1. The importance of reliable depolarisation measurements
Aerosol Working Group
Manchester, 15 February 2011
LIDAR
The importance of reliable
depolarisation measurements
Franco Marenco

2. volcanic ash flight (Irish Sea) PR2: “qualitative plot” non-ash
Extinction: analysed data, quantitative, GOOD
Depolarisation ratio: distinguish aerosol type
Unrealistic values (a factor of 20 higher expected)
volcanic
ash flight
(Irish Sea)

3. Aerosol extinction: Comparison with E-light ground-based
lidar in Aberystwyth (courtesy of Hugo Ricketts)
→ validation of both the instrument and the data analysis procedure

4. New lidar data inversion method
18 April 2010
(ground-based, Exeter)
BL AEROSOL
ASH
ash quantified
also when mixed
(low concentration,
but deep layer)
UTC+1
Separate ash from BL aerosol, using depolarisation
Requires: reliable depolarisation channel. At present: daytime ground-based only.

5. Depolarisation LIDAR concepts and calibration
Channel 0:
Channel 1:
Aerosol depolarisation ratio:
Molecular depolarisation ratio: → this is a well-known quantity
(but it depends on receiver bandwidth)
Volume depolarisation ratio:
Relative
depolarisation
ratio
Calibration
constant
use for
calibration
= (maximum... Cabannes line + full rotational Raman spectrum)
= (minimum.... only Cabannes line)
= (Leosphere: bandwidth 0.36 nm, or 28.6 cm-1)
(see Behrendt and Nakamura, 2002)

6. What if there is a channel cross-talk?
For a highly depolarising target,
For a weakly depolarising target,
(e.g. Rayleigh scattering
used in calibration)
Can’t be neglected!

7. What if there is a channel cross-talk?
volcanic ash
expected ~30%
measured 2.5%!!
K* calibrated here (area
affected by cross-talk)

8. Depolarisation LIDAR corrected for cross-talk
γ = cross-talk (Leosphere undocumented) ~ 0.025
δm =
Problem 1: cross-talk is undocumented

9. Problem 2: cross-talk varies during measurement
Ground-based
In flight

10. Cross-talk lidar tests
Temperature and humidity data loggers (Signatrol iButtons) have been inserted in lidar to monitor operating environment.
Data can be logged with a 10-minute resolution for 2 months.

11. Ground-based lidar test
Some weak (less worrying) “diurnal cycle” also here
This is the unprocessed and un-calibrated ratio of the ┴ signal to the ║ signal.
The atmospheric target is Rayleigh scattering.
Note the “diurnal cycle”!
3 days continuous measurements

12. Ground-based lidar test
Temperature dependence
of cross-talk is highlighted
(although the temperature variation is rather small).
Scattering due to condensation seems ruled out (relative humidity inside lidar is low).

13. What causes the T-dependence of the cross-talk?
It does not seem to be an issue of varying detector efficiency (1. detectors are after the optical separation of channels; 2. a varying detector efficiency would equally affect strongly + weakly depolarising targets).
It is unlikely to be a rotation of the polarisation state of emitted light (the laser is thermally regulated).
Possible candidate trouble-makers:
Polarising beam splitters: a mechanical misalignment with temperature?
Narrow-band interference filters? This would affect δm rather than γ, but the result would be similar

14. What next?
Airborne depolarisation/temperature tests during February-April (data loggers will be in place for COALESC and the early FENNEC week in April, and possibly during later campaigns).
The second (civil contingency) lidar has a new depolarisation design: compare the two instruments both on the ground and in flight.
Depending upon this comparison, we may opt for an upgrade of the first lidar by Leosphere (not before July due to campaigns).
After solving the problem of the variable part of the channel cross-talk, we’ll need to know the value of its constant part, for a correct data processing (→ either Leosphere, or laboratory work at Exeter).