Ozone in the Arctic lower troposphere during winter and spring 2000 (ALERT2000)

A summary of the temporal and vertical characteristics of ozone in the Arctic boundary layer as observed during winter and spring 2000 near Alert, Nunavut, Canada (82°N, 62°W) is presented. The measurements were made during the Polar Sunrise Experiments ALERT2000. Particular attention is given to identifying chemical and atmospheric characteristics of short-lived (<2 days) ozone depletion events that occur during dark and sunlit periods in the Arctic boundary layer. During these events the atmospheric boundary layer becomes turbulent and as a result atmospheric layers aloft, exceeding 100 m in depth, can become fully devoid of ozone. It is shown that these depletion episodes are often associated with air masses, whose origin is primarily in Eurasia, laden with chemical species of anthropogenic origin. Nevertheless, it is postulated that ozone depletion is largely driven by halogen chemistry, in particular involving bromine compounds, and this occurs during the transport of air masses across the Arctic Ocean. A detailed analysis of a period in mid April suggests that, later in the season during 24-h sunlit periods, locally occurring ozone depletion chemistry is an important process, and we speculate that photolysis of bromoform of marine origin is the fuse that starts a local “bromine explosion”. A severe ozone depletion episode occurred in late April. During this prolonged episode, lasting 14 consecutive days, the atmospheric column extending from the surface to about 1400 m remained almost completely free of ozone. We present and discuss evidence that atmospheric thermodynamics and air mass transport history explain the dynamics of ozone depletion episodes in the high Arctic.