Abstract
We present smoke lidar measurements from the Canadian fires of 2017. The advected smoke layers over Europe are detected at both tropospheric and stratospheric heights, with the latter presenting non-typical values of the Particle Linear Depolarization Ratio (PLDR) with strong wavelength dependence from the UV to the Near-IR. Specifically, the PLDR values are of the order of 22, 18 and 4% at 355, 532 and 1064 nm respectively. In an attempt to interpret these results, we apply the hypothesis that smoke particles have near-spherical shapes. Scattering calculations with the T-matrix code support other findings in the literature ([1]- [2]), showing that the near-spherical shape (or closely similar shapes as in [2]), is the only shape that has been shown to reproduce the observed PLDR and Lidar Ratio (LR) values of the stratospheric smoke particles at the three measurement wavelengths.
Highlights
Recent studies show that the Particle Linear Depolarization Ratio (PLDR) of smoke presents large variability ([3] - [7])
As it can be seen from the Range Corrected Signal (RCS) at 1064 nm, the stratospheric smoke layer lies between 15 and 16 km, while there is a smoke layer in the troposphere at 5-6 km
No significant difference has been detected for the Lidar Ratio (LR) values between the tropospheric and stratospheric layers, while this does not hold true for the PLDR values
Summary
Since the PLDR is indicative of particle shape, this variability may be attributed to (a) smoke aerosol mixing with other aerosol types, (b) particles’ aging through various atmospheric processes, (c) particle water uptake at different relative humidity conditions. Burton et al (2015) [8], showed air-borne HSRL measurements of smoke at 8 km altitude, originating from Canadian fires, revealing PLDR values of the order of 20, 9 and 1.8 % at 355, 532 and 1064 nm, respectively. These are the first reported measurements of smoke PLDR spectral dependence.
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