Radiative Effects of Atmospheric Aerosols

							Radiative Effects of Atmospheric
  Aerosols and Regional Haze

             Jin Xu
        DAS Science Talk
        February 17, 2004
                    Aerosol Radiative Effects

• Regional Haze, Air Quality and Visibility (COHA, FAQS)

• Photochemical Reaction (Atlanta Supersite)

• Photosynthesis and Crop Yields (ChinaMAP)

• Climate Change - Whitehouse Effect (ACE-Asia, ChinaMAP)

         Directly     -   Scattering & Absorption of Solar
                          Radiation

         Indirectly -     Modifying Cloud Properties
       Scattering and Absorption of Light by Aerosols




                                                                 I=Light
 Io=Light                  L=Path Length                         Detector (W/m2)
 Source (W/m2)
                    I      ext L     ( sp   ap  eg ) L
                       e          e
                    I0

  ( sp   ap ) * L;          sp /( sp   ap );              (b, g )
                   Scattering Model of an Aerosol Layer
                                                    F r a c tio n r e f le c te d u p w a r d
                                                                  r  (1  e   ) 




                                F r a c tio n a b s o r b e d =     (1   ) (1  e   )




                                                                     F r a c tio n s c a tte r e d d o w n w a r d
                                                                           (1   ) (1  e   )
          F r a c tio n tr a n s m itte d = e  

      T o ta l d o w n w a r d tr a n s m itte d f r a c tio n t= e   +  (1   ) (1  e   )
      T o ta l r e f le c te d o f f s u r f a c e =  s t (  s = s u r f a c e a lb e d o )


                                            F0= incident solar flux (wm-2)
                             t s       2
F   F0 (1  Ac )Ta [(r        )   s ] Ac= fraction of the surface covered by clouds
                     2

                            1sr           Ta= fractional transmittance of the atmosphere
                Aerosol Scattering and Absorption Coefficients

                               D p ,max

              sp ( )             scat ( D p ,  , ri ) m ( D p ) dD p
                               D p ,min

                               D p ,max

              ap ( )             abs ( D p ,  , ri ) m ( D p ) dD p
                               D p ,min
          Where:
           = Wavelength (m)
          Dp = Particle Diameter (m)
          scat, abs = Mass Scattering and Absorption Efficiencies (m2/g)
          ri = Refractive Index
          m(Dp) = Aerosol Mass Size Distribution


Note: Aerosol Extinction Depends on Wavelength (Ångstrom Exponent, å = - d log ext / d log  ),
Chemical Composition, and Size
 Major Aerosol Chemical Species that Contribute to
               the Light Extinction
• Sulfate Aerosols
  SO2 from Fossil Fuel Combustion
• Carbonaceous Aerosols
   – Organic Compounds (OC)
     Biomass Burning, Automobile Emissions, Fossil Fuel
     Combustion, Gas-to-particle Conversion of Hydrocarbons
   – Elemental Carbon (EC) (Absorption, Warming Effect)
     Incomplete Combustion of Fossil and Biomass Fuels
• Mineral Dust Aerosols
  Desert Dust, Construction, Road Dust
• Nitrate Aerosols
  Industrial and Automobile Emissions
Visibility Impairment of Aerosols Based on Aerosol
             Chemical Speciation Data

• Bext = 3F(RH)[Sulfate] + 3F(RH)[Nitrate] +
  4[OMC] + 10[LAC] + 1[Soil] + 0.6[CM]+
  10 (Rayleigh Gas Scattering)
• Visual Range (V.R.) = K/Bext
   Where K is the Koschmieder Coefficient – the log of the contrast
   threshold of the human eye, K = 3 – 3.9
  GOES View of the Dust Streak Across North America,
                      April 17




GOES10 view of dust streak on the   GOES8 view of dust streak on the
  morning of April 17                 evening of April 17

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          Transport of the Asian dust to the United States




The common weather conditions are usually associated
with the upper low pressure trough / cut-ff low and
surface low pressure system (low formed by a strong
cyclonic vortex) over northeast China and north Korea
[Kim et al., 2002]. Under this weather conditions, Asian
dust can move fast along the zonal wind distribution
due to the jet streak [Kim et al., 2002].

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                                         Origin of the Asian Dust




Strong low pressure system sitting in northeast Mongolia caused surface wind speeds to be as high as ~30 m/s
Given suitable weather conditions, dust can be lifted from the dry surface of the Asian Gobi desert region and
transported to the United States in about 7-10 days.                                                             34