1 Ground-based Remote Ground-based Remote Sensing of Aerosols Sensing of Aerosols Pawan K Bhartia Pawan K Bhartia Laboratory for Atmospheres Laboratory for Atmospheres NASA Goddard Space Flight Center NASA Goddard Space Flight Center Maryland, USA Maryland, USA
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1 Ground-based Remote Sensing of Aerosols Pawan K Bhartia Laboratory for Atmospheres NASA Goddard Space Flight Center Maryland, USA.
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Ground-based Remote Sensing Ground-based Remote Sensing of Aerosolsof Aerosols
Pawan K BhartiaPawan K BhartiaLaboratory for AtmospheresLaboratory for Atmospheres
NASA Goddard Space Flight CenterNASA Goddard Space Flight CenterMaryland, USAMaryland, USA
Note: For internally mixed aerosols, AOT depends largely on REAL()=n(), while 0 depends on IMG()=k()
Scattered radiance
Usually written simply as
ISSAOS 2008ISSAOS 2008 55
Solar Extinction TechniqueSolar Extinction Technique
0
€
I = I0e−mτ
m = secθ0
τ = τ Rayl + τ aer + τ gasI0
I
Rayl can be estimated with ±0.005 accuracy if surface press is known.
ozone is the most important gaseous absorber for <1 m. At >0.34 m it can be estimated with ±0.005 accuracy using available satellite data.
To estimate AOT to ±0.01 accuracy, I/I0 should be known to the 1-2% level
aer =aerosol extinction optical thickness (AOT)
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Micro-physical Meaning of AOTMicro-physical Meaning of AOT
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n(r)=particle size distributionQext=aerosol ext coefficient, varies with , and refr index (n)ext=ext cross-section of particles
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ext = σ ext∫ N z( )dz
N= particle number densityext is total effective area of particles,varies with
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ext = πr 2Qext∫ r( )n(r)dr
n=1.55
n=1.40
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-dependence of -dependence of or or
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If n r( ) = n0rr0
( )−ν
then σ ext = σ 0λλ 0
( )− ν −2( )
= σ 0λλ0
( )−α
larger implies that mean particle size is smaller, varies from 2-4 in earth’s atmosphere.
is called the Angstrom coeff. Varies between 0-2.
Power-law (Junge) size distribution For typical bimodal size distribution
Power law
0.3 0.5 2
=3.8, =1.8
ISSAOS 2008ISSAOS 2008 88
Estimation of size distribution Estimation of size distribution from from -dependence of -dependence of or or
wt fns20.34
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ext = 34
Qext
r∫ ∂V∂ ln r d ln r
= W λ ,r( )∫ ∂V∂ ln r d ln r
€
∂V∂ ln r is sensitive to a limited
range of particle volumes As W moves to right with increase in itsamples larger particles
=
issue: W is very sensitive to REAL(), which varies significantly.
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Sky-Radiance MethodSky-Radiance Method
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In single - scattering approx :
L = L Rayl + I 0
4πcosθ τ aerΡ Θ( )ϖ 0
I0
L
L=path radiance (watt/m2/nm/steradian)P()=Scattering Phase function
P() contains information about REAL(), particle size distribution, and particle shape
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Scattering Phase FunctionScattering Phase Function
Phase fn Phase fn Ratio
P(n=1.55)/P(n=1.43)
P(reff=0.49)/P(reff=0.25)
scatt is minimum for =120˚ sens to n is max at =120˚sens to size is max at 0˚ and 160˚
Inaccessible from ground
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Almucantar TechniqueAlmucantar Technique
=0
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cosΘ = cosθ02 + sinθ0
2 cosφReundant meas allow quality control, and minimizes cloud contamination. Inversion of almucantar radiances provides size dist & refractive index
Redundant meas
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Estimation of SSA from Estimation of SSA from Almucantar measurementsAlmucantar measurements
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L ≈ LRayl + I 0
4π cosθ τ aerϖ 0Ρ Θ( )
cosθI 0
L − LRayl( )dΩ∫ ≈ τ aerϖ 0
Given size distr and refractive index derived from the Almucantar technique, L can be calculated at all scattering angles. Given aer from the direct sun technique, SSA can be estimated.
Conceptual Basis
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SSA from the Total Hemispheric SSA from the Total Hemispheric Irradiance MethodIrradiance Method
A detector with flat plate diffuser measures T. R can be estimated from the asymmetric factor (related to asymmetry in scattering phase function in backward and forward hemispheres). A is related to abs=ext(1-).
Reflected
Diffuse
Absorbed
Direct
Flat plate diffuser
I0
0
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I0 = I tot + Iabs + IreflI absI 0
=1− I totI 0
−I reflI 0
A =1−T − R
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Instruments & NetworksInstruments & Networks
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Cimel SunphotometerCimel Sunphotometer
http://www.cimel.fr/index_us.html
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AeAerosol rosol RoRobotic botic NetNetwork work (AERONET)(AERONET)
http://aeronet.gsfc.nasa.gov/
Established by NASA/GSFC & LOA-PHOTONS (CNRS)Many collaborating agencies & institutes
Key Products multi-spectral AOT Size Distribution Refractive Index SSA
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CalibrationCalibration
slope=ln(v0-vstd)
-std=ln(v0-vstd)/m
-std
1/m=cos0
•••
••
•lnv0
ln(I/I0)=ln(v/v0)=-m
lnv
m=sec0
Langley plot Methodclean site approx const during the day
Measures total and diffuse irradiance on a flat plate. direct=total-diffuse Direct/total is not affected by instrument calibration, and I0 cancels out. Since direct can be calibrated using standard methods, the calibration can be transferred to total. However, since direct=I0cos0e-m one needs to know the “cosine response” of the diffuser accurately.
Provides multi-spectral AOT and SSA
ISSAOS 2008ISSAOS 2008 2020
Shadow-band vs Almucantar for Shadow-band vs Almucantar for measuring SSAmeasuring SSA
Shadow-band Shadow-band + simpler, faster, and less noisy+ simpler, faster, and less noisy- less accurate (needs asymmetry factor)less accurate (needs asymmetry factor)- requires cloudless skyrequires cloudless sky
AlmucantarAlmucantar+ more accurate + more accurate
- slower, noisier, and more complex- slower, noisier, and more complex
- possible in presence of broken clouds- possible in presence of broken clouds
Diect-sun irradiance measurements from Diect-sun irradiance measurements from sun photometers provide sun photometers provide -dep AOT, which -dep AOT, which provide some aerosol size info.provide some aerosol size info.
Almucantar measurements from AERONET Almucantar measurements from AERONET provide particle size, shape, and refractive provide particle size, shape, and refractive index.index.
SSA can be obtained from almucantar, but SSA can be obtained from almucantar, but hemispherical irradiance provides greater hemispherical irradiance provides greater sampling and precision, and go into the sampling and precision, and go into the UV. UV.