Given that these measurements were made in the Arctic, which can be subject to anthropogenic pollutants, the next question was – are these natural or man-made effects on ozone levels? The answer lay in Antarctic measurements, which showed that these features of rapid ozone loss, now known as Ozone Depletion Events (ODEs) were also measured at research stations in coastal Antarctica (see Figure 1). These data confirmed that ODEs were a natural phenomenon.
Ongoing research in both polar regions progressively built the now familiar picture of ODEs. They occur only during the spring; sometimes ozone concentrations drop from normal amounts to near-zero within the space of a few minutes; sometimes ozone loss can be less complete, and can occur more gradually; ozone can remain suppressed for several days; vertical profiling using balloons has shown that the ozone loss can extend up to ~1500 m in altitude and that they are contained within the planetary boundary layer by a capping inversion. An important observation was that ODEs are associated with transport of air masses over sea ice. |
The mechanism that is driving surface ozone loss is known as the “Bromine Explosion”, and is autocatalytic in bromine in the sense that one gaseous bromine going into the system results in two gas-phase bromines coming out (Figure 2). Bromide is liberated from the quasi-liquid layer on some sort of sea ice surface (exactly which is still open to debate). The possible candidates include sea salt aerosol, sea salt deposited onto the snowpack, or frost flowers – delicate dendritic ice crystals which are highly saline and grow as new sea ice forms. Interestingly, elevated BrO has also been measured in other locations where large areas of exposed salt exist, such as salt pans and the Dead Sea.
ODEs have been significant for atmospheric chemists for a number of reasons. They highlighted new reaction pathways that are important for halogen chemistry in the boundary layer. They have a potential radiative role that could be relevant on a regional scale, if air that is low in ozone is mixed to higher altitudes where ozone is particularly radiatively important. Finally, they were completely unanticipated, and so provide a nice example that the natural world still holds surprises for us! |