Scientists flew missions over Antarctica in 1987 to measure ozone levels. They found high ozone levels in pockets of upper atmosphere, especially near the ozone hole. Ozone levels were 2-3 times higher near the hole compared to background levels. Ozone-rich air rarely went below 9 km altitude. Background ozone in the surrounding troposphere was typically 20-50 ppbv. Initially, ozone levels decreased beneath the hole, but later, they increased as the hole deepened.