µm) fume: solid particles generated from vapour state (usually after volatilization from melted substances) (D < 1 µm) haze: water droplets, pollutants, and dust (D < 1 µm)

  8. Chemical composition Tropospheric aerosols contain: • sulfate • ammonium • nitrate • sodium • chloride • trace metals • carbonaceous material • crustal elements • water after Seinfeld&Pandis, 1998

  9. CCN Without particles, no clouds would form! • Cloud Condensation Nuclei: particles that become activated and grow to droplets in the presence of supersaturated water vapour • for marine stratiform clouds, the supersaturation is 0.1-0.5%; minimum particle diameter is 50-140 nm • CCN number concentrations: < 100 cm-3 in remote marine areas to > 10000 cm-3 in polluted areas

  10. Cloud condensation Liquid Water Content (LWC): L = 0.05-3 gH2O/m3 Droplet size:r = 1 µm - 50 µm

  11. Absorption equilibrium A(g) A(aq) for dilute solutions: [A(aq)] = HA· pA partial pressure of A in gas-phase (atm) aqueous-phase concentration (mol L-1) Henry coefficient (mol L-1 atm-1) Gas/Aqueous phase partitioning soluble fraction of A in aqueous phase:

  12. Gas/Aqueous-phase partitioning very soluble gases: H2O2, HNO3, NO3

  13. pH Upon dissolution in water, several species will form ions, e.g. H2O, and CO2. Water: H2O H+ + OH- ; equilibrium constant K = [H+][OH-] (at 298K, only 2 µmol/L ions versus 55.5 mol/L H2O) pH = -log10[H+]

  14. Dissolution Generally: K = [X+][Y-] / [X•Y] TABLE6.4

  15. CO2(g) Khc Kc2 Kc1 H+ + CO32- H+ + HCO3- CO2•H2O CO2(aq) = bicarbonate carbonate CO2 uptake by the oceans total dissolved carbon: effective Henry coefficient

  16. CO2 uptake by the oceans (2)

  17. SO2 uptake by droplets

  18. Other species of interest: SO2 HSO3- SO32-, NH3 NH4+, HNO3 NO3- Acidity of (clean) rainwater The atmospheric CO2 concentration has an influence on the acidity of rain water: Electro neutrality demands that This can be rearranged to: With given temperature and pCO2, [H+] can be computed, from which all other ion concentrations can be deduced. For T=298K and pCO2 = 350 ppm, pH = 5.6

  19. S(IV)  S(VI) oxidation The conversion of dissolved SO2 to sulfate is the most important chemical transformation in cloud water. If one S(IV) ion is consumed in a reaction, it will quickly be replaced, because the equilibrium between SO2•H2O, HSO3-, and SO32- is established very fast (milliseconds), and because the dissociation of dissolved SO2 enhances its solubility. Pathways for S(IV) to S(VI) conversion include reaction with O3, H2O2, O2 (catalized by Mn(II) and Fe(III)), OH, NO3, ... Examples: S(IV) + O3 S(VI) + O2(slow in gas-phase, rapid in aqueous-phase) HSO3- + H2O2 SO2OOH- + H2O , followed by SO2OOH- + H+ H2SO4

  20. Aerosol size distribution Consider only spherical shape ... number density distribution surface distribution volume distribution

  21. Deliquescence and (re)cristallisation The thermodynamic phase of an aerosol particle depends on the humidity. Dry particles will remain solid until the relative humidity reaches a threshold. The, the particle spontaneously absorbs water and grows (deliquescence). Subsequent drying leads to recristallisation, but at much lower relative humidities (Hysteresis effect).

  22. Observations • Aerosols predominantly internally mixed • Externally mixed contribution The Hamburg Aerosol Model (HAM) in ECHAM5 1) Introduction - Requirements • Prediction of size-distribution, composition and mixing-state essential • Aerosol-cloud interaction • Sink processes • Radiative effects • Computational constraints • Balance in complexity: GCM vs. Aerosol Model

  23. 1. Discretize aerosol distribution in bins and calculate their temporal development: The Hamburg Aerosol Model (HAM) in ECHAM5 • Prescribe aerosol distribution function and calculate the temporal development of its moments: Aerosol Modelling - Methods

  24. MODES IN M7 SOLUBLE / MIXED INSOLUBLE NUCLEATION (r < 0.005 µm) 1N1, MSO4 AITKEN (0.005 µm < r < 0.05 µm) 2N2, MSO4, MBC, MOC 5N5, MBC, MOC Mixing State of the compounds: ACCUMULATION (0.05 µm < r < 0.5 µm) 3N3, MSO4, MBC, MOC, MSS, MDU 6N6, MDU • Sulfate • Black Carbon • Organic Carbon • Sea Salt • Dust The Hamburg Aerosol Model (HAM) in ECHAM5 COARSE (0.5 µm < r ) 4N4, MSO4, MBC, MOC, MSS, MDU 7N7, MDU Aerosol Representation in HAM • Resolve aerosol distribution by 7 log-normal modes • Each mode is described by three moments: • Number, Median Radius  Mass, Standard Deviation (fixed)  Reduction of the number of transported tracers to 28

  25. Optische Dichte von Aerosolen JANUAR JULI from Chin et al., 2002

  26. The Hamburg Aerosol Model (HAM) in ECHAM5 Mixing State

  27. ECHAM5 ECHAM5 The Hamburg Aerosol Model (HAM) in ECHAM5 IMPROVE IMPROVE Mount Rainier (122 W, 47 N) Mamoth Cave (86 W, 37 N) Surface Particle Mass

  28. The Hamburg Aerosol Model (HAM) in ECHAM5 70 S – 20 S Total aerosol number annual mean Pacific profile; Averaged over 70S - 20S and 130 E - 90 W Pacific measurement composite(From Clarke and Kapustin; JAS; 2002) Number Concentrations

  29. Bibliography Material for this lecture comes mostly from • Seinfeld, J., and Pandis, S., Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, Wiley, New York, …, 1998.

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