1st European Lidar Conference (ELC) 2018, 3 - 5/7 Thessaloniki, Greece
Interpreting the observed Particle Linear Depolarization Ratio spectral dependence of aged smoke using T-matrix simulations ANNA GIALITAKI 1, A. TSEKERI 1, V. AMIRIDIS 1, E. MARINOU 1, 2, E. GIANNAKAKI 3, S. SOLOMOS 1, R. CEOLATO 4, M. KOTTAS 1, E. TETONI 1, H. BAARS 5 AND R. ENGELMANN 5
Contact:
[email protected] 1
Institute for Space Applications and Remote Sensing, National Observatory of Athens, Athens, Greece, 2 Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany, 3 National and Kapodistrian University of Athens, 4 ONERA, The French Aerospace Lab, Toulouse, France, 5 Leibniz Institute for Tropospheric Research (TROPOS), Leipzig, Germany
We present lidar measurements of aged tropospheric and stratospheric smoke plumes, originating from the large scale Canadian fires in August 2017. Retrieval of the optical properties showed that for the stratospheric layer the values of Particle Linear Depolarization Ratio (PLDR) deviate from typical ones at both measurement wavelengths, while this is not the case for tropospheric smoke. Furthermore, for both tropospheric and stratospheric smoke, the PLDR value at 355nm is significantly larger than 532nm indicating a strong wavelength dependence. In general, smoke particles are assumed to be spherical, thus not expected to cause high depolarization. In order to interpret our results, we utilize T-matrix simulations focusing on particle shape. We use typical refractive indices and effective radii of aged smoke particles, assuming near-spherical and Chebyshev particles, based on findings that clusters of soot aggregates might form in aged smoke plumes. The observed PLDR and lidar ratio (LR) values are reproduced successfully using near spherical particles with an aspect ratio of 0.9 and 0.95 for stratospheric and tropospheric smoke particles respectively. Polly XT NOA Range Corrected signal at 1064nm
Near spherical particles
Stratospheric layer
Tropospheric layer
Stratosphere
Troposphere
a/b = 0.9
a/b = 0.95
m = 1.45 + 0.03i reff = 0.45 μm
m = 1.45 + 0.025i reff = 0.45 μm
Polly XT NOA Volume Linear Depolarization Ratio at 532nm Stratospheric layer
Tropospheric layer
Optical properties of tropospheric and stratospheric smoke Layer width PLDR 355 (km) (%) 5.8 - 6.7
8.7±0.9
PLDR 532 (%)
LR 355 (sr)
LR 532 (sr)
EAE (355/532)
3.4±0.1
53.2± 5
53.9±15.6
1.15±0.7
Chebyshev particles 12.2 - 13
28.9±0.7
15.4±0.3
41.2±8.9
61.8±17.5
-0.44±0.09
21/8/2017, 22:00-24:00 UTC
Τ2 (0.10)
Τ4 (0.10)
Τ6 (0.10)
Τ2 (-0.10)
Τ4 (-0.10)
Τ6 (-0.10)
Chebyshev particles graphics source: https://books.google.gr/books?id=qIg6AQAAIAAJ
Summary - Conclusions: • Tropospheric and stratospheric smoke plumes originating from large scale fires were observed with the portable polarization, Raman lidar of the National Observatory of Athens. • The PLDR of smoke in the troposphere and stratosphere showed a strong wavelength dependence at 355 and 532 nm. More importantly, the PLDR values of stratospheric smoke are exceptionally high. • Simulations of the PLDR, LR and extinction related Angstrom exponent (EAE) were conducted with the use of T-matrix code for a range of microphysical properties corresponding to aged smoke particles, assuming both spheroids and Chebyshev particles. • For the stratospheric layer it is found that the aged smoke optical properties are reproduced if we consider near spherical particles with an aspect ratio of 0.9. • For the tropospheric layer PLDR and LR values were reproduced assuming particles with an aspect ratio of 0.95. Unfortunately, the simulated EAE deviates significantly from the measured value. This is a point for further investigation. • The retrieved imaginary part of the refractive index indicates slightly higher absorption of stratospheric smoke. • Simulations made with more complex morphologies (Chebyshev particles) showed that only the near-spherical Chebyshev particles with minimum surface roughness are able to reproduce the PLDR wavelength dependence of the stratospheric smoke. This is supported also from the findings of Ceolato et al. (2018 - in preparation), who used even more complicated morphologies (soot fractal aggregates) aiming to reproduce this behavior. • Chebyshev particles experiencing the maximum surface roughness reproduced successfully the PDLR behavior in the troposphere. Although there was no simulation able to reproduce also the LR. • Further investigation of the particular tropospheric and stratospheric processes associated with smoke particles aging that may lead to these observations, is underway. REFERENCES Burton et al., Atmospheric Chemistry and Physics, tomos, pages, (2015) Bi et al., Optics Express , (2018) Ceolato et al., ELS conference proceedings (2018) Hansen, J.E., Travis, L.D., Light scattering in planetary atmospheres (1974) Space Science Reviews Mishchenko, M.I., Travis, L.D, Journal of Quantitative Spectroscopy and Radiative Transfer, ( http://www.giss.nasa.gov/&crmim ), 1998 Mishchenko et al., (2016) Applied Optics
ACKNOWLEDGEMENTS The research leading to these results has received funding from: ACTRIS-2 under grant agreement no. 654109 from the European Union’s Horizon 2020 research and innovation programme ECARS under grant agreement No 602014 from the European Union’s Horizon 2020 Research and Innovation programme European Research Council under the European Community's Horizon 2020 research and innovation framework program / ERC Grant Agreement 725698 (D-TECT) Stavros Niarchos Foundation
