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

2. 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

3. 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

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

5. 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

6. 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

7. 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

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

9. 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.

10. 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.

11. 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.

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

13. 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

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

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

16. 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

17. 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

18. J UV laser
Lidar equation 7

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

20. Lidar equation 9

21. 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

22. 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

23. 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

24. 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

25. 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

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

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

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

29. FEATURING:
LIGHT CHARACTERS 3/3
EXTINCTED
PHOTONS

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

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

32. LIDAR
LASER EMISSION
laser

33. SCENE II THE UPWARD TRAVEL
“experiencing …”

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

35. 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

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

37. MIE BACK-SCATTERING … immutable identity … O2 aerosol particle N2 N2H O
H2O H O
H2O N N2 N N2 O O2 N N2
… immutable identity …

38. 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 …

39. RAMAN N2 BACK-SCATTERINGH 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

40. RAMAN O2 BACK-SCATTERINGH O
H2O N N2 N N2
aerosol particle O O2 H O
H2O N N2 O O2 N N2
… added values …

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

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

43. … again M.I.A. …

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

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

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

47. … 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

48. 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

49. Scattering (elastic & inelastic)
Lidar physical processes 2

50. 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

51. 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

52. 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:
Scattering 4
Lidar physical processes 5

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

54. Back-scattering cross section Mie (aerosol) scattering
Back-scattering cross sections
Physical process
Back-scattering cross section
Mie (aerosol) scattering
10-8  cm2 sr-1
Rayleigh scattering
10-27 cm2 sr-1
Raman scattering
10-30 cm2 sr-1
receiver
laser
Lidar physical processes 7

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

56. LIDAR … aerosol devoted
Aerosol lidar

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

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

59. 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

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

61. Aerosol lidar
UV Raman lidar L’Aquila

62. 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

63. 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

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

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

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

68. SULFATE AEROSOLS Qext(r,m,) Qbck(r,m,)
Volodymyr Bazhan ScatLab Project

69. CLOUD DROPLETS Qext(r,m,) Qbck(r,m,)
Volodymyr Bazhan ScatLab Project

70. naer(r)

71. naer(r)
see
G. Feingold
CLOUD MODEL

72. lognormal naer(r)
SULFATE AEROSOLS
CLOUD DROPLETS

73. CLOUD DROPLETS

74. LIDAR SIGNAL SIMULATOR