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Transcript of Characterizing the Radiative Effects of Black Carbon Internal Mixing Charles Li Group Meeting...
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Characterizing the Radiative Effects of Black Carbon Internal Mixing
Charles Li
Group Meeting Presentation
October 1, 2014
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Black Carbon (BC) Direct Radiative
Forcing:
• +0.71 W m-2 (+0.08, +1.27)
(1750-2005) Bond et al. [2013]
• +0.60 W m-2 (+0.2, +1.1)
(1750-2010) IPCC-AR5
Large Uncertainties associated with
direct radiative forcing of BC!
Background
IPCC-AR5
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Background
Difference of BC AAOD between AERONET observations and AeroCom models. (Koch et al., 2009; Bond et al., 2013)
Absorption Aerosol Optical Depth
(AAOD, τa)
MAC = Mass Absorption Coefficient
nm = mass concentration
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Background
(Oshima et al., 2012; IPCC-AR5)
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Background
(Bond et al., 2013)
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Background
• Internal mixing between black carbon (BC) and other aerosol
species, e.g. sulfate and organic carbon (OC)
Credit to Adachi et al. [2010]
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Background
Particle-level observations, due to BC internal mixing, MAC
is enhanced by
• 1.8 ~ 2 for secondary organic aerosol (SOA) & BC (Schnaiter et al.,
2005)
• 1.2 ~ 1.6 near large cities (Knox et al., 2009)
• 1.4 in biomass burning plumes (Lack et al., 2012)
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Research Question
How does black carbon internal mixing affect aerosol
climate forcing?
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Background
Radiative forcing due to BC internal mixing from model results:
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Internal Mixing II:Core Shell
Internal Mixing I:Homogeneous
Radiation
External Mixing
BC
Internal Mixing III:Maxwell-Garnet (MG)Approximation
• +0.51 W m-2 (Jacobson, 2001)
• +0.50 W m-2 (Lesins et al., 2002)
• +0.39 W m-2 (Liao and Seinfeld, 2005)
• +0.17 W m-2
(Chylek et al., 1995)
• +0.27 W m-2 (Jacobson, 2001)
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Background
• 2 × CO2 :
• 2 × Sulfate :
• 2 × BC (at different altitudes):
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(Hansen et al., 2005)
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Specific Question and Aim I
How does BC internal mixing influence surface forcing
and atmospheric absorption additional to top of the
atmosphere (TOA) radiative forcing?
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Mie Calculation
Radiative Transfer Module
Atmospheric-Chemistry
Model
Radiative ForcingParticle-level Radiative Properties Aerosol distribution
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Background
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Specific Question and Aim II
Is it possible to provide a more efficient framework to
study BC internal mixing with reduced complexities?
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Method
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Mie Theory Calculation
Comprehensive Radiative Transfer
Model
Particle-level Radiative Properties
Layer-level Radiative Forcing
Simplified Radiative Transfer Model
• Captures major characteristics;• Saves computational cost;• Examines radiative forcing varied with
variables e.g. mixing ratios/states, aerosol species, RH, hygroscopicity.
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I. DEFINING RADIATIVE FORCING DUE TO INTERNAL MIXING.
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Method: GFDL Standalone Radiative Transfer Model
Definition:
RF(BC + Sulfate) = RF(All) – RF(no BC & Sulfate)
RF(BC) = RF(All) – RF(no BC)
RF(Sulfate) = RF(All) – RF(no Sulfate)
Standalone Radiative
Transfer Model
Radiative Fluxes (RF)
• Radiative Properties
• Aerosol
distribution• Meteorological
condition
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Radiative Fluxes: INT vs. EXT
Surface Radiative Flux TOA Radiative Forcing Atmospheric AbsorptionBC+Sulfat
eBC Sulfat
eBC+Sulfate
BC Sulfate
EXT -2.70 -0.94 -1.73 -1.72 +0.20
-1.90 +0.98
INT -3.20 -1.45 -2.22 -1.26 +0.66
-1.44 +1.94
Global mean clear-sky radiative fluxes using aerosol climatology in 1999 :
≅ + ≅ +
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Radiative Fluxes: INT vs. EXT
Surface Radiative Flux TOA Radiative Forcing Atmospheric AbsorptionBC+Sulfat
eBC Sulfat
eBC+Sulfate
BC Sulfate
EXT -2.70 -0.94 -1.73 -1.72 +0.20
-1.90 +0.98
INT -3.20 -1.45 -2.22 -1.26 +0.66
-1.44 +1.94
Global mean clear-sky radiative fluxes using aerosol climatology in 1999 :
≠ + ≠ +
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RF(BC + Sulfate) = RF(All) – RF(no BC & Sulfate)
RF(BC) = RF(All) – RF(no BC)
RF(Sulfate) = RF(All) – RF(no Sulfate)
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Surface Radiative Flux TOA Radiative Forcing Atmospheric AbsorptionBC+Sulfat
eBC Sulfat
eBC+Sulfate
BC Sulfate
EXT -2.70 -0.94 -1.73 -1.72 +0.20
-1.90 +0.98
INT -3.20 -1.45 -2.22 -1.26 +0.66
-1.44 +1.94
Radiative Fluxes: INT vs. EXT
Global mean clear-sky radiative fluxes using aerosol climatology in 1999 :
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Radiative Fluxes: INT vs. EXT
Surface Radiative Flux TOA Radiative Forcing Atmospheric AbsorptionBC+Sulfat
eBC Sulfat
eBC+Sulfate
BC Sulfate
EXT -2.70 -0.94 -1.73 -1.72 +0.20
-1.90 +0.98
INT -3.20 -1.45 -2.22 -1.26 +0.66
-1.44 +1.94
Global mean clear-sky radiative fluxes using aerosol climatology in 1999 :
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Radiative
Fluxes: INT
vs. EXT
Global mean clear-sky radiative fluxes using aerosol climatology in 1999
-0.50 Wm-2
+0.46 Wm-2
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Nonlinear effect due to internal mixing• Previous studies:
α ≅ 2 (Jacobson, 2001)α ≅ 1.3 (Bond et al., 2011)
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Nonlinear effect due to internal mixing
TOA
Each color has 8 marks denoting RF based on model year 1860,1890,1910,1930,1950,1970,1990,1999.
Clear-sky
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Nonlinear effect due to internal mixing• Assumption behind previous studies:
• Actually, in the case of BC and sulfate mixing:nonlinear cross term!
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II. CHARACTERIZING INTERNAL MIXING ON PARTICLE LEVEL
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Mie Calculation: BC/Sulfate Mixing
Simulations Difference in Calculation
Ext. Mixing Mix of radiative properties (BC, Sulfate+water) post MIE
Int. Mixing Mix of Refractive Indices (BC, Sulfate+water) before MIE
Homogeneous Mixing
Magnitude of estimations:
External Spherical & Aggregated
< Core/shell & MG
< Homo. Internal
(Lesin et al., 2002; Bond et al., 2006; Jacobson,
2006)
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Radiative Properties Of The ParticlesMAC MSC
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Radiative Properties Of The Particles
Effect of internal mixing
at the particle level:
• Slight increase in
extincetion
• Enhanced absorption
• Reduced scattering
• Forward scattering
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III. CHARACTERIZING INTERNAL MIXING ON LAYER LEVEL
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Relationship between particle-level and layer-level effects
Two-layer Simplified RTM
Mie Calculation
λ—wavelength, RH—relative humidity, σ—mass ratio
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Radiative Properties Of The Aerosol Layer
• Absorbance dominates the
difference between layer-
level radiative properties of
INT vs. EXT • MAC is the key particle-
level factor that determines
this difference.
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Method : Simplified Radiative Transfer Model
Mie Calculation
Standalone Radiative
Transfer Model
GFDL Climate Model
Two-layer Simplified RTM
Radiative FluxesRadiative Properties
Radiative Forcing
• Aerosol distribution• Meteorological
condition
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Method: Simplified Radiative Transfer Model
…
Top of Atmosphere
…
…
Surface
Aerosol Layer
Multi-scattering
One Dimensional Two-layer Aerosol Radiative Transfer Model
Radiative properties of the aerosol layer:
t—transmittance
a—absorbance
r—reflectance.
F0—insolation
Ac—cloud fractionTa—transmittance
Rs—surface albedo(Chylek and Wong, 1995)
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Simplified Radiative Transfer Model• Assumption I: eliminate high-order term• Approximated radiative fluxes:
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Simplified Radiative Transfer Model• Radiative forcing due to internal mixing:
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Simplified Radiative Transfer Model• As was shown• Then, effects of internal mixing will be
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Simplified vs. Comprehensive Model
Each color has 8 marks denoting RF based on model year 1860,1890,1910,1930,1950,1970,1990,1999.
Clear-sky Clear-sky
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Simplified vs. Comprehensive Model
Assume Rs falls between 0.3 and 0.4,
• Simplified model well captured the relative magnitude of radiative energy.
• Internal mixing evenly captures extra energy from TOA (positive RF) and surface (negative RF), while retaining them in the atmosphere.
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Particle-level Absorption Enhancement
In most source regions,
sulfate mass ratio is
between 80% and 98%:
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Absorption Enhancement
Each color has 8 marks denoting RF based on model year 1860,1890,1910,1930,1950,1970,1990,1999.
Simplified model:
Comprehensive model:
Particle-level:
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Radiative Fluxes due to internal mixing
F0 = 342 W m-2Ta = 0.79Rs = 0.45
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Important Role Of Water
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Missing role of OCAerosol mass concentration over West Africa in model year 1999
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IV. THREE-SPECIES INTERNAL MIXING: BC, SULFATE AND OC
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Three-species internal mixing
Mixing Description
All EXT BC, Sulfate(+water), and OC(+water) are all externally
mixed
BCSUL INT BC and Sulfate(+water) are internally mixed, while
OC(+water) is externally mixed with them.
All INT BC, Sulfate(+water), and OC(+water) are all internally
mixed
σsul—mass ratio of sulfate to BCσoc—mass ratio of OC to BC
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Three-species internal mixing: MAC
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Changing OC/BC Mixing Ratio
• When changing OC mixing
ratio towards BC, normalized
RF calculated by BCSUL INT
is a good approximation to All
INT
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Changing BC/Sulfate Mixing Ratio
• The difference between
BCSUL INT and All INT is
susceptible to changing
Sulfate/BC mixing ratio.
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Changing BC/Sulfate Mixing Ratio
• Consider the global mean column density of the three species together as about 7 mg m-2. • Then, if we assume σsul = 80%, the bias between All INT and BCSUL INT is compared with the bias between BCSUL INT and All EXT
Unnegligible!
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Summary of current results• Internal mixing evenly captures extra energy from TOA and surface, while retaining them in the atmosphere.• Enhancement of the absorbing ability (a factor of 2~3) is the dominant factor in determining the difference between INT and EXT.• Effects of internal mixing is strongest at mass mixing ratio of 60% sulfate, and has an important contribution from water.• Internal mixing significantly enhances and alters vertical heating profile, that may result in hydrological response.• Three-species internal mixing has an important contribution, especially for studying the changing sulfate/BC mixing ratio.
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V. LIMITATIONS
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From instantaneous radiative forcing to effective radiative
forcing:
• Fast feedbacks—semi-direct effects on clouds
• Missing component in the current framework: vertical heating
profile due to internal mixing
Limitations
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Possible fast feedbacksVertical heating rates
Forcing:
• Strong atmospheric heating at
750mb and near surface
Possible effects:
• Enhanced convection near
surface
• Prohibited convection beyond
750mb
• Increased low cloud at 800 mb
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Follow-up work (current project)
• Implement internal mixing between three aerosol species: BC, sulfate and OC in the radiative module of the GFDL climate model.
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THANK YOU!
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