The pixels, colour coded by date, indicate doubling of aerosol extinction with respect to December 2019 levels for data where aerosol to molecular extinction ratio is 1 or higher. The black circles with date-colour filling indicate the locations of high amounts of water vapour and/or carbon monoxide detected by MLS (see “Methods”). The black contour encircles the locations of aerosol bubble detections by CALIOP lidar (see Fig. 5a and Supplementary Fig. 3). The cross marks the latitude-altitude extent of the stratospheric cloud detected by CALIOP on the 1st January (see Fig. 5a). The grey solid and black dashed curves indicate respectively the zonal-mean 380 K isentrope and the lapse-rate tropopause for the January–March 2020 period.