rad_bc_mixing_group_meeting_10_1_2014

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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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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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