Figure 4

Long-lasting and large ozone holes, that often bring ozone-depleted air over the northern most part of the continent in late spring, have been occurring at about the same frequency during the last two decades. (a) Changes from 1996–2005 to 2011–2020 in the ozone hole area averaged from 1 September to 15 October. (b) Changes from 1996–2005 to 2011–2020 in the ozone hole area averaged from 16 October to 30 November. (c) Daily estimates of the ozone hole area averaged over two periods: 1996–2005 (blue line) and 2011–2020 (red line). (d) Progress of ozone hole area averaged from 1 September to 15 October (red line) and from 16 October to 30 November (blue line). (e) Number of days with a very large ozone hole area (defined according to the 90th percentile) counted over two periods: from 1 September to 15 October (red line) and from 16 October to 30 November (blue line). (f) Number of days with an ozone hole area larger than 20 million km² counted over two periods: from 1 September to 15 October (red line) and from 16 October to 30 November (blue line). Bold lines in plots (d–f) show 11-year centered moving averages. In plots (a,b) the ozone hole was estimated considering the area within which, on average over the decades either 1996–2005 or 2011–2020, daily TOC values were lower than 220 Dobson units (DU) on twenty or more days per period. Data from the TOMS instrument on the Earth Probe satellite and from the OMI instrument onboard the Aura satellite were used in plots (a,b). Data produced by the Laboratory for Atmospheres at NASA's Goddard Space Flight Center were used in plots (c–f). The plots were generated using Python’s Matplotlib library⁷¹.