Lecture 16: Aerosol Light Scattering and Cloud Nucleation
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
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
1 Health Effects of Particles From FP&P • “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
Visibility Degradation I • Particles can From Jacob scatter and absorb radiation • These effect limit atmospheric visibility
2 Direct Attenuation of Radiation
-t×m From F-P&P & S. Nidkorodov I = I0 × e
t = tsg + tag + tsp + tap I ≡ radiation intensity (e.g., F)
I0 ≡ radiation intensity above atmosphere m ≡ air mass t ≡ attenuation coefficient due to – absorption by gases (ag) – scattering by gases (sg) – scattering by particles (sp) – absorption by particles (ap)
Rayleigh scattering t ∝λ-n -4 sp tsg ∝λ much
more Deep UV – O, N2, O2 complex Mid UV & visible – O3 Near IR – H2O Infrared – CO2, H2O, others tag ∝ σ
Scattering by Gases • Purely physical process, not absorption • Approximation: 5 2 4 tsg =1.044⋅10 ⋅(n0λ −1) / λ • Strongly increases as λ decreases • Reason why “sky is blue” during the day From Turco
3 Gas vs. Particle / Scat. Vs. Abs.
From FP&P
• Denver Brown Cloud (late 70’s): 7% NO2, 93% particles
Light Scattering by Particles
From FP&P
Size Parameter Rayleigh scattering: D << λ p πD Mie scattering: Dp ≈λ α = Geometric scattering: Dp >> λ λ
4 Nephelometer
From FP&P
Mie Scattering I
I λ2 (i + i ) I(θ , R) = 0 1 11 8π 2 R2
• i1 and i11 are “Mie intensity parameters” • Functions of α, θ, & m • m: refractive index = c/v • Imaginary part of m represents absorption
From FP&P
5 Absorption by EC
From FP&P
Relative Importance of Abs. vs. Scat.
From FP&P
6 Mie Scattering vs. α From FP&P •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 Rayleigh scattering, which is symmetric
From FP&P Mie Scattering vs. size
• This is for a single particle • Note Rayleigh limit as d6 at small sizes
7 Mie Scattering per unit volume
From FP&P • 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 vs 1000 1 µm particles)
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
8 Scattering vs. Fine Particles
From FP&P
Scattering vs. Fine & Coarse Part.
From FP&P
9 Mass Scattering Efficiencies
From FP&P
• Be careful with these as they depend on size dist. & state of mixing
Dependence of Scattering on RH
From FP&P
10 Why Dependence on RH is so Strong
From FP&P
Deliquescence and Efflorescence
1.7
Dp / (Dp)dry
1.3
40 80
RH (%)
Compound RHC (%) RHD (%)
(NH4)2SO4 40 80
NH4HSO4 10 40
H2SO4 00
From Don Collins NH4NO3 28 62 Texas A&M U NaCl 42 75
11 Deliquescence for (NH4)2SO4 From FP&P
Deliquescence Points
From FP&P
12 Deliquescence vs. Composition
From FP&P
Importance of H2SO4 Neutralization From FP&P
13 Extension of hygroscopic growth to cloud droplet formation
e e e ⎛ 4σ ⎞ sd = sd c = γ x exp⎜ ⎟ = RH (at equilibrium) e e e w w ⎜ R ρ TD ⎟ s s flat s pure ⎝ w w ⎠
As the RH approaches and exceeds 100%, the solution droplet becomes increasingly dilute Æ
γw approaches 1.0 and the droplet volume can be directly related to the amount of water.
e ⎛ 2σ 3iMW m ⎞ ⎛ A B ⎞ sd = exp⎜ − w ⎟ = exp − ⎜ 3 ⎟ ⎜ 3 ⎟ Köhler Curve es ⎝ RwρwTr 4πρwMWsr ⎠ ⎝ r r ⎠ 1/ 2 d ⎛ e ⎞ ⎛ 3B ⎞ ⎜ sd ⎟ = 0 ⇒ r = ⎜ ⎟ dr ⎜ e ⎟ c A ⎝ s ⎠ ⎝ ⎠ Pure 1x solute water 1/ 2 ⎛ e ⎞ ⎛ 4A3 ⎞ ln⎜ sd ⎟ = ln(S +1) ≈ S = ⎜ ⎟ ⎜ e ⎟ c c ⎜ 27B ⎟ ⎝ s ⎠c ⎝ ⎠ S
Example: Calculate the critical supersaturation of a 0.08 µm diameter ammonium sulfate particle 0% 2x solute 2σ 2(0.075 N )( J ) A = = m N m =1.0919×10−9 m ρ R T (1000 kg )(461 J )(298 K) w w m3 kg K m = 4 πρ r 3 = 4 π (1760 kg )(0.04×10−6 m)3 = 4.7183×10−19 kg Radius s 3 w p 3 m3 3iMW m 3(3)(0.018 kg )(4.7183×10−19 kg) B = w s = mol = 4.608×10−23 m3 4πρ MW 4π (1000 kg )(0.132 kg ) From Seinfeld, J. H., and S. N. Pandis, w s m3 mol Atmospheric Chemistry and Physics, 1/ 2 1/ 2 ⎛ 4A3 ⎞ ⎛ 4(1.0919×10−9 m)3 ⎞ John Wiley, New York, 1998. S = ⎜ ⎟ = ⎜ ⎟ = 0.00205 = 0.205% ⇒ RH =1+ S =100.205% c ⎜ 27B ⎟ ⎜ 27(4.608×10−23 m3 ) ⎟ c ⎝ ⎠ ⎝ ⎠ 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
14 1.008
1400 1.006
1.004 1200
1.002 RH (fraction) 1000
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0.998 800
0.01 0.1 1 10 r (µm)
1 600 Height (meters)
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0 6 7 8 9 6 1 RH (fraction) 0.01 0.1 1 10 r (µm) From Don Collins, Texas A&M U.
1.008
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1.004 1200
1.002 RH (fraction) 1000
1.000
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0.01 0.1 1 10 r (µm)
1 600 Height (meters)
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0 6 7 8 9 6 1 RH (fraction) 0.01 0.1 1 10 r (µm) From Don Collins, Texas A&M U.
15 1.008
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1.004 1200
1.002 RH (fraction) 1000
1.000
0.998 800
0.01 0.1 1 10 r (µm)
1 600 Height (meters)
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0 6 7 8 9 5 1 RH (fraction) 0.01 0.1 1 10 r (µm) From Don Collins, Texas A&M U.
1.008
1400 1.006
1.004 1200
1.002 RH (fraction) 1000
1.000
0.998 800
0.01 0.1 1 10 r (µm)
1 600 Height (meters)
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0 6 7 8 9 5 1 RH (fraction) 0.01 0.1 1 10 r (µm) From Don Collins, Texas A&M U.
16 1.008
1400 1.006
1.004 1200
1.002 RH (fraction) 1000
1.000
0.998 800
0.01 0.1 1 10 r (µm)
1 600 Height (meters)
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0 6 7 8 9 5 1 RH (fraction) 0.01 0.1 1 10 r (µm) From Don Collins, Texas A&M U.
1.008
1400 1.006
1.004 1200
1.002 RH (fraction) 1000
1.000
0.998 800
0.01 0.1 1 10 r (µm) 1 600 Height (meters)
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0 6 7 8 9 5 1 RH (fraction) 0.01 0.1 1 10 r (µm) From Don Collins, Texas A&M U.
17 1.008
1400 1.006
1.004 1200
1.002 RH (fraction) 1000
1.000
0.998 800
0.01 0.1 1 10 r (µm) 1 600 Height (meters)
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0 6 7 8 9 5 1 RH (fraction) 0.01 0.1 1 10 r (µm) From Don Collins, Texas A&M U.
1.008
1400 1.006
1.004 1200
1.002 RH (fraction) 1000
1.000
0.998 800
0.01 0.1 1 10 r (µm) 1 600 Height (meters)
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0 6 7 8 9 5 1 RH (fraction) 0.01 0.1 1 10 r (µm) From Don Collins, Texas A&M U.
18 1.008
1400 1.006
1.004 1200
1.002 RH (fraction) 1000
1.000
0.998 800
0.01 0.1 1 10 r (µm) 1 600 Height (meters)
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0 6 7 8 9 5 1 RH (fraction) 0.01 0.1 1 10 r (µm) From Don Collins, Texas A&M U.
1.008
1400 1.006
1.004 1200
1.002 RH (fraction) 1000
1.000
0.998 800
0.01 0.1 1 10 r (µm) 1 600 Height (meters)
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0 6 7 8 9 5 1 RH (fraction) 0.01 0.1 1 10 From Don Collins, Texas A&M U. r(µm)
19 1.008
1400 1.006
1.004 1200
1.002 RH (fraction) 1000
1.000
0.998 800
0.01 0.1 1 10 r (µm) 1 600 Height (meters)
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0 6 7 8 9 5 1 RH (fraction) 0.01 0.1 1 10 From Don Collins, Texas A&M U. r(µm)
1.008
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1.004 1200
1.002 RH (fraction) 1000
1.000
0.998 800
0.01 0.1 1 10 r (µm) 1 600 Height (meters)
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0 6 7 8 9 5 1 RH (fraction) 0.01 0.1 1 10 From Don Collins, Texas A&M U. r(µm)
20 1.008
1400 1.006
1.004 1200
1.002 RH (fraction) 1000
1.000
0.998 800
0.01 0.1 1 10 r (µm) 1 600 Height (meters)
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0 6 7 8 9 5 1 RH (fraction) 0.01 0.1 1 10 From Don Collins, Texas A&M U. r(µm)
1.008
1400 1.006
1.004 1200
1.002 RH (fraction) 1000
1.000
0.998 800
0.01 0.1 1 10 r (µm)
1 600 Height (meters)
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0 6 7 8 9 5 1 RH (fraction) 0.01 0.1 1 10 r (µm) From Don Collins, Texas A&M U.
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