Depolarization and lidar ratios at 355, 532, and 1064 nm and microphysical properties of aged tropospheric and stratospheric Canadian wildfire smoke

Abstract

Abstract. We present spectrally resolved optical and microphysical properties of western Canadian wildfire smoke observed in a tropospheric layer from 5–6.5 km height and in a stratospheric layer from 15–16 km height during a record-breaking smoke event on 22 August 2017. Three polarization/Raman lidars were run at the European Aerosol Research Lidar Network (EARLINET) station of Leipzig, Germany, after sunset on 22 August. For the first time, the linear depolarization ratio and extinction-to-backscatter ratio (lidar ratio) of aged smoke particles were measured at all three important lidar wavelengths of 355, 532, and 1064 nm. Very different particle depolarization ratios were found in the troposphere and in the stratosphere. The obviously compact and spherical tropospheric smoke particles caused almost no depolarization of backscattered laser radiation at all three wavelengths (<3 %), whereas the dry irregularly shaped soot particles in the stratosphere lead to high depolarization ratios of 22 % at 355 nm and 18 % at 532 nm and a comparably low value of 4 % at 1064 nm. The lidar ratios were 40–45 sr (355 nm), 65–80 sr (532 nm), and 80–95 sr (1064 nm) in both the tropospheric and stratospheric smoke layers indicating similar scattering and absorption properties. The strong wavelength dependence of the stratospheric depolarization ratio was probably caused by the absence of a particle coarse mode (particle mode consisting of particles with radius >500 nm). The stratospheric smoke particles formed a pronounced accumulation mode (in terms of particle volume or mass) centered at a particle radius of 350–400 nm. The effective particle radius was 0.32 µm. The tropospheric smoke particles were much smaller (effective radius of 0.17 µm). Mass concentrations were of the order of 5.5 µg m−3 (tropospheric layer) and 40 µg m−3 (stratospheric layer) in the night of 22 August 2017. The single scattering albedo of the stratospheric particles was estimated to be 0.74, 0.8, and 0.83 at 355, 532, and 1064 nm, respectively.

Extreme levels of Canadian wildfire smoke in the stratosphere over central Europe on 21–22 August 2017

“…We used a value of 70 sr for 532 nm. This lidar ratio of 70 sr was measured with our Raman lidars after sunset on 22 August 2017 (Haarig et al, 2018). The uncertainty of the extinction profile obtained is almost directly dependent on the lidar ratio uncertainty.…”

“…At these stratospheric heights the visibility is usually several hundreds of kilometers. Peak mass concentrations were of the order of 70-100 µg m −3 in the lower stratosphere over central Europe around noon on 22 August 2017 (Haarig et al, 2018). Particle extinction values close to 500 Mm −1 in combination with lidar ratios around 70 sr correspond to backscatter ratios (total-to- Rayleigh backscatter) of up to 25-30 at 532 nm at a height of 15 km.…”

“…The 532 nm volume linear depolarization ratio was mostly between 0.1 and 0.2 at heights from 12 to 16 km. The relationship between particle shape and particle linear depolarization ratio is discussed in Haarig et al (2018).…”

“…2.1, the uncertainty in the particle extinction coefficients and AOT(s) values is almost directly proportional to the uncertainty in the lidar ratio assumption. We used a smoke lidar ratio of 70 r in the extinction coefficient retrieval (3-17 km height range) as measured with several Raman lidars at Leipzig on the evening of 22 August 2017, (Haarig et al, 2018) and also found over Hohenpeissenberg and Kosetice on the nights on 21-22 August and 22-23 August 2017, respectively. By assuming a realistic smoke lidar ratio from 60 to 80 sr for 532 nm, the uncertainty in the extinction profiles is of the order of 15 %.…”

“…The height profile of the particle extinction coefficient at 532 nm over Leipzig on 22 August 2017, measured with lidar close to noon (red profile), when the optically densest stratospheric smoke layers crossed the lidar site, and at nighttime (blue profile). The nighttime observations from three lidars will be discussed in detail in Haarig et al (2018). The data analysis is the same as in of 18 km.…”