Each spring, after several dark winter months, the sun begins to rise over the Arctic. At around the same time, the local surface-level ozone begins to decline. Researchers have long suspected that two chemical elements, bromine and chlorine, touch off this ozone destruction, but the source of these chemicals remained unknown. Now new research, reported Friday in the journal Science, reveals that they likely come from sea salt. The findings thus indicate that complex interactions among the sun, sea salt and snow are naturally depleting the near-surface ozone in the Arctic.
Scientists first noted a link between polar sunrise and Arctic ozone removal in 1986. Since then, they have discovered a number of chemicals that lay dormant in the snow but react with sunlight to form pollutants capable of affecting the makeup of the atmosphere. In order to examine how bromine and chlorine figure into this equation, Paul Shepson of Purdue University and his colleagues went to the Arctic last year to measure the levels of these elements in the snow during polar sunrise. They found that in mid-March, as the sun rose over the Arctic, bromine and chlorine levels in the snow began to fall. Their presence in the air, however, increased. Sea salt, it turns out, is the only source of bromine in the Arctic. "Because three quarters of the earth's surface is covered by ocean," Shepson notes, "we've uncovered a process we need to understand much better in terms of our ability to model ozone in the atmosphere."
Although it applies to a naturally occurring process, the new research suggests that the combination of snow and salt in urban areas might also affect the atmosphere. "The ingredients from what we observe in the Arctic are sometimes present in high concentrations on the streets of urban areas during the winter months," Shepson observes. "I wouldn't want to make predictions, but there could be a variety of chemical interactions that occur in urban, road-salted environments that we need to understand."