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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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