Abstract
In August 2019, a 4-km thick wildfire smoke layer was observed in the lower stratosphere over Leipzig, Germany, with a ground-based multiwavelength Raman lidar. The smoke was identified by the smoke-specific spectral dependence of the extinction-to-backscatter ratio (lidar ratio) measured with the Raman lidar. The spaceborne CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) lidar CALIOP (Cloud–Aerosol Lidar with Orthogonal Polarization) detected the smoke and classified it as sulfate aerosol layer (originating from the Raikoke volcanic eruption). In this article, we discuss the reason for this misclassification. Two major sources for stratospheric air pollution were active in the summer of 2019 and complicated the CALIPSO aerosol typing effort. Besides intense forest fires at mid and high northern latitudes, the Raikoke volcano erupted in the Kuril Islands. We present two cases observed at Leipzig, one from July 2019 and one from August 2019. In July, pure volcanic sulfate aerosol layers were found in the lower stratosphere, while in August, wildfire smoke dominated in the height range up to 4–5 km above the local tropopause. In both cases, the CALIPSO aerosol typing scheme classified the layers as sulfate aerosol layers. The aerosol identification algorithm assumes non-spherical smoke particles in the stratosphere as consequence of fast lifting by pyrocumulonimbus convection. However, we hypothesize (based on presented simulations) that the smoke ascended as a results of self-lifting and reached the tropopause within 2–7 days after emission and finally entered the lower stratosphere as aged spherical smoke particles. These sphercial particles were then classified as liquid sulfate particles by the CALIPSO data analysis scheme. We also present a successful case of smoke identification by the CALIPSO retrieval method.
Highlights
In the summer of 2019, the Raikoke volcano in the Kuril Islands (48.3°N, 153.3°E, see map in Figure 1) erupted and emitted ash and volcanic gases over the North Pacific (Kloss et al, 2021; Vaughan et al, 2021)
We show that the CALIPSO aerosol-typing worked well and successfully identified the volcanic sulfate aerosol type in July 2019 and failed in August 2019
Distinction of volcanic sulfate and wildfire smoke particles in the stratosphere by means of the CALIPSO aerosol classification scheme is based on depolarization ratio (PLDR532) measurements in the case of prominent aerosol layers
Summary
In the summer of 2019, the Raikoke volcano in the Kuril Islands (48.3°N, 153.3°E, see map in Figure 1) erupted and emitted ash and volcanic gases over the North Pacific (Kloss et al, 2021; Vaughan et al, 2021). Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) lidar CALIOP (Cloud–Aerosol Lidar with Orthogonal Polarization) (Winker et al, 2009) classified most of the detected stratospheric aerosol layers as sulfate aerosol layers (of volcanic origin) This result perhaps is not surprising because very low values of the 532 nm particle linear depolarization ratio PLDR were measured with CALIOP, suggesting the presence of spherical, liquid sulfuric-acidcontaining water droplets (Kim et al, 2018). The latest status of the computation of backscatter and extinction coefficients and lidar ratios at 355 and 532 nm for stratospheric smoke is explained in detail in TABLE 1 | Particle optical properties observable with the spaceborne dual-wavelength CALIOP (532 and 1,064 nm) and the ground-based triple-wavelength Polly (operated at Leipzig until end of August 2019 and aboard the icebreaker Polarastern since end of September 2019).
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