The upper atmosphere decrease lags because it takes years for air from high and low altitudes to mix. NOAA Ozone levels over Antarctica Sept. 12, 2000. The black area is where there is no daylight. Even though ozone-destroying substances are decreasing, the ozone hole is going to be around for a while as these chemicals slowly disappear from the upper atmosphere. Total recovery of the ozone hole back to levels observed before 1980 will take at least 50 years.
Volcanoes are another wild card.
Volcanoes that shoot large amounts of material into the stratopshere affect ozone because sulfur from the volcanoes forms high-altitude clouds that can have the same effect as polar stratospheric clouds. When no materials from volcanoes are in the stratosphere, upper atmospheric weather is the big factor, along with the amounts of ozone-destroying substances in the air, determining that the size, depth and life time of ozone holes. The stratosphere over Antarctica is cold enough during all winters for a hole to form, but it varies from year to year depending on the weather. Hofmann says the 2000 ozone hole probably started breaking up sooner than usual because the polar vortex broke down earlier than usual. These instruments can measure ozone even during the almost six months of winter darkness at the Pole.