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.