Lecture 16: Aerosol Light Scattering and Cloud Nucleation · Mie Scattering vs. α •Visible Mie...

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1 Required Reading: FP&P Section 9.A.4 and 9.C.1.d Atmospheric Chemistry CHEM-5151 / ATOC-5151 Spring 2005 Prof. Jose-Luis Jimenez Lecture 16: Aerosol Light Scattering and Cloud Nucleation Outline of Lecture We study aerosols because of effects on: Health Ecosystems (acid rain) Visibility Climate Today Aerosol light scattering Aerosol water uptake Subsaturated Influence in light scattering Supersaturated: cloud formation

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Transcript of Lecture 16: Aerosol Light Scattering and Cloud Nucleation · Mie Scattering vs. α •Visible Mie...

  • 1

    Required Reading: FP&P Section 9.A.4 and 9.C.1.d

    Atmospheric ChemistryCHEM-5151 / ATOC-5151

    Spring 2005Prof. Jose-Luis Jimenez

    Lecture 16: Aerosol Light Scattering and Cloud Nucleation

    Outline of Lecture• We study aerosols because of effects on:

    – Health– Ecosystems (acid rain)– Visibility– Climate

    • Today– Aerosol light scattering– Aerosol water uptake

    • Subsaturated– Influence in light scattering

    • Supersaturated: cloud formation

  • 2

    Health Effects of Particles• “Harvard six-

    city study” (1993)

    • Mortality increases with fine particle concentration

    • Disputed for a decade, now considered proven– New Dutch

    study shows even greater risk

    • Mechanism still uncertain

    From FP&P

    Visibility Degradation I• Particles can

    scatter and absorb radiation

    • These effect limit atmospheric visibility

    From Jacob

  • 3

    Direct Attenuation of Radiation

    I ≡ radiation intensity (e.g., F)I0 ≡ radiation intensity above atmospherem ≡ air masst ≡ attenuation coefficient due to

    – absorption by gases (ag)– scattering by gases (sg)– scattering by particles (sp)– absorption by particles (ap)

    apspagsg

    m-t

    ttttt eI I

    +++=×= ×0

    Rayleigh scatteringtsg ∝ λ-4

    Deep UV – O, N2, O2Mid UV & visible – O3

    Near IR – H2OInfrared – CO2, H2O, others

    tag ∝ σ

    tsp ∝ λ-n

    much more

    complex

    From F-P&P & S. Nidkorodov

    Scattering by Gases• Purely physical

    process, not absorption

    • Approximation:

    • Strongly increases as λdecreases

    • Reason why “sky is blue” during the day

    From Turco

    420

    5 /)1(10044.1 λλ −⋅⋅= ntsg

  • 4

    Gas vs. Particle / Scat. Vs. Abs.

    • Denver Brown Cloud (late 70’s): 7% NO2, 93% particles

    From FP&P

    Light Scattering by Particles

    Rayleigh scattering: Dp > λ

    Size Parameter

    λπα D=

    From FP&P

  • 5

    NephelometerFrom FP&P

    Mie Scattering I

    22111

    20

    8)(),(

    RiiIRI

    πλθ +=

    • i1 and i11 are “Mieintensity parameters”• Functions of α, θ, & m• m: refractive index = c/v• Imaginary part of m represents absorption

    From FP&P

  • 6

    Absorption by ECFrom FP&P

    Relative Importance of Abs. vs. Scat. From FP&P

  • 7

    Mie Scattering vs. α•Visible Mie scattering reaches peak efficiency for particles ≈ 0.30 – 0.70 µm. Because this is close to typical ambient particle sizes, Mie scattering is the most important in the atmosphere. •Mie scattering can only be treated analytically for spherical homogeneous particles.•Mie scattering mostly occurs in forward direction, as opposed to Rayleighscattering, which is symmetric

    From FP&P

    Mie Scattering vs. size

    • This is for a single particle• Note Rayleigh limit as d6 at small sizes

    From FP&P

  • 8

    Mie Scattering per unit volume

    • A single 10 µm particle scatters much more than a 1 µm particle• But the reverse is true per unit volume (one 10 µm particle vs1000 1 µm particles)

    From FP&P From FP&P

    Scattering in Atmosphere

    • By coincidence, mass concentration is largest for particles that are most efficient scatterers• Scattering by fine mode dominates total scattering in most conditions• Some exceptions such as dust storms

    From FP&P

  • 9

    Scattering vs. Fine ParticlesFrom FP&P

    Scattering vs. Fine & Coarse Part.From FP&P

  • 10

    Mass Scattering EfficienciesFrom FP&P

    • Be careful with these as they depend on size dist. & state of mixing

    Dependence of Scattering on RHFrom FP&P

  • 11

    Why Dependence on RH is so StrongFrom FP&P

    Dp / (Dp)dry

    RH (%)

    40 80

    1.7

    1.3

    Compound RHC (%) RHD (%)

    (NH4)2SO4 40 80

    NH4HSO4 10 40

    H2SO4 0 0

    NH4NO3 28 62

    NaCl 42 75

    Deliquescence and Efflorescence

    From Don CollinsTexas A&M U

  • 12

    Deliquescence for (NH4)2SO4From FP&P

    Deliquescence Points

    From FP&P

  • 13

    Deliquescence vs. Composition

    From FP&P

    Importance of H2SO4 NeutralizationFrom FP&P

  • 14

    Extension of hygroscopic growth to cloud droplet formation

    )(4exp mequilibriuatRHTDR

    xee

    ee

    ee

    wwww

    pures

    c

    flats

    sd

    s

    sd =⎟⎟⎠

    ⎞⎜⎜⎝

    ⎛==

    ρσγ

    As the RH approaches and exceeds 100%, the solution droplet becomes increasingly dilute γw approaches 1.0 and the droplet volume can be directly related to the amount of water.

    2/13

    2/1

    33

    274)1ln(ln

    30

    exp4

    32exp

    ⎟⎟⎠

    ⎞⎜⎜⎝

    ⎛=≈+=⎟⎟

    ⎞⎜⎜⎝

    ⎟⎠⎞

    ⎜⎝⎛=⇒=⎟⎟

    ⎞⎜⎜⎝

    ⎟⎠⎞

    ⎜⎝⎛ −=⎟⎟

    ⎞⎜⎜⎝

    ⎛−=

    BASS

    ee

    ABr

    ee

    drd

    rB

    rA

    rMWmiMW

    TrRee

    cccs

    sd

    cs

    sd

    sw

    w

    wws

    sd

    πρρσ

    Radius

    S

    Pure water1x solute

    2x solute0%

    Köhler Curve

    %205.1001%205.000205.0)10608.4(27

    )100919.1(4274

    10608.4)132.0)(1000(4

    )107183.4)(018.0)(3(343

    107183.4)1004.0)(1760(

    100919.1)298)(461)(1000(

    ))(075.0(22

    2/1

    323

    392/13

    32319

    1936343

    34

    9

    3

    3

    3

    =+=⇒==⎟⎟⎠

    ⎞⎜⎜⎝

    ⎛××

    =⎟⎟⎠

    ⎞⎜⎜⎝

    ⎛=

    ×=×

    ==

    ×=×==

    ×===

    −−

    −−

    cc

    molkg

    mkg

    molkg

    sw

    sw

    mkg

    pws

    KkgJ

    mkg

    mNJ

    mN

    ww

    SRHmm

    BAS

    mkg

    MWmiMWB

    kgmrm

    mKTR

    A

    ππρ

    ππρ

    ρσ

    Example: Calculate the critical supersaturation of a 0.08 µm diameter ammonium sulfate particle

    From Seinfeld, J. H., and S. N. Pandis, Atmospheric Chemistry and Physics, John Wiley, New York, 1998.

    From Don Collins, Texas A&M U.

    Example of Activation in a Cloud• The next set of slides shows the evolution of the size (i.e. water

    uptake) of two particles of initial dry sizes of 150 and 300 nm.• As the air rises in the subsaturated atmosphere below the cloud,

    the absolute humidity stays constant, but RH increases as T decreases. The particles eventually deliquesce and keep taking up water

    • When the particles enter the cloud, which is supersaturated in H2O they keep growing.

    • S reaches a point higher than Scrit for the larger particle, which leads to activation of that particle

    • S always stays below S’crit for the smaller particle, so that one remains unactivated througout the cloud. This is called the “interstitial” aerosol. Typically 10-50% of the particles activate and the rest remain as interstitial.

    • Finally, if the air containing the particles goes beyond the top of the cloud, both particles lose most water because the air is again subsaturated

    • This animation was prepared by Don Collins at Texas A&M Univ

  • 15

    1400

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    0

    Hei

    ght (

    met

    ers)

    6 7 8 91

    RH (fraction)

    1.008

    1.006

    1.004

    1.002

    1.000

    0.998

    RH

    (fra

    ctio

    n)

    0.01 0.1 1 10r (µm)

    6

    7

    8

    9

    1

    RH

    (fra

    ctio

    n)

    0.01 0.1 1 10r (µm) From Don Collins, Texas A&M U.

    1400

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    met

    ers)

    6 7 8 91

    RH (fraction)6

    7

    8

    9

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    RH

    (fra

    ctio

    n)

    0.01 0.1 1 10r (µm)

    1.008

    1.006

    1.004

    1.002

    1.000

    0.998

    RH

    (fra

    ctio

    n)

    0.01 0.1 1 10r (µm)

    From Don Collins, Texas A&M U.

  • 16

    1400

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    met

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    6 7 8 91

    RH (fraction)5

    6

    7

    8

    9

    1

    RH

    (fra

    ctio

    n)

    0.01 0.1 1 10r (µm)

    1.008

    1.006

    1.004

    1.002

    1.000

    0.998

    RH

    (fra

    ctio

    n)

    0.01 0.1 1 10r (µm)

    From Don Collins, Texas A&M U.

    1400

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    Hei

    ght (

    met

    ers)

    6 7 8 91

    RH (fraction)5

    6

    7

    8

    9

    1

    RH

    (fra

    ctio

    n)

    0.01 0.1 1 10r (µm)

    1.008

    1.006

    1.004

    1.002

    1.000

    0.998

    RH

    (fra

    ctio

    n)

    0.01 0.1 1 10r (µm)

    From Don Collins, Texas A&M U.

  • 17

    5

    6

    7

    8

    9

    1

    RH

    (fra

    ctio

    n)

    0.01 0.1 1 10r (µm)

    1.008

    1.006

    1.004

    1.002

    1.000

    0.998

    RH

    (fra

    ctio

    n)

    0.01 0.1 1 10r (µm)

    1400

    1200

    1000

    800

    600

    400

    200

    0

    Hei

    ght (

    met

    ers)

    6 7 8 91

    RH (fraction)

    From Don Collins, Texas A&M U.

    5

    6

    7

    8

    9

    1

    RH

    (fra

    ctio

    n)

    0.01 0.1 1 10r (µm)

    1.008

    1.006

    1.004

    1.002

    1.000

    0.998

    RH

    (fra

    ctio

    n)

    0.01 0.1 1 10r (µm)

    1400

    1200

    1000

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    600

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    Hei

    ght (

    met

    ers)

    6 7 8 91

    RH (fraction)

    From Don Collins, Texas A&M U.

  • 18

    1400

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    0

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    ght (

    met

    ers)

    6 7 8 91

    RH (fraction)

    1.008

    1.006

    1.004

    1.002

    1.000

    0.998

    RH

    (fra

    ctio

    n)

    0.01 0.1 1 10r (µm)

    5

    6

    7

    8

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    1

    RH

    (fra

    ctio

    n)

    0.01 0.1 1 10r (µm) From Don Collins, Texas A&M U.

    1400

    1200

    1000

    800

    600

    400

    200

    0

    Hei

    ght (

    met

    ers)

    6 7 8 91

    RH (fraction)

    1.008

    1.006

    1.004

    1.002

    1.000

    0.998

    RH

    (fra

    ctio

    n)

    0.01 0.1 1 10r (µm)

    5

    6

    7

    8

    9

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    RH

    (fra

    ctio

    n)

    0.01 0.1 1 10r (µm) From Don Collins, Texas A&M U.

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    1.008

    1.006

    1.004

    1.002

    1.000

    0.998

    RH

    (fra

    ctio

    n)

    0.01 0.1 1 10r (µm)

    5

    6

    7

    8

    9

    1

    RH

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    ctio

    n)

    0.01 0.1 1 10r (µm)

    1400

    1200

    1000

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    600

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    0

    Hei

    ght (

    met

    ers)

    6 7 8 91

    RH (fraction)

    From Don Collins, Texas A&M U.

    1.008

    1.006

    1.004

    1.002

    1.000

    0.998

    RH

    (fra

    ctio

    n)

    0.01 0.1 1 10r (µm)

    5

    6

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    RH

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    0.01 0.1 1 10r (µm)

    1400

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    Hei

    ght (

    met

    ers)

    6 7 8 91

    RH (fraction)

    From Don Collins, Texas A&M U.

  • 20

    5

    6

    7

    8

    9

    1

    RH

    (fra

    ctio

    n)

    0.01 0.1 1 10r (µm)

    1.008

    1.006

    1.004

    1.002

    1.000

    0.998

    RH

    (fra

    ctio

    n)

    0.01 0.1 1 10r (µm)

    1400

    1200

    1000

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    600

    400

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    0

    Hei

    ght (

    met

    ers)

    6 7 8 91

    RH (fraction)

    From Don Collins, Texas A&M U.

    1.008

    1.006

    1.004

    1.002

    1.000

    0.998

    RH

    (fra

    ctio

    n)

    0.01 0.1 1 10r (µm)

    5

    6

    7

    8

    9

    1

    RH

    (fra

    ctio

    n)

    0.01 0.1 1 10r (µm)

    1400

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    1000

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    400

    200

    0

    Hei

    ght (

    met

    ers)

    6 7 8 91

    RH (fraction)

    From Don Collins, Texas A&M U.

  • 21

    1.008

    1.006

    1.004

    1.002

    1.000

    0.998

    RH

    (fra

    ctio

    n)

    0.01 0.1 1 10r (µm)

    5

    6

    7

    8

    9

    1

    RH

    (fra

    ctio

    n)

    0.01 0.1 1 10r (µm)

    1400

    1200

    1000

    800

    600

    400

    200

    0

    Hei

    ght (

    met

    ers)

    6 7 8 91

    RH (fraction)

    From Don Collins, Texas A&M U.

    5

    6

    7

    8

    9

    1

    RH

    (fra

    ctio

    n)

    0.01 0.1 1 10r (µm)

    1.008

    1.006

    1.004

    1.002

    1.000

    0.998

    RH

    (fra

    ctio

    n)

    0.01 0.1 1 10r (µm)

    1400

    1200

    1000

    800

    600

    400

    200

    0

    Hei

    ght (

    met

    ers)

    6 7 8 91

    RH (fraction)

    From Don Collins, Texas A&M U.