The unprecedented 2017–2018 stratospheric smoke event: Decay phase and aerosol properties observed with EARLINET
Authors: Baars Holger , Ansmann Albert , Ohneiser Kevin , Haarig Moritz , Engelmann Ronny , Althausen Dietrich , Hanssen Ingrid , Gausa Michael , Pietruczuk Aleksander , Szkop Artur , Stachlewska Iwona S. , Wang Dongxiang , Reichhardt Jens , Skupin Annett , Mattis Ina , Trickl Thomas , Vogelmann Hannes , Navas-Guzmán Francisco , Haefele Alexander , Acheson Karen , Ruth Albert A. , Tatarov Boyan , Müller Detlef , Hu Qiaoyun , Podvin Thierry , Goloub Philippe , Vesselovski Igor , Pietras Christophe , Haeffelin Martial , Fréville Patrick , Sicard Michaël , Comerón Adolfo , Fernández García Alfonso Javier , Molero Menéndez Francisco , Córdoba-Jabonero Carmen , Guerrero-Rascado Juan Luis , Alados-Arboledas Lucas , Bortoli Daniele , Costa Maria João , Dionisi Davide , Liberti Gian Luigi , Wang Xuan , Sannino Alessia , Papagiannopoulos Nikolaos , Boselli Antonella , Mona Lucia , D'Amico Giuseppe , Romano Salvatore , Perrone Maria Rita , Belegante Livio , Nicolae Doina , Grigorov Ivan , Gialitaki Anna , Amiridis Vassilis , Soupiona Ourania , Papayannis Alexandros , Mamouri Rodanthi-Elisaveth , Nisantzi Argyro , Heese Birgit , Hofer Julian , Schechner Yoav Y. , Wandinger Ulla , Pappalardo Gelsomina 
Published: July 2019

Abstract. Six months of stratospheric aerosol observations with the European Aerosol Research Lidar Network (EARLINET) from August 2017 to January 2018 are presented. The decay phase of an unprecedented, record-breaking stratospheric perturbation caused by wild fire smoke is reported and discussed in terms of geometrical, optical, and microphysical aerosol properties. Enormous amounts of smoke (mainly soot particles) were injected into the upper troposphere and lower stratosphere over fire areas in western Canada on 12 August 2017 during strong thunderstorm-pyrocumulonimbus activity. The stratospheric smoke plumes spread over the entire northern hemisphere in the following weeks and months. Twenty-eight European lidar stations from northern Norway to southern Portugal and the Eastern Mediterranean monitored the strong stratospheric perturbation on a continental scale. The main smoke layer (over central, western, southern, and eastern Europe) was found between 15 and 20 km height since September 2017 (about two weeks after entering the stratosphere). Thin layers of smoke were detected to ascent to 22–24 km height. The stratospheric aerosol optical thickness at 532 nm decreased from values > 0.25 on 21–23 August 2017 to 0.005–0.03 until 5–10 September, and was mainly 0.003–0.004 from October to December 2017, and thus still significantly above the stratospheric background (0.001–0.002). Stratospheric particle extinction coefficients (532 nm) were as high as 50–200 Mm−1 until the beginning of September and of the order of 1 Mm−1 (0.5–5 Mm−1) from October 2017 until the end of January 2018. The corresponding layer mean particle mass concentration was of the order of 0.05–0.5 μg cm−3 over the months. Soot is an efficient ice-nucleating particle (INP) at upper tropospheric (cirrus) temperatures and available to influence cirrus formation when entering the tropopause from above. We estimated INP concentrations of 50–500 L−1 until the first days in September and afterwards 5–50 L−1 until the end of the year 2018 in the lower stratosphere for typical cirrus formation temperatures of −55 °C and ice supersaturation values of 1.15. The measured profiles of the particle linear depolarization rato indicated the predominance of non-spherical soot particles. The 532 nm depolarization ratio decreased with time in the main smoke layer from values of 0.15–0.25 (August–September) to values of 0.05–0.10 (October–November) and