Bromine Enters the Equation

Associate Professor Daniel J. Jacob Tackles the Case of the Disappearing Ozone as

In the quest to determine what controls rapidly falling atmospheric ozone levels, scientists often place blame on big corporate polluters and lazy Americans who love their cars. But a new theory proposed by a Harvard researcher explains how a seemingly innocent element, bromine, eats "ozone-eater."

Everyone knows ozone is necessary for protection from the sun's harmful ultraviolet rays. The phenomenon of Arctic spring, when ozone levels over the Arctic suddenly drop, leaving a hole, is also well-known. But scientists still wonder exactly what ozone does, how it works, and where it goes. What controls the amount of ozone in our atmosphere?

With an eye for answering these and other questions, McKay Associate Professor of Atmospheric Chemistry Daniel J. Jacob has developed a model which he says explains the sudden loss of tropospheric ozone over the Arctic each spring, and possibly over one area of the equatorial Pacific.

Public concern over ozone levels is primarily centered on the loss of stratospheric ozone, in the layer of our atmosphere existing 10 to 50 kilometers above the earth's surface. When stratospheric ozone is depleted, the dangers of severe sunburn and skin cancer due to the sun's ultraviolet radiation increase.

But tropospheric ozone levels, in the layer of the atmosphere extending from ground level to the stratosphere, act as a crucial buffer between the earth's surface and the rest of the atmosphere. It breaks down ozone-eating molecules before they reach the stratosphere, preventing them from destroying the vital ozone which protects us from the sun.

Tropospheric ozone also helps to break down a great mass of particles created by humans by way of chemical reactions known as oxidation.

Atmospheric chemists are puzzled, says Jacob, by the mechanism which controls the concentration of ozone in the troposphere, and he finds this embarrassing.

"It's one of the most noble quests in atmospheric chemistry today," Jacob says. "It's kind of the Holy Grail, to try to understand what controls tropospheric ozone, because then you will understand a lot of other things."

Scientists have long been aware of bromine's capacity for destroying ozone, which is comprised of three oxygen atoms, by grabbing one oxygen atom and binding to it. This reaction leaves oxygen in the form which we breathe.

But the mechanism by which bromine enters the atmosphere remains unclear. While bromine reacts with ozone, it also reacts with other molecules, which makes it unable to participate in the ozone-destroying reaction.

The model proposed by Jacob and Research Assistant Song-Miao Fan explains how unreactive bromine combines with acid particles present in human pollution and is then activated by light so it can "eat" more ozone.

So the culprit, it seems, is still human industry.

Using Arctic ozone data, Jacob was able to prove that a reaction catalyzed by bromine was involved in the destruction of tropospheric ozone.

"We can explain the observed depletion by a simple mechanism, dependent on the square of the bromine concentration," Jacob says. "So a small fluctuation in the amount of bromine makes a big difference."

Once he determined the validity of this model over the Arctic, Jacob tried to apply the model to the Pacific depletion, but with no success.