Abstract
Abstract. We examine the capability of near-spherical-shaped particles to reproduce the triple-wavelength particle linear depolarization ratio (PLDR) and lidar ratio (LR) values measured over Europe for stratospheric smoke originating from Canadian wildfires. The smoke layers were detected both in the troposphere and the stratosphere, though in the latter case the particles presented PLDR values of almost 18 % at 532 nm as well as a strong spectral dependence from the UV to the near-IR wavelength. Although recent simulation studies of rather complicated smoke particle morphologies have shown that heavily coated smoke aggregates can produce large PLDR, herein we propose a much simpler model of compact near-spherical smoke particles. This assumption allows for the reproduction of the observed intensive optical properties of stratospheric smoke, as well as their spectral dependence. We further examine whether an extension of the current Aerosol Robotic Network (AERONET) scattering model to include the near-spherical shapes could be of benefit to the AERONET retrieval for stratospheric smoke cases associated with enhanced PLDR. Results of our study illustrate the fact that triple-wavelength PLDR and LR lidar measurements can provide us with additional insight when it comes to particle characterization.
Highlights
Particles originating from biomass burning activities are known to have a significant effect on radiation and climate (Kaufman et al, 2002)
Our analysis shows that for the Canadian stratospheric smoke observed above Europe in August 2017, the particle linear depolarization ratio (PLDR) and lidar ratio (LR) measurements along with their spectral dependence can be successfully reproduced with the proposed model of compact near-spherical particles
The unique optical properties of transported stratospheric smoke, originating from the pyrocumulonimbus activity of the large Canadian fires in 2017, were reproduced using Tmatrix simulations and assuming near-spherical shapes for smoke. This is consistent with results of past studies showing that near-spherical particles produce PLDR values that can reach up to 100 % depending on their size and composition (Bi et al, 2018) and that smoke particles in particular, when heavily coated or even encapsulated with weakly absorbing materials, can produce large depolarization with a noticeable spectral dependence (Mishchenko et al, 2016; Ishimoto et al, 2019)
Summary
Particles originating from biomass burning activities are known to have a significant effect on radiation and climate (Kaufman et al, 2002). The measurements were performed above Denver, Colorado, with an airborne High Spectral Resolution Lidar (HSRL) instrument during the DISCOVER-AQ (Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality) field mission This particular smoke plume was found at 8 km height, originating from Pacific Northwest wildfires, and exhibited PLDR values of 20 %, 9.3 % and 1.8 % at 355, 532 and 1064 nm, respectively. Our analysis shows that for the Canadian stratospheric smoke observed above Europe in August 2017, the PLDR and LR measurements along with their spectral dependence can be successfully reproduced with the proposed model of compact near-spherical particles.
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