1. Lidar Profiling of the Atmosphere
Geraint Vaughan
University of Manchester, UK

2. Basic principles LIDAR – Light Detection and Ranging
Similar principle to RADAR – pulses of light emitted into the atmosphere and scattered back by clouds, aerosols or air molecules
Light collected by a telescope
Spectrometers or interference filters isolate wavelength concerned
Photon-counting or analogue detection
Time-of-flight gives scattering height z=2ct z

3. What can we measure with lidar?
Clouds
Aerosol
Water vapour
Minor constituents e.g. ozone, hydrocarbons
Temperature
Wind (by Doppler-shifting)
Lidars can be used from the ground, aircraft or from space

4. Properties of lidar as a remote sensing tool
Advantages
Disadvantages
Good height and time resolution
Backscattered signals readily interpreted
May be mounted on trailers or aircraft for mobile operation
Affected by cloud (light can’t get through)
Background light is a problem in daytime
Systems to observe the stratosphere tend to be large (and expensive)
Precise alignment must be maintained

5. Example: Aberystwyth aerosol/water vapour lidar
Transmitter
Nd-YAG laser
355 nm
X10 Beam expander (refracting telescope)
To atmosphere
From atmosphere
Receiver

6. Transmitter characteristics
High power pulsed laser (UV/Vis/IR)
Typical pulse energy 10 – 400 mJ
Typical rep rate 10 – 50 s-1 (much higher for excimer or copper vapour laser)
Typical pulse length 3 ns (1 m)
Linearly polarised
Usually fixed wavelength – dye lasers and some solid state lasers tuneable.
Neodymium-YAG lasers a popular choice (1.06µm, 532, 355, 266 nm)

7. Receiver characteristics
Basically, focusing mirror to collect backscattered light. Size depends on application (e.g. 10 cm for low-level work, 1 m for stratosphere/mesosphere)
Photomultipliers with photon-counting electronics best for linearity and sensitivity but dynamic range limited: analogue electronics can deliver this for large signals.
Typical range resolution 30 m (min ~ 1 m)
Time resolution can range from single pulse to several hours
All equipment can be bought off-the-shelf these days.

8. P(λ,r) = P0 A E β(λ,r) exp-{ 2 0∫ Σ(λ,r’) dr’ } r
The Lidar Equation
Transmitted pulse power
Backscatter coefficient of atmosphere r
P(λ,r) = P0 A E β(λ,r) exp-{ 2 0∫ Σ(λ,r’) dr’ } r r2
Solid angle subtended by mirror
Transmissivity of atmosphere: contributions to Σ from scattering by air and aerosols, absorption by gases
Received power
Efficiency of optics and electronics

9. Elastic scattering Simplest form of lidar λmeasured = λtransmitted
Used for aerosol/ cloud measurements below 25 km and temperature above 25 km
Can use a small (few mW) laser and 10 cm telescope for clouds
Polarisation can distinguish different kinds of particles
λmeasured = λtransmitted
β is from air molecules and particles
If there are no particles, β is from air alone and proportional to density
Stratospheric aerosol measured with polarisation lidar, 9 Dec 2001

10. Aerosol Measurements
Measure the backscatter coefficient β, usually as a ratio to the air backscatter coefficient.
Lidar backscatter ratio = total backscattered signal/ backscatter from air alone; R = βtot / βair
Backscatter from air calculated from a nearby radiosonde profile, or measured by Raman scattering or polarisation measurement – background stratospheric aerosol are spherical droplets which don’t depolarise laser beam; air does depolarise slightly.
Dec (12 hours data)
Lidar backscatter ratio measured at Aberystwyth using dual polarisation method
Aerosol extinction must either be parameterised or measured using Raman scattering

11. Temperature measurements
Above 30 km, atmosphere generally aerosol-free. Then lidar signal measures density.
Use p = ρrT and dp/dz = -ρg
Assume p and T at upper boundary of profile and solve equations by stepping down the profile. Within ~15 km effect of boundary condition negligible
Can be used to measure T up to 80 km with very powerful systems. Extension to 100 km+ possible using resonance fluorescence
Daily mean temperature measured at ALOMAR, Andoya, Jan 1998

12. Cloud measurements
Airborne lidar measurements of cirrus outflow from thunderstorm near Darwin
Backscatter (arbitrary scale)
Path of in-situ aircraft (Egrett)
Depolarisation
Measurements from ARA King Air 23/11/02 – courtesy Jim Whiteway and Clive Cook

13. Raman Scattering
Scattered light is shifted in wavelength by an amount specific to the molecule concerned
Energy is exchanged with vibrational or rotational quantum states of molecules
Used to measure water vapour, temperature and aerosol extinction
Water: vibrational Raman. Laser at 355 nm, receivers at 407 nm (H2O) and 387 nm (N2)
Temperature: Rotational Raman. Laser at 532 nm, receivers at 533 and 535 nm

14. Properties of Raman lidars
Advantages
Disadvantages
Specific to particular molecules
Ratio to N2 directly measures mixing ratio
Insensitive to extinction
Many systematic errors cancel in ratio
Raman scattering is very weak
Need large lidars
For UTLS, measurements restricted to night-time
Spectroscopic uncertainties

15. Rotational Raman Spectrum
Interference Filters
210K
290K
Wavelength, nm
Wavelength, nm
Raman scattering from nitrogen, relative intensity
Raman scattering from oxygen, relative intensity

16. Water vapour measurements, Aberystwyth Dec 9 2001

17. Differential Absorption
Used for ozone and other absorbers
Transmit two wavelengths – one weakly and one strongly absorbed
Difference in attenuation through the atmosphere gives absorber profile
For ozone, we use laser at 266 nm shifted by Stimulated Raman Scattering to 289, 299 and 316 nm.

18. DIAL method P(λ,r).r2 α β(λ,r) exp-{2 ∫ Σ(λ,r) dr }
Σ is the extinction coefficient of the atmosphere per unit length. In the absence of aerosols, Σ = σairnair + σmoleculenmolecule
By measuring at two wavelengths with a large difference in σmolecule, and taking the ratio, the effect of that molecule can be isolated.
Rat(r) = P(λ1,r)/P(λ2,r) = β(λ1,r)/β(λ2,r) exp –{ 2∫ Σ(λ1,r) - Σ(λ2,r) dr}
The backscatter coefficients vary with distance in the same way for the two wavelengths, as these are determined by air and aerosol
So d ln(Rat) /dr = - 2{σmoleculenmolecule + σair nair}
Method gives absolute concentration

19. Ozone measurements, June 5 2000
Above: tropospheric measurements from 289/299 nm pair.
Below: stratospheric measurements from 299 alone. (We now do DIAL with 299/316 for stratosphere)

20. Mobile 5-wavelength Ozone/aerosol lidar
Supplied by elight, Germany
Uses 266, 289, 299, 316 and 355 nm
Ozone and aerosol profiles 100 m – 4 km
Used on field campaigns

21. Ozone and aerosol profiles, Sept 24 2003

22. What else can you measure with DIAL?
Courtesy of National Physical Laboratory, Teddington, UK

23. Summary
Lidar technique allows continuous monitoring of profiles with good height resolution
Different scattering mechanisms permit different kinds of measurement
New technology offers more compact sources and development of transportable and mobile systems