New Insights into Aerosol Asymmetry Parameter

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New Insights into Aerosol Asymmetry Parameter

• John Ogren, Betsy Andrews, Anne Jefferson,
  Allison McComiskey, Pat SheridanNOAA/ESRL,
  Boulder, CO

http//www.cmdl.noaa.gov/aero/
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Absorption and Angular Scattering Are Both
Important to Variability of RFE
Single-scattering albedo (SSA) and backscatter
fraction (BFR) co-vary, so that total variability
is less than what would be expected from
individual variabilities.
SGP daily averages, 1996-2005
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The Asymmetry Parameter g

• Defined as the cosine weighted integral of the
  phase function, alternatively, the average cosine
  of the scattering angle
• Phase function P(T) is the probability of
  radiation being scattered in a given direction
• Values can range from -1 (180-degree backwards
  scattering) to 1 (0-degree forward scattering),
  with a value of 0.7 commonly used in radiative
  transfer models.
• Single value that describes the angular
  scattering properties of aerosols,
  computationally efficient for radiative transfer
  modeling
• Controlled by particle size, shape, and
  refractive index
• No method exists for direct measurement
• Polar nephelometers, which measure P(T), lack
  sensitivity
• Indirect methods are used to derive g

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Schemes for Deriving
Aerosol size distribution Chemistry (Refractive
Index)

• Limitations/Assumptions
• Spherical particles

Mie theory
Inversion (e.g, King et al ., 1978)
Asymmetry Parameter
Spectral Aerosol Optical Depth Chemistry
(Refractive Index)
Inversion (e.g, Fiebig and Ogren, 2006)
Wiscombe and Grams, (1976) relationship
Spectral light scattering (total, back) Light
absorption Size cut information
Back-scattering fraction
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Comparison of Schemes

• Results from ARM 2003 Aerosol IOP
• A 10 change in g leads to a 19 percent change
  in TOA forcing
• Source Andrews et al., JGR, 2006.

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Estimation from Backscatter Fraction
Assumption of Henyey-Greenstein phase function
gives a reasonable first estimate of g, but with
systematic differences compared to Fiebig
retrieval
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Climatology at ARM Sites
asymmetry parameter
Daily averages for 2001-2004 at 450, 550, and 700
nm wavelengthSource Fiebig and Ogren, J.
Geophys. Res., 2006, submitted.
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Climatology of g for Marine Aerosols

• RH CMDL Data lt40 AERONET Data at ambient RH
• Asymmetry parameter from literature and CMDL
  estimated from empirical relationship to bfr
• ?532-550 nm from literature ?550 nm for CMDL
  Data ?550 nm for AERONET Data

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RH-dependence at SGP
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Vertical Profiles over SGP
Values from five years of flights with IAP
airplane, for Dp lt 1 ?m.
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Effect of Cloud Scavenging
Cloud scavenging preferentially removes larger
particles, leaving behind particles with
systematically lower values of g in interstitial
air
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Sensitivity of Forcing to Asymmetry Parameter
Deficit of diurnally-averaged downwelling solar
irradiance relative to value for g0.7 at SGP.
Dashed curves indicate values calculated using
observed wavelength dependence of asymmetry
parameter. The box-whisker plot represents the
frequency distribution of asymmetry parameter
(550 nm) at SGP from in-situ measurements.
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Summary and Conclusions

• There is no method for direct measurement of
  asymmetry parameter
• There are several approaches to deriving
  asymmetry parameter from measurements of aerosols
  and radiation
• Different measurement approaches yield systematic
  differences in values and climatologies
• There is a large range in asymmetry parameter
• Natural atmospheric variability (size
  distribution)
• Measurement error
• Aerosol radiative forcing is sensitive to the
  variability of g
• Process-level research is needed to reconcile the
  differences among the various approaches for
  determining asymmetry parameter