V.3 AEROSOL LIDAR THEORY Vincenzo Rizi vincenzo.rizi@aquila.infn.it CETEMPS Dipartimento di Fisica...

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V.3 AEROSOL LIDAR THEORY

Vincenzo Rizi vincenzo.rizi@aquila.infn.itCETEMPS

Dipartimento di FisicaUniversità Degli Studi dell’Aquila

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

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

Lidar remote sensing

detector

Interaction between radiation and object

radiation source

signal propagation

Data acquisition and analysis

radiation propagation

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

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

)tan()tan(

)tan()tan()tan()tan(

trrtrt hhdh

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

duration pulselaser

light of speed

flight of time

range max. range resolution

LIDAR ARCHITECTURE

TRANSMITTERRADIATION SOURCE

RECEIVER LIGHT COLLECTION AND DETECTION

SYSTEM CONTROL AND DATA ACQUISITION

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.

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.

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.

Time of flight (sec)

Lidar equation 1

LIDAR RETURN

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!

detector

radiation source

signal propagation

Lidar equation 3

detector

radiation source

signal propagation

sT o , sT ,

Lidar equation 4

sT o ,

sso ,,

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

BooooooS NsGd

sTsssTNRN

,,,,,,

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

BooooooS NsGd

sTsssTNRN

,,,,,,

shotlaser single ofenergy

Lidar equation 7

lsephotons/pu 107.6 12o oN

J UV laser

BooooooS NsGd

sTsssTNRN

,,,,,,

(s)T(s)T(s)T,s)T(λ ooo λabs

λmolo

sion transmisabsorption gas )(

sion transmisscattering aerosol )(

@ sion transmisscattering molecular )(

(s)T(s)T(s)Ts)T(λ λabs

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

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)(

siabsabsabs

aerextaer

molmolmol

dssnsT

dsrsnmrQrdrsT

dssnsT

Lidar equation 9

BooooooS NsGd

sTsssTNRN

,,,,,,

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

particle aerosolan of efficiency ringbackscatte Mie ),,(

),(),,(4

(s,r)n

rsnmrQrdrs

aerbckaer

BooooooS NsGd

sTsssTNRN

,,,,,,

receiver

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

BooooooS NsGd

sTsssTNRN

,,,,,,

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

Lidar equation 12

BooooooS NsGd

sTsssTNRN

,,,,,,

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

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.

A PARTIAL REPRESENTATION(a physics-ological drama)

LASER EMITTEDPHOTON

ELASTICALLYBACK-SCATTERED

PHOTON

FEATURING:LIGHT CHARACTERS 1/3

NON-ELASTICALLYBACK-SCATTERED

PHOTONS

LIGHT CHARACTERS 2/3FEATURING:

EXTINCTED PHOTONS

LIGHT CHARACTERS 3/3FEATURING:

aerosol particle

ATMOSPHERE

LOCATION:

SCENE ITHE LASER EMISSION

“leaving together …”

LIDAR LASER EMISSION

SCENE IITHE UPWARD TRAVEL

(s)T(s)T,s)T(λ oo λaer

λmolo

“experiencing …”

aerosol particle

MIE EXTINCTION

… lost …

aerosol particle

MOLECULAR EXTINCTION

… lost …

SCENE IIILOCAL BACK-SCATTERING

“mission accomplished! but …”

N2aerosol particle

MIE BACK-SCATTERING

… immutable identity …

aerosol particle

MOLECULAR BACK-SCATTERING

… preserving the identity … apparently …

aerosol particle

RAMAN N2 BACK-SCATTERING

… deep changes …

aerosol particle

RAMAN O2 BACK-SCATTERING

… added values …

aerosol particle

RAMAN H2O BACK-SCATTERING

… apparently new?…

SCENE IVTHE DOWNWARD TRAVEL

(s)T(s)Ts)T(λ λaer

λmol ,

“on the way back …”

… again M.I.A. …

SCENE VDETECTION

“several … at home with different stories …”

TELESCOPE

LIDAR RECEIVER

… carrying back … a vanishing footprint. …

SCENE VIFINAL FATE

“figuring out the intimate experiences … a new vision”

INTO THELIDAR

RECEIVER

Rayleig

H2 O Raman

… something … useful … remains …

… wrong way for me! …

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

Scattering (elastic & inelastic)

Scattering 1

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

Scattering 2

Raman Raman

Raman scatteringelastic collision of photons with molecules: molecular rotations, vibrations, electronic transitions change of of incoming radiation ( R <104 cm-1)

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.

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

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

back-scattering

extinction

scattering

aerosolback-scattering

aerosol extinction

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

receiver

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

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

Example ...o=355nm

~32nm ~53nm

LIDAR … aerosol devoted

Aerosol lidar

Aerosol lidar i.e., stratospheric aerosols

backscattering increase

Aerosol lidar

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 9 9 1 -1 9 9 9 sca tte rin g ra tio @ 3 5 1 n m

Pinatubo eruptionstarting

June 1991

163 profiles

Aerosol lidar – CETEMPS - Università Degli Studi dell’Aquila o=351nm; 1991-1999

Aerosol lidar

Raman aerosol lidar

Rayleigh/Mie signal

N2 Raman/anelastic signal

i.e., tropospheric aerosol

more backscattering

more attenuationno backscattering

Aerosol lidar

Lidar setup ( =24)

Parabolic

mirror

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

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

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

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

0 .1 0

1 .0 0

cloud transmission

cloud backscattering

Nitrogen Raman

Air/aerosol Rayleigh

1/2 hour measurementsnighttimeSept. 2001

Aerosol lidar

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 )

1 E + 1

1 E + 2

1 E + 3

1 E + 4

1 E + 5

1 E + 6

E la s tic

R am an

Rayleigh/Mie

Aerosol lidar

0 4E-6 8E-6

bcks coeff. (m-1sr-1 )

0 4E-4 8E-4

ext. coeff. (m-1)

0 25 50 75 100lr (sr)

... from Raman N2

… from Rayleigh/Mie (Bcks coeff.)/(ext. Coeff.)

HOW? Lecture V.4Aerosol lidar

particle aerosolan of efficiency extinction Mie ),,(

),(),,()(0

(s,r)n

rsnmrQrdrs

aerextaer

particle aerosolan of efficiency eringbackscattt Mie ),,(

),(),,(4

(s,r)n

rsnmrQrdrs

aerbckaer

EXAMPLES: UV LIDAR (=355nm)SULFATE AEROSOLS, CLOUD DROPLETS, …

Aerosol signature in the LIDAR measurements

),(),,()(0

2 rsnmrQrdrs aerextaer

),(),,(4

2 rsnmrQrdrs aerbckaer

SULFATE AEROSOLS Qext(r,m,) Qbck(r,m,)

Volodymyr Bazhan ScatLab Project

CLOUD DROPLETS Qext(r,m,) Qbck(r,m,)

Volodymyr Bazhan ScatLab Project

naer(r)

seeG. FeingoldCLOUD MODEL

lognormal naer(r)

Ndrrn md

exp)log(2

log5exp

2 meff rr

SULFATE AEROSOLS

CLOUD DROPLETS

3/10 16.0 cmparticlesNmr deff

3/2.136 8.14 cmdropletsNmr deff

)(saer

aeraer

023.0 1

CLOUD DROPLETS

110015.0 srmaer

LIDAR SIGNAL SIMULATOR

V.3 AEROSOL LIDAR THEORY Vincenzo Rizi vincenzo.rizi@aquila.infn.it CETEMPS Dipartimento di Fisica...

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