V.3 AEROSOL LIDAR THEORY Vincenzo Rizi vincenzo.rizi@aquila.infn.it CETEMPS Dipartimento di Fisica Università Degli Studi dell’Aquila Italy

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

OUTCOMES Upon 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

Interaction between radiation and object signal propagation radiation propagation radiation source detector Data acquisition and analysis Lidar remote sensing

LIDAR HISTORY modern lidar searchlight receiver CW light pulsed laser Lidar started in the pre-laser times in 1930s with searchlight beams, and then quickly evolved to modern lidars using nano-second laser pulses. modern lidar searchlight receiver CW light pulsed laser receiver lidar history 1

searchlight CW light h receiver r t ht hr d 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 h receiver r t ht hr d lidar history 2

modern lidar s s pulsed laser receiver range max. range resolution 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. s s pulsed laser receiver range max. range resolution lidar history 3

LIDAR ARCHITECTURE TRANSMITTER RADIATION SOURCE RECEIVER LIGHT COLLECTION AND DETECTION SYSTEM CONTROL AND DATA ACQUISITION

TRANSMITTER It 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 photons It consists of telescopes, filters, collimating optics, photon detectors, discriminators, etc. The receiver can spectrally distinguish the returned photons.

SYSTEM CONTROL AND DATA ACQUISITION It 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.

LIDAR RETURN Lidar equation 1 returned photons over a number of laser pulses Time of flight (sec) Lidar equation 1

UV-VIS … restrictions! LIDAR EQUATION 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. UV-VIS … restrictions! Lidar equation 2

Lidar equation 3 Interaction between radiation and object signal propagation radiation propagation radiation source detector Data acquisition and analysis Lidar equation 3

Lidar equation 4 Interaction between radiation and object signal propagation radiation propagation radiation source detector Data acquisition and analysis Lidar equation 4

Emitted laser photon number Laser photon transmission through medium Probability of a transmitted photon to be scattered Scattered photon transmission through medium Probability of a scattered photon to be collected Lidar system efficiency and geometry factor Lidar equation 5

The expected photon counts are proportional to the product of the 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. Lidar equation 6

J UV laser Lidar equation 7

The transmission, T(,s), is the relative fraction of propagating photons () that travels a distance s without interacting with the medium. Lidar equation 8

Lidar equation 9

The volume backscatter coefficient is the probability per unit distance travel that a photon (o) is (back-) scattered into wavelength , in unit solid angle. 1m Lidar equation 10

s s 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. A receiver Lidar equation 11 Modern Mechanix, 3, 1933

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 s laser receiver Lidar equation 12

returned photons along a number of laser pulses It is the the expected photon counts due to background noise (i.e., solar light) and detector/electronic noise. Time of flight (sec) Lidar equation 13

Different Forms of Lidar Equation physical process Mie, Rayleigh, Raman, etc. Lidar equation may change form to best fit for each particular physical process and lidar application. Lidar equation 14

A PARTIAL REPRESENTATION (a physics-ological drama)

FEATURING: LIGHT CHARACTERS 1/3 ELASTICALLY BACK-SCATTERED PHOTON LASER EMITTED PHOTON

FEATURING: LIGHT CHARACTERS 2/3 NON-ELASTICALLY BACK-SCATTERED PHOTONS

FEATURING: LIGHT CHARACTERS 3/3 EXTINCTED PHOTONS

LOCATION: ATMOSPHERE O O2 N N2 N N2 O O2 H O H2O aerosol particle N N2

SCENE I THE LASER EMISSION “leaving together …”

LIDAR LASER EMISSION laser

SCENE II THE UPWARD TRAVEL “experiencing …”

MIE EXTINCTION … lost … aerosol particle O2 N2 N2 O2 O N N2 N H2O H2O

MOLECULAR EXTINCTION … lost … H2O N2 O N2 H O2 N2 N O2 N N2 O H2O N O aerosol particle … lost … O O2 N N2

LOCAL BACK-SCATTERING SCENE III LOCAL BACK-SCATTERING “mission accomplished! but …”

MIE BACK-SCATTERING … immutable identity … O2 aerosol particle N2 N2 H O H2O H O H2O N N2 N N2 O O2 N N2 … immutable identity …

MOLECULAR BACK-SCATTERING aerosol particle O O2 N N2 N N2 N N2 N N2 H O H2O H O H2O N N2 N N2 O O2 … preserving the identity … apparently …

RAMAN N2 BACK-SCATTERING H O H2O O O2 N N2 aerosol particle N N2 N N2 H O H2O N N2 N N2 … deep changes … O O2 N N2

RAMAN O2 BACK-SCATTERING H O H2O N N2 N N2 aerosol particle O O2 H O H2O N N2 O O2 N N2 … added values …

RAMAN H2O BACK-SCATTERING aerosol particle O O2 H O H2O N N2 O O2 N N2 … apparently new?…

SCENE IV THE DOWNWARD TRAVEL “on the way back …”

… again M.I.A. …

“several … at home with different stories …” SCENE V DETECTION “several … at home with different stories …”

… carrying back … a vanishing footprint. … LIDAR RECEIVER TELESCOPE

“figuring out the intimate experiences … a new vision” SCENE VI FINAL FATE “figuring out the intimate experiences … a new vision”

… something … useful … remains … INTO THE LIDAR RECEIVER Rayleigh-Mie N2 Raman H2 O Raman … wrong way for me! … … something … useful … remains … signal signal signal range range range

LIDAR PHYSICAL PROCESS Interaction 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/extinction Extinction = Scattering + Absorption Lidar physical processes 1

Scattering (elastic & inelastic) Lidar physical processes 2

Rayleigh scattering wavelength ()  particle size (r) [gas molecules] inversely proportional to 1/4. Blue sky, red sunset/sunrise 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 Scattering 2 Lidar physical processes 3

Raman scattering elastic collision of photons with molecules: molecular rotations, vibrations, electronic transitions change of  of incoming radiation ( R <104 cm-1) 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. Raman Raman Scattering 3 Lidar physical processes 4

Mie scattering r small cloud droplets, aerosols 1/. Affect long visible wavelengths 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. 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 Scattering 4 Lidar physical processes 5

LIDAR scattering back-scattering aerosol back-scattering aerosol extinction extinction Lidar physical processes 6

Back-scattering cross section Mie (aerosol) scattering 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 laser Lidar physical processes 7

Example ...o=355nm ~21nm ~32nm ~53nm Lidar physical processes 8

LIDAR … aerosol devoted Aerosol lidar

i.e., stratospheric aerosols backscattering increase Aerosol lidar i.e., stratospheric aerosols backscattering increase Aerosol lidar

Aerosol lidar – CETEMPS - Università Degli Studi dell’Aquila o=351nm; 1991-1999 163 profiles Pinatubo eruption starting June 1991 Aerosol lidar

i.e., tropospheric aerosol N2 Raman/anelastic signal Raman aerosol lidar i.e., tropospheric aerosol o Rayleigh/Mie signal N2 Raman/anelastic signal more backscattering o+N2 more attenuation no backscattering Aerosol lidar

Lidar setup XeF   ( =24) LASER Parabolic mirror 20cm Aerosol lidar UV Raman lidar L’Aquila

Aerosol lidar UV Raman lidar L’Aquila

Main characteristics 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) Aerosol lidar

Real Raman signal in presence of a cloud Nitrogen Raman 1/2 hour measurements nighttime Sept. 2001 cloud transmission Air/aerosol Rayleigh cloud backscattering Aerosol lidar

UV Raman lidar – CETEMPS - Università Degli Studi dell’Aquila o=351nm; N2=382nm (N2) Rayleigh/Mie Raman Aerosol lidar

(Bcks coeff.)/(ext. Coeff.) ... from Raman N2 … from Rayleigh/Mie (Bcks coeff.)/(ext. Coeff.) HOW? Lecture V.4 Aerosol lidar

Aerosol signature in the LIDAR measurements EXAMPLES: UV LIDAR (=355nm) SULFATE AEROSOLS, CLOUD DROPLETS, …

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)

naer(r) see G. Feingold CLOUD MODEL

lognormal naer(r) SULFATE AEROSOLS CLOUD DROPLETS

CLOUD DROPLETS

LIDAR SIGNAL SIMULATOR