Group Meeting 10/1/14
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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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Background
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!
IPCC-AR5
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Background
Absorption Aerosol Optical Depth
(AAOD, τa)
MAC = Mass Absorption Coefficient
nm
= mass concentration
Difference of BC AAOD between AERONET
observations and AeroCom models. (Koch
et al., 2009; Bond et al., 2013)
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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:
Radiation
Internal Mixing II:
Core Shell
Internal Mixing I:
Homogeneous
•
+0.51 W m-2
•
+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)
(Jacobson, 2001)
•
Internal Mixing III:
Maxwell-Garnet (MG)
Approximation
•
BC
+0.27 W m-2
(Jacobson, 2001)
External Mixing
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Background
• 2 × CO2 :
• 2 × Sulfate :
• 2 × BC (at different altitudes):
(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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Background
Mie
Calculation
Radiative
Transfer
Module
AtmosphericChemistry
Model
Particle-level
Radiative Properties
Radiative Forcing
Aerosol distribution
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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
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
• Radiative
Properties
• Aerosol
distribution
• Meteorological
condition
Standalone
Radiative
Transfer Model
Radiative
Fluxes (RF)
Definition:
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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Radiative Fluxes: INT vs. EXT
Global mean clear-sky radiative fluxes using aerosol climatology in 1999 :
Surface Radiative Flux
TOA Radiative Forcing
BC+Sulfat BC
e
≅
BC+Sulfat BC
e
≅
Sulfat
+e
EXT
-2.70
-0.94
-1.73
-1.72
INT
-3.20
-1.45
-2.22
-1.26
Atmospheric
Sulfate Absorption
+
+0.2 -1.90
0
+0.6
6
-1.44
+0.98
+1.94
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Radiative Fluxes: INT vs. EXT
Global mean clear-sky radiative fluxes using aerosol climatology in 1999 :
Surface Radiative Flux
TOA Radiative Forcing
BC+Sulfat
e
BC+Sulfat
e
EXT
-2.70
INT
-3.20
BC
Sulfat
e
≠ -0.94 + -1.73
-1.45
-2.22
RF(BC + Sulfate)
-1.72
-1.26
BC
Atmospheric
Sulfate Absorption
≠ +0.2 + -1.90
0
+0.6
6
-1.44
+0.98
+1.94
= RF(All) – RF(no BC & Sulfate)
RF(BC)
= RF(All) – RF(no BC)
RF(Sulfate)
= RF(All) – RF(no Sulfate)
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Radiative Fluxes: INT vs. EXT
Global mean clear-sky radiative fluxes using aerosol climatology in 1999 :
Surface Radiative Flux
TOA Radiative Forcing
Atmospheric
Sulfate Absorption
BC+Sulfat
e
BC
Sulfat
e
BC+Sulfat
e
BC
EXT
-2.70
-0.94
-1.73
-1.72
+0.2
0
-1.90
+0.98
INT
-3.20
-1.45
-2.22
-1.26
+0.6
6
-1.44
+1.94
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Group Meeting 10/1/14
Radiative Fluxes: INT vs. EXT
Global mean clear-sky radiative fluxes using aerosol climatology in 1999 :
Surface Radiative Flux
TOA Radiative Forcing
Atmospheric
Sulfate Absorption
BC+Sulfat
e
BC
Sulfat
e
BC+Sulfat
e
BC
EXT
-2.70
-0.94
-1.73
-1.72
+0.2
0
-1.90
+0.98
INT
-3.20
-1.45
-2.22
-1.26
+0.6
6
-1.44
+1.94
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Global mean clear-sky radiative fluxes using aerosol climatology in 1999
+0.46 Wm-2
Radiative
Fluxes: INT
vs. EXT
-0.50 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
Clear-sky
TOA
Each color has 8 marks denoting RF based on model year 1860,1890,1910,1930,1950,1970,1990,1999.
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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
Magnitude of estimations:
External Spherical & Aggregated
< Core/shell & MG
< Homo. Internal
Homogeneous
Mixing
(Lesin et al., 2002; Bond et al., 2006; Jacobson,
2006)
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Radiative Properties Of The Particles
MAC
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
Mie
Calculation
λ—wavelength, RH—relative humidity, σ—mass ratio
Two-layer
Simplified RTM
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Radiative Properties Of The Aerosol Layer
• Absorbance dominates the
difference between layerlevel radiative properties of
INT vs. EXT
• MAC is the key particlelevel factor that determines
this difference.
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Method： Simplified Radiative Transfer Model
Radiative Properties
Mie
Calculation
Radiative Fluxes
Standalone
Radiative
Transfer Model
GFDL Climate
Model
• Aerosol distribution
• Meteorological
condition
Two-layer
Simplified RTM
Radiative Forcing
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Method: Simplified Radiative Transfer Model
Top of Atmosphere
F0—insolation
One Dimensional Two-layer Aerosol
Radiative Transfer Model
…
Ac—cloud fraction
Ta—transmittance
…
Aerosol Layer
Multi-scattering
…
Surface
Rs—surface albedo
Radiative properties of the aerosol layer:
t—transmittance
a—absorbance
r—reflectance.
(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
Clear-sky
Clear-sky
Each color has 8 marks denoting RF based on model year 1860,1890,1910,1930,1950,1970,1990,1999.
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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
Comprehensive model:
Particle-level:
Simplified model:
Each color has 8 marks denoting RF based on model year 1860,1890,1910,1930,1950,1970,1990,1999.
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Radiative Fluxes due to internal mixing
F0 = 342 W m-2
Ta = 0.79
Rs = 0.45
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Important Role Of Water
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Missing role of OC
Aerosol 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
All EXT
Description
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 BC
σoc—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
Unnegligible!
compared with the bias between BCSUL INT and All EXT
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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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Limitations
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
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Possible fast feedbacks
Vertical 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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