CALIPSO Aerosol-Typing Scheme Misclassified Stratospheric Fire Smoke: Case Study From the 2019 Siberian Wildfire Season

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 detected the smoke aerosol layer and classified it as sulfate aerosol layer (originating from a 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. We also present a successful case of smoke identification by the CALIPSO retrieval scheme. The sulfate vs smoke discrimination is mainly based on the observed layer mean particle linear depolarization ratio at 532~nm (PLDR532). If PLDR532 < 0.075, the aerosol is classified as sulfate aerosol and if PLDR532 is in the range from 0.075-0.1, the aerosol is classified as smoke. In the case of the lidar observations in July and August 2019, PLDR532 was always < 0.05.