Lidar for Atmospheric Remote sensing Philippe Keckhut et Andrea Pazmiño LATMOS, Institut Pierre Simon Laplace, CNRS-UPMC-UVSQ, Paris, France Historical Overview Lidar Basics The Lidar Equation Lidar systems Summary Outline

Historical Overview (1) 1930 Synge proposed a method to determine the atmospheric density with an antiaircraft searchlight and a telescope (bistatic configuration) 1936 First reported results of density profiles: Duclaux (3.4 km), Hulbert (28 km) 1938 First reported use of a monostatic configuration for cloud base height, using a pulsed light source (Bureau) 1953 First retrieval of temperature profiles from density profiles (Elterman) Emitted beam Detector field of view ~km Monostatic co-axial Monostatic bi-axial Bistatic Transmitter & receiver collocated (pulsed light source  range of scattering) Receiver’s FOV scanned along the transmitted beam (geometry  range of scattering)

Historical Overview (2) 1956 Friedland et al. reported the first pulsed monostatic system for atmospheric density measurements Early 1960s Invention of laser  powerful new light source for lidar systems 1962 First use of laser in a lidar system (Smullins & Fiocco) 1977 First ozone measurements by lidar (Mégie et al.) Gérard Mégie

Historical Overview (3) Present Networks of ground-based lidar systems as NDSC, EARLINET, etc Lidars on aircraft Space-based lidar (ALISSA, LITE, … CALIPSO program)

LIDAR: LIght Detection And Ranging Lidar Basics (1) LIDAR: LIght Detection And Ranging Active remote sensing technique for measuring atmospheric parameters (T, , wind and different constituents: H2O, O3, …, clouds, aerosols) Same principle as radar but 0.1 <  < 10 m Principle: emission of a light beam that interacts with the medium & detection of radiation backscattered towards the instrument Interactions with the Atmosphere: elastic (Rayleigh, Mie, Resonance scattering) inelastic (Raman scattering, Fluorescence) Absorption

Lidar Basics (2) + = Differential Absorption & Scattering Some Optical Interactions of Relevance to Laser Environmental Sensing Elastic Interactions Inelastic Interactions Rayleigh Scattering Mie Scattering Virtual Level Vibrationally Excited Level  ~ d  Mie = C/a  ~ d  Mie = C/a Raman Scattering  >> d  Rayleigh = C/4  ~ d  Mie = C/a Interaction with the quantized vibrational & rotational energy levels of the molecule Absorption + = Differential Absorption & Scattering

Block diagram of a generic lidar system Lidar Basics (3) Block diagram of a generic lidar system Emitted light Backscattered light Laser Beam expander (optional) Transmitter Light collecting telescope Optical filtering Receiver Synchronization control Optical to electrical transducer Electrical recording system Detector & Recording

Complete altitude scattering profile Lidar Basics (4) Ranging of pulsed monostatic lidar Time Altitude Aerosol layer 2 layer 1 Z1 Z2 Laser beam Scattered light T1=2.Z1/C T2=2.Z2/C Signal Rayleigh scattering Mie Noise level Emission impulsion tup tdown tup + tdown = Each light pulse fired  Complete altitude scattering profile z = ct/2  1 s 150 m

The lidar equation (1)  Number of photons detected by a lidar system o instrumental parameters o geophysical variables  If Ne is the total number of photons emitted by the laser at L Total number of photons transmitted into the atmosphere transmission coefficient of optics (0-1)  The number of photons available to be scattered at the distance r optical transmission of the atmosphere at L along the laser path to the range r  The number of photons backscattered, per unit solid angle due to scattering of type i, from the range interval R1 to R2 backsatter coefficient for scattering of the type i and L

The lidar equation (2)  Number of photons incident in the collecting optic of the lidar due to scattering of the type i area of the collecting optic overlap factor wavelength of the scattered light decreasing illuminance of the telescope by the scattered light  The number of photons N(s,r) after the detection transmission coefficient of the reception optics at s quantum efficiency of the detector at s

The lidar equation (3) o L = s  Ta(L) = Ta(s)  In many cases, approximations allow simplification of lidar equation … o L = s  Ta(L) = Ta(s) o integral range  cte, during acquisition (t = 2 (R2-R1)/c) o (r)  1  Then, the lidar equation … instrumental dependency atmospheric dependency where contribution of molecules & particles optical depth extinction coefficient of molecules & particles cross section of constituent k at L concentration of constituent k

Rayleigh-Mie Aerosol Lidar (1) Receiver contribution of aerosols Transmitter Application of inversion method to the lidar equation (Klett)  p(,z), p(,z) (hypothesis on p(,z)/p(,z) ) Polarization technique: measure the polarization ratio  indication of aerosols shape (liquid or solid) Multi-wavelength lidar  Spectral dependence of aerosol optical thickness (AOT) Detector (polarization technique) Measurements of aerosols & clouds in the troposphere & lower stratosphere

Aerosol and molecular scattering (2) Both molecular and aerosol contribution are present Aerosols are identified through their vertical shape Aerosol analysis consists in estimating Molecular contribution Aerosol attenuation

Rayleigh-Mie Aerosol Lidar (3) Measurements in the troposphere (Pietras et al., 2004) ln(Nr2) 15000 10000 5000 flag 15000 10000 5000 Altitude [m] Altitude [m] 8 9 10 11 12 13 14 15 8 9 10 11 12 13 14 15 Time [UTC] Time [UTC] Temporal evolution of lidar signal at 532 nm (linear polarization component) corrected in distance [ln(Nr2)] for April 1st 2003, (left panel). Classification of the atmospheric layers: noise (flag 0), zone with molecules (flag 1), ABL (flag 2), zone with particles (flag 3 & 4), and indefiended zones (flag > 4) Transmitter: Nd:YAG at 532 nm (second harmonic) & linear polarization + expander Receiver: 2 telescopes (0.1-7 km & 2-15 km) Detection: 532 nm linear & cross polarization components par PMT, 1064 nm par avalanche photodiodes Vertical resolution: 15 m & temporal resolution: 30’ Classification of the atmospheric structure from backscattering lidars signals corrected from noise & total overlapping: (Identification of atmospheric boundary layer (ABL), the zones with particles (aerosols & clouds) & finally the zones with molecules)

Multiwavelenght lidar (4) Médiane du nuage filtré Minimum de la fonction de coût No = 7.71 cm-3 rm = 0.29 µm σ = 1.45

Size distribution -> Aerosol surface and volume

Temperature measurements (5) Required pure molecular scattering Density and pressure are relative measurements Temperature is absolute

Rayleigh Lidar (6) Temperature measurements (Alpers et al., 2004) (a) (background corrected) raw lidar backscatter profiles with the Rayleigh/Mie/Raman (RMR) and Potassium (K) lidar at Kühlungsborn, Germany on 23 February 2003. (b) Temperatures profiles retrieved from (a) Transmitter: Nd:YAG at 532 nm (second harmonic) & 355 nm (third harmonic) for T measurements 532 nm high Rayleigh signal : 4 telescopes of 50 cm diameter  40-90 km (blocking chopper at 40 km) 532 nm low Rayleigh signal : 1 telescope of 50 cm diameter  20-50 km (blocking chopper at 20 km) 1 h integration time Vertical resolution of 1 km & a heigh-variable smooth filter (0.6-3 km width) Statistical T error < 10 %

To discriminate species: Raman scattering (7) Raman consists in a spectral shift of the returned wavelength Raman shift is characterized by the molecules considered Only attenuation of the bean is required Technique useful for pollution

Raman Lidar (8) Spectral shift of the returned wavelength (Raman= L   ) Raman shift is characterized by the considered molecules (unique spectral signature) The vibrational Raman lines are generally selected for detection  concentrations High-quality of narrow-band interference filters High-blocking filter for elastic backscatter of molecules & aerosols Small cross-section of Raman scattering  molecules with a relatively high abundance (H2O, N2, O2) X Measurements of temperature using Raman scattering from N2 X Cloud & aerosols can also be studied by this technique H2O Raman Lidar: q(z) H2O mixing ratio is specified as: Atmospheric transmission at Ram Differential Raman backscattering cross sections for water vapor & nitrogen Calibration constant Lidar Raman signal for nitrogen & water vapor k Mass H2O / Dry air mass

H2O Raman: Calibration (9) H2O at 660 nm N2 at 607 nm Filters for Rayleigh rejection At 532 nm Beam spliter Sky background calibration during daytime (SZA=60°) 2 channels: H2O and N2

Raman Lidar (10) Water vapor measurements in the lower troposphere (Tarniewicz et al., 2003) a) 29/10/2002 b) (a) Comparisons with collocated radiosonde. Data are summed over 20 minutes. (b) Height time series for the water vapor mixing ratio for night 29 October 2002. Profiles are summed over 5 minutes. (right column). Vertical resolution is variable from 50 to 500 m in order to maintain a good signal to noise ratio Good agreement between lidar and RS water vapor mixing ratio below 5 km Same water vapor structures are seen by the two instruments. Slight overestimation of lidar profile after 4 km due to an undetermined instrumental bias. Relative precision of lidar < 5% (up to 2 km) & 10% (up to 4 km)  requirements for boundary layer applications.

Lidar Retrieval for O3 Measurements (11) DIfferential Absorption Lidar technique for stratospheric ozone measurements Measurements of the stratospheric ozone vertical distribution Two laser wavelengths (on, off) characterized by a different ozone absorption cross section (UV spectral range, great ozone absorption) Self calibrating technique, no instrumental constants Rayleigh & Mie differential scattering Rayleigh & Mie differential extinction Absorption by others constituents (SO2, NO2) O3 number density differential O3 absorption cross-section number of detected photons at i background radiation at i correction term

OHP stratospheric ozone DIAL system (12) Multiple-fiber collector concept Mechanical chopper optical fibers Beam expanders Moveable fiber mounts for the alignment of the XeCl laser radiation spectrometer 4 Collecting mirrors:  0.53 m, F 1.5 m, Ap. F/3 Ref. line: 3rd harmonic Continuum Nd:Yag (355 nm) Abs. radiation : XeCl Lambda Physics EMG 200 Excimer laser (308 nm)

Example of ozone profile (13) Courtesy S. Godin-Beekmann Ozone measurements performed during the night Temporal resolution 3 – 4 hours Require clear skies

Wind measurements (14) Wind is based on the Doppler shift of the return signal

Limitations (1) Dynamic of the signal : 5-6 orders of magnitudes Emission-reception geometry Parallax defocalisation Noise and signal-induced-noise

Lidar subsystems (2) Transmitter sub-system Strategy of measurement laser beam expander Strategy of measurement  choice of the laser source Altitude range and concentration to be detected Specific Wavelengths (absorption) Energy & repetition rate Concentration and spectral characteristics of other gases Reliability, ease of operation in monitoring applications Operating costs Common examples: Gas laser (ex: excimer laser) optical medium: gas of molecules only stables in an excited state. Electrical discharge Solid-state laser (ex: Nd:YAG) Impurity ions (Nd3+) in a glassy material (YAG). Optically pumped by a flash lamp  stimulated emission (1.06 m)

Lidar subsystems (3) Receiver sub-system Collection of scattered laser light back from the atmosphere and focuses it to a smaller spot   ~ 10 cm, lenses or mirrors (close range)   ~ few meters, mirrors (middle & upper atmosphere) 4 fibers grating 387 nm 308 nm high & low energy 355 nm 347 nm 332 nm  spectral filtering schemes: centered in a specific wavelength (dichroic, gratings, mirrors, narrowband interference filters  < 1 nm)  separation based on polarization (aerosols)  protection of detector (fast mechanical shutter, electrical gating) Processing of scattered laser light chopper

Lidar subsystems (4) Signal Detection & Acquisition sub-system Conversion of light into an electrical signal & recording in electronic device  Photomultipliers Tubes (PMTs) are generally used in incoherent lidar systems (direct detection) Output of PMT: current pulses produced by photons + thermal emission of electrons (dark current) 2 Techniques: - Photon counting Mode (individual pulses) - Analog Mode (multitude of pulses) To the electronic device Hamamatsu Selecting the PMT: - PMT structure  optical measurement conditions - Photocathode Quantum Efficiency  high QE in the wavelength range - Gain  > 106 - Dark count  lower detection limit - Response time  maximum count rate, time resolution

Lidar subsystems (5) Signal Detection & Acquisition sub-system Photo Counting Preamplifier  amplification + pulse shape (ringing) Main amplifier (if it is necessary) High speed comparator (discriminator)  remove a substantial number of the dark current Pulse shape & counter x Generally for low signals detection Hamamatsu Typical Photon Counting System Analog Detection Pulse pair resolution of the detector 10 to 100 MHz Fast analog-to-digital converter x Generally for tropospheric measurements Coherent Detection Mixing of backscattered laser light with light from local oscillator on a photomixer  radio frequency (RF) signal Frequency of RF signal  Doppler shift of the scattered laser light  wind velocity x Frequency stability & short laser pulse length is required

Photon counting (6) t t Measurement = Histogram D Improvements = increase the number of collected photons Size of the telescope Laser power Vertical resolution Temporal resolution t t