Conceptually Characterizing the Radiative Effects of Black Carbon Internal Mixing

METHODOLOGY

MOTIVATIONS

RESULTS

SUMMARIES

Xiaoyuan Li1 (xl4@princeton.edu), Yi Ming2 (yi.ming@noaa.gov), and Denise L. Mauzerall1,3 (mauzeral@princeton.edu) 1Department of Civil and Environmental Engineering, Princeton University; 2Geophysical Fluid dynamics Laboratory (GFDL/NOAA), Princeton NJ; 3Woodrow Wilson School of Public and International Affairs, Princeton University

REFERENCES  Adachi,  K.,  S.  H.  Chung,  and  P.  R.  Buseck  (2010),  Shapes  of  soot  aerosol  parBcles  and  implicaBons  for  their  effects  on  climate,  J.  Geophys.  Res.  Atmos.,  doi:10.1029/2009JD012868.  Hansen,  J.,  M.  Sato,  et  al.  2005b.  Efficacy  of  climate  forcings.  J.  Geophys.  Res.,  110,  D18104,  doi:10.1029/2005/JD005776.  

Paper #: A53H-3307

Mie  Theory  CalculaBon  

GFDL  Standalone  RadiaBve  Transfer  

Model  

Par+cle-­‐level    Radia+ve  Proper+es  

Layer-­‐level    Radia+ve  Forcing  

Simplified  Radia+ve  Transfer  Model  

…  

Top  of  Atmosphere  

…  

…  

Surface  

Aerosol  Layer  MulB-­‐sca[ering  

F0—insolaBon  

Ac—cloud  fracBon  Ta—transmi[ance  

Rs—surface  albedo  

One Dimensional Two-layer Conceptual Radiative Transfer Model

t—transmittance a—absorbance r—reflectance

!"#!!!" = !! !− !! !!![ !+!!!!

!− !!!− !!]!

!"#!!!" = !! !− !! !! [(!− !!)!

!− !!!− ! ]!

!"#!!!" = !! !− !! !! [! !+ !!!!− !!!

]!

I.  Defini+on  of  radia+ve  effects  due  to  internal  mixing            

∆!"!"# = !!"!"# − !"!"#!!"!"# = !"!" + !"!"# + ∆!"!"# !

∆!"!"# = ∆!"!"# !", !"# ≠ ! ∗ !"!" !!

II.  Par+cle-­‐level  radia+ve  proper+es  due  to  internal  mixing    

Scenarios   Mie  Calcula+on  Descrip+on  

External  Mixing   Mix  of  radiaBve  properBes  (BC,  Sulfate+water)  post  MIE  

Internal  Mixing   Mix  of  RefracBve  Indices  (BC,  Sulfate+water)  before  MIE  

ü  Enhanced  absorp+on  ü  Reduced  scaGering  

III.  Simplifying  the  conceptual  radia+ve  transfer  model      

backscattering,factor,! = !!!! ,

Simplified  RTM    

Mie  CalculaBon  

Mass$Extinction$Coefficient$!"# = !"#(!,!",!)$

optically)thin)layer)! ≪ 1)

Single'Scattering'Albedo'! = !(!,!",!)'Asymmetry(Factor(! = !(!,!",!)(

! = 1− ! !!"#!!"#!"#$ = !"#!!"#!"#$!

! = !!!!!"#!!"#!"#$ = !!!"#!!"#!"#$!

λ—wavelength,  RH—relative  humidity,  σ—mass  ratio  

∆!!"# = !"#!"# −!"#!"# !∆!!"# = !!"#!"#!"# − !!"#!"#!"# !!

∆!!"# ≅ 0.133 ∗ ∆!!"# !

On  layer  level,  change  in  absorbance  due  to  internal  mixing  dominates  over  reflectance.  

Given                                                                      and                                ,  the  conceptual  radiaBve  transfer  model  can  be  simplified  as:        

! = 7.5!!!

! = 5!!!

∆!!"# ≅ 0.133 ∗ ∆!!"# ! !!!! ≪ 1!

! = !! ! = 2!!

Internal  mixing  enhances  atmospheric  absorpBon  by  increasing  TOA  forcing  and  decreasing  surface  forcing  the  same  amount.  

!"#!!∆!"!"# ≅ !! 1− !! !!!(2!!)∆!!"#!!"#!!∆!"!"# ≅ −!! 1− !! !! 1+ !! ∆!!"#!!"#!!∆!"!"# ≅ !! 1− !! !! 1+ !! + 2!!!! ∆!!"# !

Calcula4on  from  standalone  model  shows  linear  rela4onship  between  the  three  forcing  components  due  to  internal  mixing.  The  simplified  model  provides  a  good  approxima4on  when  surface  albedo  falls  between  0.3  and  0.4.  

!"#!∆!"!"# ≅ −! ∗ !"#!∆!"!"# ≅ ! ∗ !"#!∆!"!"#!

V.  Applica+on  II:  internal  mixing  between  BC,  sulfate  and  OC  

Mixing  Scenario   Mie  Calcula+on  Descrip+on  

All  EXT   BC,  Sulfate(+water),  and  OC(+water)  are  all  externally  mixed  

BCSUL  INT   Only  BC  and  Sulfate(+water)  are  internally  mixed  

All  INT   BC,  Sulfate(+water),  and  OC(+water)  are  all  internally  mixed  

TOA  forcing  would  be  underes+mated  by  0.21  W/m2  if  OC  is  missing  from  internal  mixing  (given  sulfate/BC  raBo  of  80%,  and  global  mean  aerosol  column  density  of  7  mg/m2).  

(Credits  to  Adachi  et  al.,  2010)  

AbsorpBon  Aerosol  OpBcal  Depth  (AAOD,  τa)              AAOD  is  largely  underes+mated  by  models  in  Asia  and  Africa,  which  is  partly  due  to  poor  implementaBon  of  black  carbon  (BC)  internal  mixing.  

!! = !"# ∙ !! ∙ !"!

!!

MAC  –Mass  AbsorpBon  Coefficient  

nm          –mass  concentraBon  

Gaps  in  exis+ng  studies:  Ø  Mainly  focus  on  top  of  the  atmosphere  (TOA)  

radiaBve  forcing  of  BC  internal  mixing,  without  addressing  verBcal  energy  redistribuBon.  

Ø  ComputaBonal  costs  are  large  using  3D  aerosol  models  to  perform  each  calculaBon.  

Climate sensitivity:

2 × CO2 :

2 × Sulfate :

2 × BC (at different altitudes):

∆!!∆! = !.!"#!℃/(!/!!)!

∆!!∆! = !.!""!!"!!.!"#!℃/(!/!!)!

∆!!∆! = !.!"!!℃/(!/!!)!

Main  findings:  ²  Internal  mixing  enhances  atmospheric  absorpBon  by  

increasing  TOA  forcing  and  decreasing  surface  forcing  the  same  amount.  

²  Change  of  layer  absorbance  due  to  internal  mixing  dominates  over  that  of  reflectance.  It  provides  a  good  approximaBon  that  leads  to  a  validated  simplified  radiaBve  transfer  framework.  

²  Using  our  new  framework,  we  esBmate  a  global  average  increase  of  0.21  W/m2  when  internal  mixing  of  BC  with  sulfate  and  OC  is  included  relaBve  to  a  case  where  internal  mixing  with  OC  is  absent.  

(Credits  to  Hansen  et  al.,  2005)  

RESEARCH QUESTIONS

I.    How   does   BC   internal   mixing   influence   surface  forcing   and   atmospheric   absorp>on   as   well   as  TOA  radia+ve  forcing?  

II.   How  can  we  develop  a  more  efficient  framework  to  study  the  radiaBve  effects  of  BC  internal  mixing  with  reduced  complexi>es?  

ü  More  Forward  sca[ering  ü  Slight  increase  in  ex+nc+on    

MAC   MEC