Comment on "Stratospheric Aerosol Composition Observed by the Atmospheric Chemistry Experiment Following the 2019 Raikoke Eruption" by Boone et al.

Based on satellite observations in the Arctic stratosphere at latitudes from 61 degrees to 66 degrees N in the second half of 2019, Boone et al. (2022, ) provide the impression that the aerosol in the upper troposphere and lower stratosphere (UTLS) over the entire Arctic consisted of sulfate aerosol originating from the Raikoke volcanic eruption in the summer of 2019. Here, we show that this was most probably not the case and the aerosol layering conditions were much more complex. By combining the stratospheric aerosol typing results of Boone et al. (2022, ) with lidar observations at 85 degrees-86 degrees N of Ohneiser et al. (2021, ) of a dominating wildfire smoke layer in the UTLS height range, we demonstrate that the Arctic UTLS aerosol most likely consisted of Siberian wildfire smoke in the lower part and sulfate aerosol in the upper part of the aerosol layer which extended from 7 to 19 km height and was well observable until May 2020. The smoke- and sulfate-related aerosol optical thickness (AOT) fractions were about 0.7-0.8 and 0.2-0.3, respectively, according to our analysis. The sulfate AOT is in good agreement with model-based predictions of the Raikoke sulfate AOT. In the summer of 2019, the Raikoke volcano erupted. A stratospheric aerosol layer formed from the SO2 emitted into the lower stratosphere. At the same time, intense and long-lasting Siberian wildfires occurred in July and August 2019 and led to an extremely high level of smoke pollution in the Arctic. The smoke filled the entire troposphere. Boone et al. (2022, ) analyzed satellite observation between 61 degrees and 66 degrees N in the second half of 2019 and suggested that the aerosol in the upper troposphere and lower stratosphere (UTLS) over the entire Arctic consisted of volcanic sulfate aerosol, while Ohneiser et al. (2021, ) showed, based on lidar observations at latitudes of 85 degrees-86 degrees N in the fall of 2019, that wildfire smoke dominated in the UTLS aerosol layer, at least up to 12 km height in the central Arctic. In this commentary, we show that the UTLS aerosol over the Arctic most probably consisted of both, smoke particles in the lower part and sulfate aerosol in the upper part of the layer extending from 7 to 19 km height. The smoke and sulfate particles contributed to the overall Arctic UTLS aerosol optical thickness by about 70%-80% and 20%-30%, respectively. The impression given by Boone et al. that the 2019 Arctic upper troposphere and lower stratosphere (UTLS) aerosol was composed of sulfate only was not supported by lidar data The UTLS aerosol layer most likely contained Siberian wildfire smoke in the lower part and Raikoke sulfate aerosol in the upper part in the autumn of 2019 UTLS aerosol optical thickness fractions were about 0.7-0.8 (smoke) and 0.2-0.3 (sulfate)