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Characterizing the Radiative Effects of Black Carbon Internal Mixing Charles Li Group Meeting Presentation October 1, 2014 Group Meeting 10/1/14 1

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Black Carbon (BC) Direct Radiative Forcing: W m -2 (+0.08, +1.27) ( ) Bond et al. [2013] W m -2 (+0.2, +1.1) ( ) IPCC-AR5 Large Uncertainties associated with direct radiative forcing of BC! Background 2 IPCC-AR5 Group Meeting 10/1/14

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Background 3 Difference of BC AAOD between AERONET observations and AeroCom models. (Koch et al., 2009; Bond et al., 2013) Absorption Aerosol Optical Depth (AAOD, τ a ) MAC = Mass Absorption Coefficient n m = mass concentration Group Meeting 10/1/14

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Background (Oshima et al., 2012; IPCC-AR5) 4 Group Meeting 10/1/14

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Background (Bond et al., 2013) 5 Group Meeting 10/1/14

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Background 6 Internal mixing between black carbon (BC) and other aerosol species, e.g. sulfate and organic carbon (OC) Credit to Adachi et al. [2010] Group Meeting 10/1/14

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Background 7 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) Group Meeting 10/1/14

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Research Question How does black carbon internal mixing affect aerosol climate forcing? 8 Group Meeting 10/1/14

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Background 9 Radiative forcing due to BC internal mixing from model results: Group Meeting 10/1/14 Internal Mixing II: Core Shell Internal Mixing I: Homogeneous Radiation External Mixing BC Internal Mixing III: Maxwell-Garnet (MG) Approximation W m -2 (Jacobson, 2001) W m -2 (Lesins et al., 2002) W m -2 (Liao and Seinfeld, 2005) W m -2 (Chylek et al., 1995) W m -2 (Jacobson, 2001)

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Background 10 2 × CO 2 : 2 × Sulfate : 2 × BC (at different altitudes): Group Meeting 10/1/14 (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? Group Meeting 10/1/14 11

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Mie Calculation Radiative Transfer Module Atmospheric- Chemistry Model Radiative Forcing Particle-level Radiative Properties Aerosol distribution 12 Group Meeting 10/1/14 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? Group Meeting 10/1/14 13

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Method Group Meeting 10/1/14 14 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. 15 Group Meeting 10/1/14

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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) 16 Standalone Radiative Transfer Model Radiative Fluxes (RF) Radiative Properties Aerosol distribution Meteorological condition Group Meeting 10/1/14

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Radiative Fluxes: INT vs. EXT Surface Radiative FluxTOA Radiative ForcingAtmospheric Absorption BC+Sulfat e BCSulfat e BC+Sulfat e BCSulfate EXT INT Global mean clear-sky radiative fluxes using aerosol climatology in 1999 : 17 ≅ + Group Meeting 10/1/14

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Radiative Fluxes: INT vs. EXT Surface Radiative FluxTOA Radiative ForcingAtmospheric Absorption BC+Sulfat e BCSulfat e BC+Sulfat e BCSulfate EXT INT Global mean clear-sky radiative fluxes using aerosol climatology in 1999 : 18 ≠ + Group Meeting 10/1/14 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 FluxTOA Radiative ForcingAtmospheric Absorption BC+Sulfat e BCSulfat e BC+Sulfat e BCSulfate EXT INT Radiative Fluxes: INT vs. EXT Global mean clear-sky radiative fluxes using aerosol climatology in 1999 : 19 Group Meeting 10/1/14

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Radiative Fluxes: INT vs. EXT Surface Radiative FluxTOA Radiative ForcingAtmospheric Absorption BC+Sulfat e BCSulfat e BC+Sulfat e BCSulfate EXT INT Global mean clear-sky radiative fluxes using aerosol climatology in 1999 : 20 Group Meeting 10/1/14

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Radiative Fluxes: INT vs. EXT Global mean clear-sky radiative fluxes using aerosol climatology in Wm Wm Group Meeting 10/1/14

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Nonlinear effect due to internal mixing 22 Previous studies: α ≅ 2 (Jacobson, 2001) α ≅ 1.3 (Bond et al., 2011) Group Meeting 10/1/14

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Nonlinear effect due to internal mixing 23 TOA Each color has 8 marks denoting RF based on model year 1860,1890,1910,1930,1950,1970,1990,1999. Clear-sky Group Meeting 10/1/14

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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! 24 Group Meeting 10/1/14

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II. CHARACTERIZING INTERNAL MIXING ON PARTICLE LEVEL 25 Group Meeting 10/1/14

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Mie Calculation: BC/Sulfate Mixing 26 SimulationsDifference in Calculation Ext. MixingMix of radiative properties (BC, Sulfate+water) post MIE Int. MixingMix 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) Group Meeting 10/1/14

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Radiative Properties Of The Particles 27 MAC MSC Group Meeting 10/1/14

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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 28 Group Meeting 10/1/14

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III. CHARACTERIZING INTERNAL MIXING ON LAYER LEVEL 29 Group Meeting 10/1/14

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Relationship between particle-level and layer-level effects Two-layer Simplified RTM Mie Calculation 30 λ—wavelength, RH—relative humidity, σ—mass ratio Group Meeting 10/1/14

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Radiative Properties Of The Aerosol Layer 31 Absorbance dominates the difference between layer- level radiative properties of INT vs. EXT MAC is the key particle- level factor that determines this difference. Group Meeting 10/1/14

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Method ： Simplified Radiative Transfer Model Mie Calculation Standalone Radiative Transfer Model GFDL Climate Model Two-layer Simplified RTM Radiative Fluxes Radiative Properties Radiative Forcing Aerosol distribution Meteorological condition 32 Group Meeting 10/1/14

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Method: Simplified Radiative Transfer Model 33 … 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. F 0 —insolation A c —cloud fraction T a —transmittance R s —surface albedo (Chylek and Wong, 1995) Group Meeting 10/1/14

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Simplified Radiative Transfer Model Assumption I: eliminate high-order term Approximated radiative fluxes: 34 Group Meeting 10/1/14

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Simplified Radiative Transfer Model Radiative forcing due to internal mixing: 35 Group Meeting 10/1/14

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Simplified Radiative Transfer Model As was shown Then, effects of internal mixing will be 36 Group Meeting 10/1/14

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Simplified vs. Comprehensive Model 37 Each color has 8 marks denoting RF based on model year 1860,1890,1910,1930,1950,1970,1990,1999. Clear-sky Group Meeting 10/1/14

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Simplified vs. Comprehensive Model 38 Assume R s 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. Group Meeting 10/1/14

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Particle-level Absorption Enhancement 39 In most source regions, sulfate mass ratio is between 80% and 98%: Group Meeting 10/1/14

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Absorption Enhancement 40 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: Group Meeting 10/1/14

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Radiative Fluxes due to internal mixing 41 F 0 = 342 W m -2 T a = 0.79 R s = 0.45 Group Meeting 10/1/14

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Important Role Of Water 42 Group Meeting 10/1/14

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Missing role of OC Aerosol mass concentration over West Africa in model year Group Meeting 10/1/14

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IV. THREE-SPECIES INTERNAL MIXING: BC, SULFATE AND OC 44 Group Meeting 10/1/14

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Three-species internal mixing 45 MixingDescription 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 INTBC, Sulfate(+water), and OC(+water) are all internally mixed σ sul —mass ratio of sulfate to BC σ oc —mass ratio of OC to BC Group Meeting 10/1/14

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Three-species internal mixing: MAC 46 Group Meeting 10/1/14

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Changing OC/BC Mixing Ratio 47 When changing OC mixing ratio towards BC, normalized RF calculated by BCSUL INT is a good approximation to All INT Group Meeting 10/1/14

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Changing BC/Sulfate Mixing Ratio 48 The difference between BCSUL INT and All INT is susceptible to changing Sulfate/BC mixing ratio. Group Meeting 10/1/14

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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 σ sul = 80%, the bias between All INT and BCSUL INT is compared with the bias between BCSUL INT and All EXT 49 Unnegligible! Group Meeting 10/1/14

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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. 50 Group Meeting 10/1/14

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V. LIMITATIONS 51 Group Meeting 10/1/14

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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 52 Group Meeting 10/1/14

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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 53 Group Meeting 10/1/14

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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. 54 Group Meeting 10/1/14

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THANK YOU! 55 Group Meeting 10/1/14

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