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V.3 AEROSOL LIDAR THEORY

V.3 AEROSOL LIDAR THEORY. Vincenzo Rizi vincenzo.rizi@aquila.infn.it CETEMPS Dipartimento di Fisica Università Degli Studi dell ’ Aquila Italy. OUTCOMES Upon the lecturer ability you will be able to: understand how LIDAR techniques are used to characterize atmospheric aerosols

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V.3 AEROSOL LIDAR THEORY

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

  2. 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

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

  4. LIDAR HISTORY 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

  5. searchlight 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

  6. modern lidar 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

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

  8. 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.

  9. 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.

  10. 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.

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

  12. 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

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

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

  15. 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

  16. 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

  17. J UV laser Lidar equation 7

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

  19. Lidar equation 9

  20. 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

  21. 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

  22. 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

  23. 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

  24. 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

  25. A PARTIAL REPRESENTATION (a physics-ological drama)

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

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

  28. FEATURING: LIGHT CHARACTERS 3/3 EXTINCTED PHOTONS

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

  30. SCENE I THE LASER EMISSION “leaving together …”

  31. LIDAR LASER EMISSION laser

  32. SCENE II THE UPWARD TRAVEL “experiencing …”

  33. MIE EXTINCTION O2 O2 O2 N2 N2 N2 N2 N2 N2 O O O N N N N N N N N N N N N O O O aerosol particle H2O H2O aerosol particle O O H H H H … lost …

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

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

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

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

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

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

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

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

  42. … again M.I.A. …

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

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

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

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

  47. 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

  48. Scattering (elastic & inelastic) N2 Scattering 1 Lidar physical processes 2

  49. 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

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