1 ground-based remote sensing of aerosols pawan k bhartia laboratory for atmospheres nasa goddard...

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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

ISSAOS 2008ISSAOS 2008 22

LinkagesLinkages

Laboratory Measurements

In situ field Measurements

Ground-based remote sensing

Satelliteremote sensing

Direct-sun Sky-radiance Hem. Irradiance Lidar

Solar occultation Solar backscattered Lidar

Aethalometer Nephalometer Particle Counters • • • •

Chemical prop Optical prop Particle shape

A Priori information


OutlineOutline

Basic ConceptBasic Concept• Multi-spectral Solar extinctionMulti-spectral Solar extinction• Multi-angle sky radianceMulti-angle sky radiance• Polarization of sky radiancePolarization of sky radiance• Hemispherical IrradianceHemispherical Irradiance

Instruments/NetworkInstruments/Network• Cimel/AERONETCimel/AERONET• Shadow-band radiometersShadow-band radiometers


Terms, Symbols & DefinitionsTerms, Symbols & Definitions

I0 I

Spectral Irradiance (watt/m2/nm): I=I0exp(-ext)

Aerosol Optical Thickness AOText=scat+abs

Single Scattering Albedo (SSA): 0=scat/ext

Refractive Index: =n()+ik()

Note: For internally mixed aerosols, AOT depends largely on REAL()=n(), while 0 depends on IMG()=k()

Scattered radiance

Usually written simply as


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)


Micro-physical Meaning of AOTMicro-physical Meaning of AOT

€

n(r)=particle size distributionQext=aerosol ext coefficient, varies with , and refr index (n)ext=ext cross-section of particles

€

ext = σ ext∫ N z( )dz

N= particle number densityext is total effective area of particles,varies with

€

ext = πr 2Qext∫ r( )n(r)dr

n=1.55

n=1.40


-dependence of -dependence of or or

€

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


Estimation of size distribution Estimation of size distribution from from -dependence of -dependence of or or

wt fns20.34

€

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.


Sky-Radiance MethodSky-Radiance Method

€

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


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


Almucantar TechniqueAlmucantar Technique

=0

€

cosΘ = cosθ02 + sinθ0

2 cosφReundant meas allow quality control, and minimizes cloud contamination. Inversion of almucantar radiances provides size dist & refractive index

Redundant meas


Estimation of SSA from Estimation of SSA from Almucantar measurementsAlmucantar measurements

€

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


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

€

I0 = I tot + Iabs + IreflI absI 0

=1− I totI 0

−I reflI 0

A =1−T − R

1414

Instruments & NetworksInstruments & Networks


Cimel SunphotometerCimel Sunphotometer

http://www.cimel.fr/index_us.html


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


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

•• •

••

•

Std instrument MethodAny site


Further ReadingFurther Reading


Shadow Band RadiometerShadow Band Radiometer

Multi-filter Rotating Shadow-Band Radiometer (MFRSR)

Shadow-band

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


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


UV-MFRSR vs AERONETUV-MFRSR vs AERONET

SS

A

GMT (hr)

440 nm

332 nm

368 nm

AERONET (440 nm)MFRSR

Santa Cruz, Bolivia Sept 27, 2007


UV-MFRSR/AERONET AAOT UV-MFRSR/AERONET AAOT ComparisonComparison

AAOT: Aerosol Abs Optical thickness (abs)

abs=ext(1-0)

abs=0(/)-k

k=1 for black carbon =3 for desert dust >>1 for organic carbon


UV-Rotating Shadow-band UV-Rotating Shadow-band Spectroradiometer (UV-RSS)Spectroradiometer (UV-RSS)

RSS 1024 Spectroraiometer

Ref: Yankee Env. Systems

Ref: Michalski & Kiedron, ACP, 2008


SummarySummary

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.

1 ground-based remote sensing of aerosols pawan k bhartia laboratory for atmospheres nasa goddard...

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