v.3 aerosol lidar theory vincenzo rizi [email protected] cetemps dipartimento di fisica...
TRANSCRIPT
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V.3 AEROSOL LIDAR THEORY
Vincenzo Rizi [email protected]
Dipartimento di FisicaUniversità Degli Studi dell’Aquila
Italy
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Outline Notes
LIDAR technique.
Classical overview of the LIDAR technique: monitoring the fate of a bunch of coherent and undistinguishable photons travelling in a non-homogeneous atmosphere.The aerosol signatures in the LIDAR raw products (aerosol optical properties)
The architecture of LIDAR instruments (i.e., UV/Visible/Infrared - Rayleigh/Mie and Raman LIDARs) devoted to aerosol observations.
Lasers, telescopes, detectors.
The LIDAR hardware specifications.
Down- and up- sizing the different components for the best observational strategy of the various atmospheric aerosols (including clouds).
Real systems and real performances.Some examples of real systems
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OUTCOMESUpon the lecturer ability you will be able to:
• understand how LIDAR techniques are used to characterize atmospheric aerosols• perform tradeoffs among the engineering parameters of a LIDAR system to achive a given measurement capability• evaluate the performance of LIDAR systems
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Lidar remote sensing
detector
Interaction between radiation and object
radiation source
signal propagation
Data acquisition and analysis
radiation propagation
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lidar history 1
Lidar started in the pre-laser times in 1930s with searchlight beams, and then quickly evolved to modern lidars using nano-second laser pulses.
CW light
receiver
pulsed laser
receiver
searchlight
modern lidar
LIDAR HISTORY
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lidar history 2
Hulburt [1937] aerosol measurements using the searchlight technique
Johnson [1939], Tuve et al. [1935] modulated the searchlight beam with a mechanical shutter.
Elterman [1951, 1954, 1966] searchlight to a high level for atmospheric studies.
CW light
receiver
searchlight
h
hr
ht
d
tr
)tan()tan(
)tan()tan()tan()tan(
rt
trrtrt hhdh
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The first (ruby) laser was invented in 1960 [Schawlow and Townes, 1958 and Maiman, 1960].
Pulse technique (Q-Switch) McClung and Hellwarth [1962].
The first laser studies of the atmosphere were undertaken by Fiocco and Smullin [1963] for upper region and by Ligda [1963] for troposphere.
lidar history 3
pulsed laser
receiver
modern lidar
s
s
2
tcs
2
c
s
duration pulselaser
light of speed
flight of time
c
t
range max. range resolution
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LIDAR ARCHITECTURE
TRANSMITTERRADIATION SOURCE
RECEIVER LIGHT COLLECTION AND DETECTION
SYSTEM CONTROL AND DATA ACQUISITION
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TRANSMITTERIt provides laser pulses that meet certain requirements depending on application needs (e.g., wavelength, pulse duration time, pulse energy, repetition rate, divergence angle, etc).
Transmitter consists of lasers, collimating optics, diagnostic equipment.
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RECEIVER It collects and detects returned photonsIt consists of telescopes, filters, collimating optics, photon detectors, discriminators, etc.
The receiver can spectrally distinguish the returned photons.
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SYSTEM CONTROL AND DATA ACQUISITIONIt records returned data and corresponding time of flight, and provides the coordination to transmitter and receiver.It consists of multi-channel scaler which has very precise clock so can record time precisely, discriminator, computer and software.
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retu
rned
ph
oto
ns o
ver
a n
um
ber
of
laser
pu
lses
Time of flight (sec)
Lidar equation 1
LIDAR RETURN
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Lidar equation relates the received photon counts with the transmitted laser photons, the light transmission in atmosphere or medium, the physical interaction between light and objects, the photon receiving probability, and the lidar system efficiency and geometry, etc.
The lidar equation is based on the physical picture of lidar remote sensing, and derived under two assumptions: independent and single scattering.
Different lidars may use different forms of the lidar equation, but all come from the same picture.
Lidar equation 2
LIDAR EQUATION
UV-VIS … restrictions!
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detector
Interaction between radiation and object
radiation source
signal propagation
Data acquisition and analysis
radiation propagation
Lidar equation 3
Lidar equation 3
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detector
Interaction between radiation and object
radiation source
signal propagation
Data acquisition and analysis
radiation propagation
sT o , sT ,
so ,,
ooN
4
d
sGo ,
s
s
s
Lidar equation 4
Lidar equation 4
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sT o ,
sT ,
sso ,,
ooN
4
d
sGo , Lidar system efficiency and geometry factor
Emitted laser photon number
Laser photon transmission through mediumProbability of a transmitted photon to be scatteredScattered photon transmission through medium
Probability of a scattered photon to be collected
Lidar equation 5
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BooooooS NsGd
sTsssTNRN
)(,4
,,,,,,
In general, the interaction between the light photons and the particles is a scattering process.
The expected photon counts are proportional to the product of the
(1) transmitted laser photon number,
(2) probability that a transmitted photon is scattered,
(3) probability that a scattered photon is collected,
(4) light transmission through medium, and
(5) overall system efficiency.
Background photon counts and detector noise also contribute to the expected photon counts.
sN oS ,,
Lidar equation 6
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BooooooS NsGd
sTsssTNRN
)(,4
,,,,,,
oo hc
shotlaser single ofenergy
oN
Lidar equation 7
lsephotons/pu 107.6 12o oN
J UV laser
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BooooooS NsGd
sTsssTNRN
)(,4
,,,,,,
(s)T(s)T(s)T,s)T(λ ooo λabs
λaer
λmolo
sion transmisabsorption gas )(
sion transmisscattering aerosol )(
@ sion transmisscattering molecular )(
sT
sT
sT
abs
aer
mol
(s)T(s)T(s)Ts)T(λ λabs
λaer
λmol ,
The transmission, T(,s), is the relative fraction of propagating photons () that travels a distance s without interacting with the medium.
Lidar equation 8
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section cross gas absorbing th-i )(
th waveleng @ , refraction ofindex , radius of
particle aerosol an of efficiency extinction Mie),,(
@ section cross total Rayleigh
densitynumber gas absorbing th-i
ondistributi size aerosol
density number molecular catmospheri
)()(σexp)(
),(),,(exp)(
)(exp)(
0
i
0 0
2
0
i
ext
mol
i
aer
mol
i
siabsabsabs
s
aerextaer
s
molmolmol
abs
abs
mr
mrQ
n
n
n
dssnsT
dsrsnmrQrdrsT
dssnsT
Lidar equation 9
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BooooooS NsGd
sTsssTNRN
)(,4
,,,,,,
snd
ds i
i
oio
,
,,
The volume backscatter coefficient is the probability per unit distance travel that a photon (o) is (back-) scattered into wavelength , in unit solid angle.
Lidar equation 10
2
cs 1m
th waveleng @ , refraction ofindex , radius of
particle aerosolan of efficiency ringbackscatte Mie ),,(
ondistributi size aerosol
),(),,(4
1)(
0
2
mr
mrQ
(s,r)n
rsnmrQrdrs
bck
aer
aerbckaer
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BooooooS NsGd
sTsssTNRN
)(,4
,,,,,,
24 s
Ad
receiver
s
s
A
The probability that a scattered photon is collected by the receiving telescope, i.e., the solid angle subtended by the receiver aperture to the scatterer.
Lidar equation 11
Modern Mechanix, 3, 1933
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BooooooS NsGd
sTsssTNRN
)(,4
,,,,,,
o ,
)(sG
It is the optical efficiency of mirrors, lenses, filters, detectors, etc.
is the geometrical form factor, mainly concerning the overlap of the area of laser irradiation with the field of view of the receiver optics
receiver
laser
Lidar equation 12
s
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BooooooS NsGd
sTsssTNRN
)(,4
,,,,,,
BN
retu
rned
ph
oto
ns a
lon
g a
nu
mb
er
of
laser
pu
lses
Time of flight (sec)
It is the the expected photon counts due to background noise (i.e., solar light) and detector/electronic noise.
Lidar equation 13
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Different Forms of Lidar Equation
Lidar equation 14
physical processMie, Rayleigh, Raman, etc.
Lidar equation may change form to best fit for each particular physical process and lidar application.
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A PARTIAL REPRESENTATION(a physics-ological drama)
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LASER EMITTEDPHOTON
ELASTICALLYBACK-SCATTERED
PHOTON
FEATURING:LIGHT CHARACTERS 1/3
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NON-ELASTICALLYBACK-SCATTERED
PHOTONS
LIGHT CHARACTERS 2/3FEATURING:
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EXTINCTED PHOTONS
LIGHT CHARACTERS 3/3FEATURING:
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aerosol particle
H H
O
H2O N
N
N2O
O
O2
ATMOSPHERE
O
O
O2
N
N
N2
N
N
N2
LOCATION:
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SCENE ITHE LASER EMISSION
ooN
“leaving together …”
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laser
LIDAR LASER EMISSION
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SCENE IITHE UPWARD TRAVEL
(s)T(s)T,s)T(λ oo λaer
λmolo
“experiencing …”
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aerosol particle
H H
O
H2O
N
N
N2
O
O
O2
H H
O
H2O
N
N
N2
O
O
O2
aerosol particle
N
N
N2
N
N
N2
N
N
N2
N
N
N2
O
O
O2
MIE EXTINCTION
… lost …
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H H
O
H2O
N
N
N2
O
O
O2
H H
O
H2O
N
N
N2O
O
O2
aerosol particle
N
N
N2
N
N
N2
N
N
N2
N
N
N2
O
O
O2
MOLECULAR EXTINCTION
… lost …
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SCENE IIILOCAL BACK-SCATTERING
ssnd
dss i
i
oio
,
,,
“mission accomplished! but …”
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N
N
N2aerosol particle
H H
O
H2O
N
N
N2
O
O
O2
H H
O
H2ON
N
N2
N
N
N2
N
N
N2
N
N
N2
O
O
O2
MIE BACK-SCATTERING
… immutable identity …
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N
N
N2
aerosol particle
H H
O
H2ON
N
N2
O
O
O2
H H
O
H2ON
N
N2
N
N
N2
N
N
N2
N
N
N2
O
O
O2
MOLECULAR BACK-SCATTERING
… preserving the identity … apparently …
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aerosol particle
N
N
N2
H H
O
H2O
N
N
N2
O
O
O2
H H
O
H2O
N
N
N2
N
N
N2
N
N
N2
N
N
N2
O
O
O2
RAMAN N2 BACK-SCATTERING
… deep changes …
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aerosol particle
H H
O
H2O
N
N
N2
O
O
O2
H H
O
H2O
N
N
N2
N
N
N2
N
N
N2
N
N
N2
O
O
O2
RAMAN O2 BACK-SCATTERING
… added values …
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aerosol particle
H H
O
H2O
N
N
N2
O
O
O2
H H
O
H2O
N
N
N2
N
N
N2
N
N
N2
N
N
N2
O
O
O2
RAMAN H2O BACK-SCATTERING
… apparently new?…
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SCENE IVTHE DOWNWARD TRAVEL
(s)T(s)Ts)T(λ λaer
λmol ,
“on the way back …”
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… again M.I.A. …
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SCENE VDETECTION
“several … at home with different stories …”
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TELESCOPE
LIDAR RECEIVER
24 s
Ad
… carrying back … a vanishing footprint. …
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SCENE VIFINAL FATE
“figuring out the intimate experiences … a new vision”
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INTO THELIDAR
RECEIVER
range
sign
al
range
sign
al
range
sign
al
Rayleig
h-M
ie
N2 R
aman
H2 O Raman
sGo ,
… something … useful … remains …
… wrong way for me! …
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LIDAR PHYSICAL PROCESSInteraction between light and objects
• Scattering (elastic & inelastic): Mie, Rayleigh, Raman• Absorption and differential absorption• Resonant fluorescence• Doppler shift and Doppler broadening• …
Light propagation in atmosphere or medium: transmission/extinctionExtinction = Scattering + Absorption
Lidar physical processes 1
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Lidar physical processes 2
Scattering (elastic & inelastic)
N2
Scattering 1
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Rayleigh scattering is referred to the elastic scattering from atmospheric molecules (particle size is much smaller than the wavelength), i.e., scattering with no apparent change of wavelength, although still undergoing Doppler broadening and Doppler shift. However, depending on the resolution of detection, Rayleigh scattering can consist of the Cabannes scattering (really elastic scattering from molecules) and pure rotational Raman scattering.Cabannes line
Pure rotational Raman
Rayleigh
Rayleigh scatteringwavelength () particle size (r) [gas molecules]inversely proportional to 1/4. Blue sky, red sunset/sunrise
Lidar physical processes 3
Scattering 2
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Raman Raman
Raman scatteringelastic collision of photons with molecules: molecular rotations, vibrations, electronic transitions change of of incoming radiation ( R <104 cm-1)
Lidar physical processes 4
Scattering 3
Raman scattering is the inelastic scattering with rotational quantum state or vibration-rotational quantum state change as the result of scattering. The Raman scattered photons are shifted in wavelength, this shift is the signature of the stationary energy levels of the irradiated molecule. The Raman spectroscopy in a gas mixture identifies and measures the different components. Example: the nitrogen and oxygen molecules show Raman shifts (roto-vibrational transitions) of 2327cm-1 and 1556cm-1, respectively.
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Mie scattering is the elastic scattering from spherical particles [Mie, 1908], which includes the solution of Rayleigh scattering. However, in lidar field, first, Mie scattering is referred to the elastic scattering from spherical particles whose size is comparable to or larger than the wavelength. Furthermore, Mie scattering is generalized to elastic scattering from overall aerosol particles and cloud droplets, i.e., including non-spherical particles.
Mie scatteringr small cloud droplets, aerosols1/. Affect long visible wavelengths
Lidar physical processes 5
Scattering 4
Wavelength : 633 nm Dielectric : 78 nm diam. Fused Silica Incident Amplitude : 1.0 V/m Cell Size : 3 nm Workspace : 100x100x100 cells
Credits: http://bernstein.harvard.edu/research/nearfield/fdtd/FDTD%20SERS.html
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back-scattering
extinction
scattering
Lidar physical processes 6
LIDAR
aerosolback-scattering
aerosol extinction
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Back-scattering cross sections
Physical process Back-scattering cross section
Mie (aerosol) scattering
10-8 10-10 cm2 sr-1
Rayleigh scattering
10-27 cm2 sr-1
Raman scattering 10-30 cm2 sr-1
laser
receiver
Lidar physical processes 7
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3 7 0 3 7 5 3 8 0 3 8 5 3 9 0 39 5 40 0 40 5 41 0 41 5 42 0w a v e le n g th (n m )
0 .0 0
0 .0 2
0 .0 4
0 .0 6
0 .0 8
inte
nsit
y (a
.u.)
s o u rc e N d -Y A G la s e r 3 5 5 n mH 2O v a p o u r
N 2
O 2
Example ...o=355nm
~21nm
~32nm ~53nm
Lidar physical processes 8
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LIDAR … aerosol devoted
Aerosol lidar
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Aerosol lidar i.e., stratospheric aerosols
backscattering increase
z
A
zexp
12
Aerosol lidar
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0 5 0 0 1 0 0 0 1 5 0 0 2 0 0 0 2 5 0 0 3 0 0 0
d ay s s in ce 1 s t o f Jan u a ry 1 9 9 1
1 6
1 8
2 0
2 2
2 4
2 6
2 8
3 0
altit
ude
(km
)
1 9 9 1 -1 9 9 9 sca tte rin g ra tio @ 3 5 1 n m
0 .5
1 .0
1 .5
2 .0
2 .5
3 .0
3 .5
4 .0
4 .5
5 .0
5 .5
6 .0
Pinatubo eruptionstarting
June 1991
163 profiles
Aerosol lidar – CETEMPS - Università Degli Studi dell’Aquila o=351nm; 1991-1999
Aerosol lidar
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Raman aerosol lidar
Rayleigh/Mie signal
N2 Raman/anelastic signal
i.e., tropospheric aerosol
more backscattering
more attenuationno backscattering
o
o+N2
Aerosol lidar
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Lidar setup ( =24)
LAS
ER
XeF
Parabolic
mirror
20cm
The optical layout of the receiver’s beam separator. L is a 1inch plano- convex lens, BS indicates dichroic beamsplitters, I F, ND, NO and PMT labels the 2 inchesinterference filters, the interchangeable neutral densityfi lters, the notch filters and the photomultipliers,respectively. The spectral f eatures of each channel can belabelled by a representative wavelength: 351nm-Rayleigh/ Mie channel, 382nm-Nitrogen Raman channel,393nm-liquid water Raman channel, 403nm-water vaporRaman channel.
UV Raman lidar L’Aquila
Aerosol lidar
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The wavelength dependent relative
transmissions of the beam separator. These
curves have been estimated using the
manufacturer’s data sheet and the
specifi cations of the various components
(fi lters, mirror, lenses, optical fi ber, etc.).
Schematic draw of the Rayleigh/ Mie and Raman
components of the return light spectrum. The
Rayleigh/ Mie part is a reply of the laser
spectrum that has been measured, the
diff erent Raman bands have been plotted on
wavelength scale.
UV Raman lidar L’Aquila
Aerosol lidar
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capability of detecting low light levels
suppression of cross-talking between the different channels (i.e, suppression of the strong elastically backscattered light in Raman channels)
Main characteristics
Aerosol lidar
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Real Raman signal in presence of a cloud
0 1 2 3 4 5 6 7ra n g e (k m )
0 .0 1
0 .1 0
1 .0 0
sign
al *
(ra
nge)
2 (a.
u.)
0 .1 0
1 .0 0
cloud transmission
cloud backscattering
Nitrogen Raman
Air/aerosol Rayleigh
1/2 hour measurementsnighttimeSept. 2001
Aerosol lidar
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UV Raman lidar – CETEMPS - Università Degli Studi dell’Aquila o=351nm; N2=382nm (N2)
0 1 2 3 4 5 6 7 8 9 1 0a ltitu d e (k m )
0 .1
1
1 E + 1
1 E + 2
1 E + 3
1 E + 4
1 E + 5
1 E + 6
Phot
onco
unts
(a.
u.)
E la s tic
R am an
Rayleigh/Mie
Raman
Aerosol lidar
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0 4E-6 8E-6
bcks coeff. (m-1sr-1 )
1.0
1.5
2.0
2.5
3.0
alti
tud
e (
km
)
0 4E-4 8E-4
ext. coeff. (m-1)
1.0
1.5
2.0
2.5
3.0
0 25 50 75 100lr (sr)
1.0
1.5
2.0
2.5
3.0
... from Raman N2
… from Rayleigh/Mie (Bcks coeff.)/(ext. Coeff.)
HOW? Lecture V.4Aerosol lidar
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th waveleng @ , refraction ofindex , radius of
particle aerosolan of efficiency extinction Mie ),,(
ondistributi size aerosol
),(),,()(0
2
mr
mrQ
(s,r)n
rsnmrQrdrs
ext
aer
aerextaer
th waveleng @ , refraction ofindex , radius of
particle aerosolan of efficiency eringbackscattt Mie ),,(
ondistributi size aerosol
),(),,(4
1)(
0
2
mr
mrQ
(s,r)n
rsnmrQrdrs
bck
aer
aerbckaer
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EXAMPLES: UV LIDAR (=355nm)SULFATE AEROSOLS, CLOUD DROPLETS, …
Aerosol signature in the LIDAR measurements
),(),,()(0
2 rsnmrQrdrs aerextaer
),(),,(4
1)(
0
2 rsnmrQrdrs aerbckaer
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naer(r)
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naer(r)
seeG. FeingoldCLOUD MODEL
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lognormal naer(r)
drrr
r
Ndrrn md
2
2
log2
log
exp)log(2
)(
2
log5exp
2 meff rr
SULFATE AEROSOLS
CLOUD DROPLETS
3/10 16.0 cmparticlesNmr deff
3/2.136 8.14 cmdropletsNmr deff
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)(saer
)(saer
m
m
aeraer
aer
42/1
023.0 1
CLOUD DROPLETS
110015.0 srmaer
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LIDAR SIGNAL SIMULATOR