The unprecedented 2017–2018 stratospheric smoke event: decay phase and aerosol properties observed with the EARLINET
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Date
2019-07-18
Authors
Baars, H.
Ansmann, A.
Ohneiser, K.
Haarig, M.
Engelmann, R.
Althausen, D.
Hanssen, I.
Gausa, M.
Pietruczuk, A.
Szkop, A.
Journal Title
Journal ISSN
Volume Title
Publisher
European Geosciences Union (EGU)
Published Version
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 < 0.05 (December–January). The decrease of the depolarization ratio is consistent with the steady removal of the larger smoke particles by gravitational settling and changes in the particle shape with time towards a spherical form. An ascending layer with a vertical depth of 500–1000 m was detected (over the Eastern Mediterranean at 32–35° N) that ascended from about 18–19 km to 22–23 km height from the beginning of October to the beginning of December 2017 (about 2 km per month) and may be related to the increasing build up of the winter-hemispheric Brewer–Dobson circulation system.
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Keywords
Stratospheric aerosol observations , European Aerosol Research Lidar Network (EARLINET) , Wild fire smoke , INP concentrations , Smoke layer , Brewer-Dobson circulation system
Citation
Baars, H. et al (2019) 'The unprecedented 2017–2018 stratospheric smoke event: decay phase and aerosol properties observed with the EARLINET', Atmos. Chem. Phys., 19(23), pp. 15183-15198. doi: 10.5194/acp-2019-615