Physik der Atmosphäre II

Transcription

1 Physik der Atmosphäre II U. Platt SS 2009, Tuesday, 16:15-17:45, INF 229, SR 108/ Introduction - Gas-Phase Reaction Kinetics Pl Tropospheric Chemistry: O 3, H x O y, Oxidation Capacity Pl Tropospheric Chemistry: N-, S-, Halogen Cycles Pl Stratosphere, Radiation and Matter Cycles Pl Strat. Chemistry: Chapman Cycle and Extensions Pl Strat. Chemistry: Halogen Chemistry, The Ozone Hole Pl Isotopes in Atmospheric Research Pl The Carbon Cycle Pl The Hydrological Cycle Pl Liquid Phase Chemistry (atmospheric Aerosol) Pl Aerosol Physics I: Aerosol Mechanics Pl Aerosol Physics II: Particle Formation, Particle Growth Pl Measurement Techniques Pl Modeling Atmospheric Chemistry and Physics Pl

2 Aerosol 1. Introduction 2. Sources and distribution 3. Aerosol physics 4. Optical properties 5. Measurement techniques

3 1. Aerosol - Introduction correct (pedantic?) Definition: Aerosol is the mixture of suspended solid or liquid particles and the carrier gas. sloppy Definition: Suspended solid or liquid particle. Haze Particles from Pasadena, 1973 Husar & Shu 1975

4 Ranges of Aerosol Diameter, Surface Area, and Volume

5 Why should we be interested in Aerosols? Climate (Greenhouse, attenuation of solar radiation net cooling, absorption of solar radiation local heating) Light scattering (Fog, Haze, Smog) Cloud formation and properties Health effects Surface for reactions Green house heating

6 IPCC 2001 Global, Annual, Mean Radiative Forcing

7 IPCC 2007 Global, Annual, Mean Radiative Forcing

8 IPCC 2007 Direct and Indirect Aerosol Effects on Climate

9 Summer Day in the Hunsrück Mountains

10 Aerosol - Properties Number Density [particles/cm 3 ] Surface Density [cm 2 /cm 3 ] Mass Density [µg/ cm 3 ] Size Distribution Chemical Composition Optical Properties (Scattering, Absorption) Aerodynamical Properties In first approximation aerosols are treated as spherical (with an Effektive Radius r).

11 Size Distributions - Volume (Mass) vs. Number Density Production of new Particles (Gas to Particle Conversion) Coagulation of smaller Particles Number Density Dispersion of particulate material

12 Size Distribution: Radius, Surface, and Volume CCN, cloud properties Typical urban aerosol [Whitby and Sverdrup 1980] radiative effects, surface reactions chemistry, deposition

13 Morphology Annette Worringen, Paola Formenti

14 What are Aerosol Particles made of? In-situ measurement of aerosol composition at 5-19km by single particle MS. (not corrected for ionisation efficiency) Murphy, Tomson & Mahoney, Science 282, (1998) 1664-

15 Mass and number densities and mean diameters of different tropospheric types of aerosols

16 2. Sources of Tropospheric Aerosol Soil Suspension ( Dust ) Sea salt Combustion processes Volcanic eruptions Gas Particle Conversion primary aerosol particles secondary aerosol particles

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21 bubble bursting: Sea salt aerosol formation composition of sea water

22 Andreas, 1998

23 Combustion Aerosol Biomass Burning (Forest, Savannah, Agricultural Waste) Potash (K 2 CO 3 ) Partially oxidised organic compounds Soot ( Black Carbon ) Anthropogenic Combustion Similar to BB, but higher temperature...

24 Southern California October 2003

25 Aerial pictures of fires NW of Los Angeles, 26. Oct. 2003

26 Heat signatures (red) and smoke (blue haze) from numerous areas of fire burning in Bolivia, Paraguay, and Brasil

27 Heat signatures (red) and smoke (blue haze) from numerous areas of fire burning in south-central Russia and Mongolia

28 Heat signatures (red) and smoke (blue haze) from several fires burning in the Southeast US

29 Volcanoes

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31 Gas Particle Conversion Principle: Volatile species (i.e. species with high vapour pressure) react with each other to form species with much lower vapour pressure (at the same T) There are only four relevant classes of species: H 2 SO 4 + H 2 O... NH 3 + acids (H 2 SO 4, HNO 3,...) Oxidised volatile organic species (aldehydes,...) Iodine oxides

32 Life Cycle of Sulfuric Acid/Sulfate Aerosol in the Atmosphere

33 Estimated Global Aerosol Emission BUT: this is total mass and not size-resolved

34 Annual Average Source Strength (kg km-2 2 hr-1) for Different Aerosol Types (IPCC) Figure 5.2: Annual average source strength in kg km-2 hr-1 for each of the aerosol types considered here (a to g) with total aerosol optical depth (h). Shown are (a) the column average H2 SO4 production rate from anthropogenic sources, (b) the column average H2 SO4 production rate from natural sources (DMS and SO2 from volcanoes), (c) anthropogenic sources of organic matter, (d) natural sources of organic matter, (e) anthro-pogenic sources of black carbon, (f) dust ources for dust with diameters less than 2 µm, (g) sea salt sources for sea salt with diameters less than 2 µm, and (h) total optical depth for the sensitivity case CHAM/GRANTOUR model (see Section ).

35 Annual Average Source Strength (kg km-2 2 hr-1) and Optical Density for Different Aerosol Types (IPCC)

36 3. Aerosol Physics or Why don't Aerosol Particles Simply Drop to the Ground? 1. Settling velocity (v) of a particle: Stokes friction force: Gravitational force: FSt = 6πrη v (valid up to Re 0.1) r = Particle radius, η = dynamic viscosity of the fluid (air). F Equate and solve for settling velocity: v G = 4 = Mg = ρ 3 P π r g 3 ρ P = Density of the particle 4 3 π r ρpg 3 6πrη = 2 9 r 2 ρpg η r 2 Example: For r=1μm = 10-6 m we obtain v=10-4 m/s bzw. 10m/day

37 The Knudsen Number 2. Knudsen Number (Kn): n ParticleRadius r K Mean Free Path λ = = For K n << 1 the fluid can be described in good approximation as continuum, i.e. by macroskopic quantities like the viscosity η and density ρ. Example: For a particle of the accumulation mode r = 1μm, at a presure of 1 bar λ Air 65 nm thus K n = λ Air /r 0.06 << 1. For K n not << 1 the expression for Stokes friction force must be modified: F SC 1 R 0 = 6πr η v 1+ AK (A 1, valid up to K n 0.25) n 2 r 6π λ Air ηv Air Stokes Cunningham Formula Thus for large K n (i.e. for small particles or low pressure) the settling velocity becomes: v = 2ρPgλ 9η Air r r (Millikan used in 1923 for his oil droplet experiment to determine the Elementary charge the Stokes Cunningham Formula with A =

38 Slip Correction Factors Seinfeld and Pandis, 1999 Pruppacher and Klett, 1997

39 Why don't Aerosol Particles Simply Drop to the Ground? Settling Velocity: v 8 ρ Pg = 3 ρ C Air C D = drag coeff. turbulent flow D r v = 2 ρ g P r 9 η 2 laminar v 2 ρ Pgλ Air = r 9 η molecular

40 The Stopping Distance Stopping Time τ: Assuming that the particle is moving with the velocity v relative to the carrier gas (assumed to be at rest) it will experience the braking force FSt v (in the laminar case) and the braking acceleration: dv dt F = = KBr v [ = 6π rη v] M The constant K Br must have the dimension of an inverse time, thus: dv dt = v τ with τ = stopping time The above differential equation has a simple solution: () 0 t τ v t = v e With v 0 = initial velocity of the particle

41 Mobility of Aerosol Particles 3. Stopping Distance Λ: The total distance to deccelerate a particle from the initial velocity v 0 to 0 is given by: t τ vdt v0 e dt v0 0 0 Λ= = = τ 4. Mobility B of a Particle with stationary velocity B Def. ( ) = = driving force dv v F= m = m dt τ ( dv = v dt τ, see above), m = Particle mass, we obtain: v τ B = + m v = m τ Conversely: v = -B F B is (like τ) independent of the velocity of the particle (laminar case) and thus a characteristic quantity of the aerosol particle. v F

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43 Aerosol Lifetime vs. Size After Jaenicke 1978

44 Are aerosols dry or wet? deliquesced completely dissolved insoluble nucleus dry crystal surface water, quasi-liquid layer note: all filter samples of aerosols are dry!

45 Dry or wet? - cont/d Deliquescence humidity: KCl 84.2% (NH 4 ) 2 SO % NaCl 75.3% NH 4 NO % NH 4 HSO % DRH of mixed salts is always smaller than that of the individual salts shrinking growth Tang, 1997 crystallization/ effluorescence deliquescence

46 Aerosol ph Aerosols are highly concentrated particles Many heterogenous reactions, solvation are ph dependent: ph = - log 10 ([H + ]) accumulation mode: -1 1 cont. Zhu et al., remote Katoshevski et al., remote Fridlind and Jacobson, 2000 coarse mode: cont. Zhu et al., remote Fridlind and Jacobson, remote Vogt et al., 1996, Keene et al cont. Sander and Crutzen, 1996, Erickson et al cont./remote Keene and Savoie, 1999, Keene et al. 2002

47 Summary Aerosols are key players in the atmosphere: cloud microphysics radiation chemistry transport and deposition health large range/variety of.... sources.. number concentrations.. sizes.. composition, ph aged particle distributions converge to accumulation mode almost all particles have some water attached to it

48 leftovers from aerosol part I

49 Jet Drop and Film Drop Modes in Marine Aerosol (NE-Atlantic) Number Density Radius (μm) Surface Density Radius (μm)

50 Composition of Sea Water

51 Sea Salt Aerosol Formation Mechanisms

52 Evaporating Gypsum - Sea Water Droplet Polarisation microscope: The small whitish crystals consist of gypsum CaSO 4, the larger, dark crystals are NaCl.

53 Ammonium Nitrate Aerosol NH 3 (gas) + HNO 3 (gas) NH 4 NO 3 (solid) Note: = In ternary (i.e. SO 4 containing) mixtures the NH 3 and HNO 3 vapour pressures may be reduced. NH 4 NO 3 Particles NH 3 + HNO 3 Gas

54 Degradation Mechanisms for Dimethylsulfide (DMS) Low Vapour Pressure DMS Scientific Assessment of Ozone Depletion 2002, WMO Rept. No. 47

55 TOMS Image Showing the Desert Dust Girdle Source: : NASA

56 Jimenez et al. JGR 2003 Chamber Experiment on Iodine Particles

57 PSC clouds over Kiruna (Northern Sweden). Thin veils (right) are type I, thick clouds (white and coloured, left) are type II. Yellow clouds at the horizon are in the tropo-sphere being illuminated from behind. Foto: Gerd Baumgarten, Uni Bonn, Germany

58 Aerosol Lifetime vs. Size After Jaenicke 1978

59 Deliquescence and Efflorescence of NH 4 SO 4 Efflorescence Relative Humidity Deliquescence Relative Humidity

60 Evolution of Sulfur (SO 2, Sulfate) Emission

61 Tropospheric Aerosol

62 Vertical profiles of Aerosol Number Densities Densities (normalised to 1013 hpa, 273K) R. Busen, DLR Oberpfaffenhofen

63 Cunningham Slip Correction Factor