The unprecedented 2017-2018 stratospheric smoke event: Decay phase and aerosol properties observed with the EARLINET

Six months of stratospheric aerosol observationswith the European Aerosol Research Lidar Network (EAR-LINET) from August 2017 to January 2018 are presented.The decay phase of an unprecedented, record-breakingstratospheric perturbation caused by wildfire smoke is re-ported and discussed in terms of geometrical, optical, and mi-crophysical aerosol properties. Enormous amounts of smokewere injected into the upper troposphere and lower strato-sphere over fire areas in western Canada on 12 August2017 during strong thunderstorm–pyrocumulonimbus activ-ity. The stratospheric fire plumes spread over the entireNorthern Hemisphere in the following weeks and months.Twenty-eight European lidar stations from northern Norwayto southern Portugal and the eastern Mediterranean moni-tored the strong stratospheric perturbation on a continentalscale. The main smoke layer (over central, western, south-ern, and eastern Europe) was found at heights between 15and 20 km since September 2017 (about 2 weeks after en-tering the stratosphere). Thin layers of smoke were detectedat heights of up to 22–23 km. The stratospheric aerosol op-tical thickness at 532 nm decreased from values>0.25 on21–23 August 2017 to 0.005–0.03 until 5–10 September andwas mainly 0.003–0.004 from October to December 2017and thus was still significantly above the stratospheric back-ground (0.001–0.002). Stratospheric particle extinction co-efficients (532 nm) were as high as 50–200 Mm−1until thebeginning of September and on 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 concentrationwas on the order of 0.05–0.5 μg m−3over these months. Sootparticles (light-absorbing carbonaceous particles) are effi-cient ice-nucleating particles (INPs) at upper tropospheric(cirrus) temperatures and available to influence cirrus for-mation when entering the tropopause from above. We esti-mated INP concentrations of 50–500 L−1until the first daysin September and afterwards 5–50 L−1until the end of theyear 2017 in the lower stratosphere for typical cirrus for-mation temperatures of−55◦C and an ice supersaturationlevel of 1.15. The measured profiles of the particle lineardepolarization ratio indicated a predominance of nonspher-ical smoke particles. The 532 nm depolarization ratio de-creased slowly with time in the main smoke layer from val-ues of 0.15–0.25 (August–September) to values of 0.05–0.10(October–November) and<0.05 (December–January). Thedecrease of the depolarization ratio is consistent with agingof the smoke particles, growing of a coating around the solidblack carbon core (aggregates), and thus change of the shapetowards a spherical form. We found ascending aerosol layerfeatures over the most southern European stations, especiallyover the eastern Mediterranean at 32–35◦N, that ascendedfrom heights of about 18–19 to 22–23 km from the beginningof October to the beginning of December 2017 (about 2 kmper month). We discuss several transport and lifting mech-anisms that may have had an impact on the found aerosollayering structures.