Spectroscopy Course in Fall 2009 ASEN 5519 Fundamentals of Spectroscopy for Optical Remote Sensing Xinzhao Chu University of Colorado at Boulder 1 Concept of Remote Sensing Remote Sensing is the science and technology of obtaining information about an object without having the sensor in direct physical contact with the object. -- opposite to in-situ methods Radiation interacting with an object to acquire its information remotely Active SODAR: Sound Detection And Ranging Remote RADAR: Radiowave Detection And Ranging Sensing LIDAR: Light Detection And Ranging DRI, Nevada Arecibo 2 Light Detection And Ranging LIDAR is a very promising remote sensing tool due to its high resolution and accuracy. In combination of modern laser spectroscopy methods, LIDAR can detect variety of species and key parameters, with wide applications. Cloud 3 Light Detection And Ranging 120 km 75 km 30 km Ground Time of Flight Range / Altitude R = C t / 2 4 “Fancy” Lidar Architecture Transceiver (Light Source, Light Collection, Lidar Detection) Holographic Optical Element Data Acquisition (HOE) & Control System Courtesy to Geary Schwemmer 5 From Searchlight to Modern Lidar Light detection and ranging (LIDAR) started with using CW searchlights to measure stratospheric aerosols and molecular density in 1930s. Hulburt [1937] pioneered the searchlight technique. Elterman [1951, 1954, 1966] pushed the searchlight lidar to a high level and made practical devices. The first laser - a ruby laser was invented in 1960 by Schawlow and Townes [1958] (fundamental work) and Maiman [1960] (construction). The first giant-pulse technique (Q-Switch) was invented by 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. From Aerosol Detection to Spectral Analysis The first application of lidar was the detection of atmospheric aerosols and density: detecting only the scattering intensity but no spectral information. An important advance in lidar was the recognition that the spectra of the detected radiation contained highly specific information related to the species, which could be used to determine the composition of the object region. Laser- based spectral analysis added a new dimension to lidar and made possible an extraordinary variety of applications, ranging from groundbased probing of the trace-constituent distribution in the tenuous outer reaches of the atmosphere, to lower atmosphere constituents, to airborne chlorophyll mapping of the oceans to establish rich fishing areas. 6 Physical Interaction Device Observables Elastic Scattering Mie Lidar Aerosols, Clouds: by Aerosols and Clouds Geometry, Thickness Gaseous Pollutants Absorption by DIAL Atoms and Molecules Ozone Humidity (H O) Inelastic Scattering Raman Lidar 2 Aerosols, Clouds: Optical Density Elastic Scattering Rayleigh Lidar Temperature in By Air Molecules Lower Atmosphere Resonance Fluorescence Stratos & Mesos Resonance Density & Temp By Atoms Fluorescence Temperature, Wind Lidar Density, Clouds Boltzmann Distribution in Mid-Upper Atmos Doppler Shift Wind Lidar Wind, Turbulence Laser Induced Fluorescence Fluorescence Lidar Marine, Vegetation Laser Range Finder Reflection from Surfaces Topography, Target & Time of flight Laser Altimeter 7 Elastic and Inelastic Scattering virtual level Raman Absorption Fluorescence Raman Absorption Rayleigh Fluorescence Atomic absorption & Molecular elastic and inelastic scattering, (resonance) fluorescence absorption and fluorescence 8 Physical Interactions in Lidar 120 70-120 km and above 120 km: resonance fluorescence (Fe, Na, Thermosphere + K, He, O, N2 ) Doppler, Boltzmann, 100 Mesopause differential absorption lidar Airglow & Meteoric Layers Airglow, FP Interferometer PMC Fe, Na, K, Ca, OH, O, O 80 2 Molecule & aerosol scattering, Mesosphere Rayleigh and Raman integration , direct detection Doppler lidar 60 Molecular species, differential Altitude (km) Stratopause absorption and Raman lidar 40 Molecule & aerosol scattering Ozone Stratosphere High-spectral resolution lidar, PSC 20 Coherent detection Doppler lidar, Aerosols Tropopause Direct detection Doppler lidar, Clouds Troposphere Direct motion detection tech 0 (tracking aerosols, LDV, LTV) 150 200 250 300 350 Temperature (K) 9 Na Doppler (Wind & Temp) Lidar kBT D = 2 M0 v = 1 R c 12 10 150 K 0 m/s ) ) 2 2 10 m 200 K m 8 50 m/s -16 -16 250 K 100 m/s 8 D2a 6 6 4 4 D2b 2 2 Absorption Cross-Section (x10 Absorption Cross-Section (x10 0 0 -2000 -1000 0 1000 2000 -2000 -1000 0 1000 2000 Frequency Offset (MHz) Frequency Offset (MHz) Resonance Fluorescence, Frequency Analyzer in Atmosphere 10 Full-Diurnal Multiple-Beam Obs. Dr. Chiao-Yao She with CSU Na lidar [Yuan et al., JGR, 2008] 11 Large Aperture for High Precision [Chu et al., JGR, 2005] Maui, Hawaii UIUC Na Wind & Temperature Lidar Coupled with Large Telescope Momentum flux climatology [Gardner and Liu, JGR, 2007] 12 CEDAR Science: Meteor & Metal Species Na Lidar Beam Meteor from extraterrestrial [Chu et al., 2000] Meteor ablation deposits metallic atoms Lidar detection of persistent meteor trails during Leonid Shower 1998 [Plane, 2003] 13 Fe Boltzmann Temperature LIDAR J'=1 J'=2 5 0 South Pole z F J'=3 6 3d 4s4p J'=4 J'=5 372nm 374nm 368nm Electra 100% 91% 9% J=0 J=1 J=2 a5D 6 2 J=3 3d 4s 1 E 416cm J=4 P2(J = 3) g2 Rothera = exp(EkBT) P1(J = 4) g1 E/k T B = g2 P1 ln g1 P2 [Gelbwachs, 1994; Chu et al., 2002] 14 PMC Hemispheric Difference & Fe/PMC Heterogeneous Chemistry 85.5 85 South Pole Southern Hemisphere South Pole 84.5 Fe 84 Rothera 83.5 North Pole Altitude (km) 83 82.5 PMC Northern Hemisphere 82 81.5 40 50 60 70 80 90 Latitude (degree) Southern PMC are ~ 1 km Heterogeneous Removal of Higher than Northern PMC Mesospheric Fe Atoms by Earth Orbital Eccentricity PMC Ice Particles Observed and Gravity Wave Differences by the Fe Boltzmann Lidar [Chu et al., JGR, 2003, 2006] [Plane et al., Science, 2004] 15 Shuttle Formed High-Z Sporadic Fe Columbia Space Shuttle launched on Jan 16, 2003 Lyman Images from GUVI/TIMED High-Altitude Sporadic Fe layer detected by Fe Boltzmann Lidar on Jan 19, 2003 at Rothera (67.5S) Causes: Shuttle Engine Ablation! [Stevens et al., GRL, 2005] 16 DIAL & Raman Lidar for Trace Gases The atmosphere has many trace gases from natural or anthropogenic sources, like H2O, O3, CO2, NOx, CFC, SO2, CH4, NH3, VOC, etc. Can we use resonance fluorescence to detect them? Quenching effects due to collisions make fluorescence impossible in lower atmosphere for molecules. We still need spectroscopy detection - differential absorption and Raman lidars! 17 Raman Lidar for Water Vapor H2O molecules exhibit specific spectra - fingerprints! Raman lidar catches this ‘fingerprints’ and avoid the aerosol scattering in the Raman-shifted channel. Thus, only aerosol extinction will be dealt with in deriving H2O mixing ratio. 18 DIAL for Ozone in Two Decades abs = abs(ON ) abs(OFF ) Tsukuba (36N, 140E), Japan [Tatarov et al., ILRC, 2008] 19 Rayleigh + Raman Integration Lidar Hydrostatic Equation dP = gdz + Rayleigh Ideal Gas Law P = RT z Altitude (km) (z ) 1 o (r) Raman T (z) T (z ) o g(r)dr = o + (z) R z (z) Density Ratio Temperature Temperature (K) Searchlight, Falling Sphere Rayleigh Lidar, VR-Raman Lidar [Keckhut et al., 1990] 20 Direction-Detection Doppler Lidar In lower atmosphere, Rayleigh Aerosol and Mie scattering experiences Scattering Doppler shift and broadening. Molecular Scattering However, there is no frequency analyzer in the atmosphere, so the receiver must be equipped with narrowband frequency analyzers for spectral analysis. Double-Edge Filter Fringe-Imaging 21 Coherent Doppler Wind Lidar “Heterodyne” Detection from aerosol scattering: the return signal is optically mixed with a local oscillator laser, and the resulting beat signal has the frequency (except for a fixed offset) equal to the Doppler shift. fbeat = fLO fSig = f + foffset CEDAR Lidar Tutorial NOAA HRDL 22 Backscatter Cross-Section Comparison Physical Process Backscatter Mechanism Cross-Section Mie (Aerosol) Scattering 10-8 - 10-10 cm2sr-1 Two-photon process Elastic scattering, instantaneous Atomic Absorption and 10-13 cm2sr-1 Two single-photon process (absorption Resonance Fluorescence and spontaneous emission) Delayed (radiative lifetime) Molecular Absorption 10-19 cm2sr-1 Single-photon process Fluorescence From 10-19 cm2sr-1 Two single-photon process Molecule, Liquid, Solid Inelastic scattering, delayed (lifetime) Rayleigh Scattering 10-27 cm2sr-1 Two-photon process (Wavelength Dependent) Elastic scattering, instantaneous Raman Scattering 10-30 cm2sr-1 Two-photon process (Wavelength Dependent) Inelastic scattering, instantaneous 23 Mobile Solid-State Doppler Lidar NSF Major Research Instrumentation (MRI) mobile Fe-resonance/ Rayleigh/Mie Doppler lidar is an advanced resonance fluorescence lidar being developed at the University of Colorado, Boulder. It is based on Pulsed Alexandrite Ring Laser (PARL) for simultaneous measurements of temperature (30-110 km), wind (75-110 km), Fe density (75-115 km), aerosols/clouds (10-100 km), and gravity waves in both day and night through an entire year with high accuracy, precision, & resolution. 24 Extending Measurement Range Extending downward: -- Various edge-filter techniques are being developed to probe lower atmosphere wind and temperature simultaneously -- White-light lidar [Kasparian et al., 2003] Extending upward: -- Thermosphere Helium (He) lidar + -- Aurora N2 resonance lidar Driven by