How To Understand The Physics Of The Atmosphere

Transcription

1 Physics of the Atmosphere Physik der Atmosphäre SS 2014 Ulrich Platt Institut f. Umweltphysik SR 108/110, INF 229, Mi, 09:15-10:45 Übungen zur Atmosphärenphysik (MVEnv1.2) SR 108/110, INF 229, Mi, 11:00-12:00

2 Last Week The carbon cycle has a strong influence on our climate (both, in natural changes and anthropogenic changes) Important carbon reservoirs are the ocean, the land biomass, and the atmosphere Time constants in the carbon cycle range from years to 10 5 years Oceans get more acidic less carbon uptake The role of the biomass is less well known

3 Contents Introduction Literature, Structure of the atmosphere Atmospheric Radiation: Basics, Scattering Atmospheric radiation: Energy budget of the atmosphere - Climate Global circulation (Fronts, Rossby Waves) The Atmospheric Boundary Layer The Global Hydrological Cycle The Carbon Cycle Test Atmospheric Aerosol Gas-Phase Chemistry: Reaction Kinetics Ozone and Free Radicals Nitrogen, Sulfur, and Halogen Cycles The Stratosphere: Physics (Radiation and Circulation) The Stratosphere: Chemistry 1 (Chapman Cycle + Extensions) The Stratosphere: Chemistry 2 (Ozone Hole)

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

5 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

6 Ranges of Aerosol Diameter, Surface Area, and Volume

7 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

8 Source: IPCC-AR5, Fig. SPM.5 Global, Annual, Mean Radiative Forcing

9 IPCC 2007 Direct and Indirect Aerosol Effects on Climate

10 Summer Day in the Hunsrück Mountains

11 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). Aerosole size distributions: umoluasdlog r dlog rnberdensitydistribution:vmedistribution: * dn r with s n r r s 3.5 * dlog r M dn r 3 : dv r dv r dn r 4 dn r V r r r dlog r dlog r dlog r * 3 0 * * * dv r 4 M r V r r 3 Distribution dn r * * 3 P P * P *

12 Measured Aerosol Size Distributions

13 Aerosol Number Density

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

15 Comparison of Particle Sizes.

16 Aerosol Morphology Annette Worringen, Paola Formenti

17 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-

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

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

20 dust particles sea salt (spray) major sources for dispersion aerosols volcanic ashes oil droplets (forest fires) anthropogenic : industry, burning fossil fuels,...)

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

24 Andreas, 1998

25 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...

26 Southern California October 2003

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

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

29 Volcanoes

30 Principle: Gas Particle Conversion Secondary Aerosol 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 (e.g. isoprene to low vapour pressure organic acids or aldehydes,...) Iodine oxides dm/dlog(r) Mass distribution of the nucleation mode 1E radius / µm

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

32 Life Cycles of Aerosols Nucleation Dispersion Secondary aerosol Primary aerosol Heterogeneous Cloud and Fog Scavenging by Coagulation Condensation Processing Cloud Droplets (many cycles) Aged and mixed aerosol Dry deposition Wet deposition

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

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

36 3. Aerosol Physics Forces on Aerosol Particles: Drag force: F D =B -1 (u-v) = (simplest case) (6 v)/c (u-v), u: Gas velocity, v: Particle velocity Photophoretic force: in direction of the dark side (not photon pressure) Thermophoretic force: In negative direction of the T-gradient (hot to cold) Electrical Forces

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

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

39 The Stokes - Cunningham Formula 1 F D /(6 rv) F D C 6 rv ; C 1 Kn A1 A2 exp A 3 Kn A1: 1.25, A2: 0.4, A3: Kn

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

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

42 The Particle Stopping Distance 1. Stopping Time P : 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 F St v (in the laminar case) and the braking acceleration: dv FSt 6 r v KBr v dt m m The constant K Br must have the dimension of an inverse time, thus: dv v 6 r v dt P m 1 P 2. The above differential equation has a simple solution: t 0 t P v v e With v 0 = initial velocity of the particle ith6w:p m st e r oppingtim

43 Mobility of Aerosol Particles 3. Stopping Distance : The total distance to deccelerate a particle from the initial velocity v 0 to v=0 is given by: t vdt v 0 e dt v 0 P Mobility B of a Particle ithdef. (stationary) velocity B driving force dv v dv v,seabove paf m m,, m P dtw P : P v F v 0 m 6 r v P 1 m 1 B v m laminar m m 6 r 6 r case Conversely: v = -B F B is (like P ) independent of the velocity of the particle (laminar case) and thus a characteristic quantity of the aerosol particle. rticlemas,weobtain

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45 Aerosol processing by coagulation

46 Aerosol Lifetime vs. Size After Jaenicke 1978

47 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!

48 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

49 Efflourescence and Deliquescence of Natural Aerosol

50 Ice Nuclei Supesaturation over ice

51 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

52 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

53 Advanced Topics

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

55 Composition of Sea Water

56 Sea Salt Aerosol Formation Mechanisms

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

58 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

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

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

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

62 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

63 Aerosol Lifetime vs. Size After Jaenicke 1978

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

65 Evolution of Sulfur (SO 2, Sulfate) Emission

66 Tropospheric Aerosol

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

68 Cunningham Slip Correction Factor