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 << λMie scattering: Dp ≈ λGeometric 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

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)

6

7

8

9

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)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.

16

1400

1200

1000

800

600

400

200

0

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.

1400

1200

1000

800

600

400

200

0

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

800

600

400

200

0

Hei

ght (

met

ers)

6 7 8 91

RH (fraction)

From Don Collins, Texas A&M U.

18

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

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

1

RH

(fra

ctio

n)

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

19

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.

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.

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

800

600

400

200

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

1200

1000

800

600

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.