just a small explanation

About 68% of days had ground-level ozone concentration between 22.8 and 44.6 ppb (33.7 About 95% of days had ground-level ozone concentration between 11.9 and 55.5 ppb (33.7 (2 x 10.9)). Almost all days had ground-level ozone concentration between 1.0 and 66.4 ppb (33.7 (3 x 10.9)).