Bromine Enters the Equation

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.

"The concentration of bromine is much smaller in the tropics, so it does not work," he says. "We want to understand why it's so low. What controls ozone on a global scale?"

But Jacob is not discouraged. "We look for anomalies like this and try to explain them," Jacob says. "When you do, it is often indicative of a phenomenon on a larger scale."

Jacob has some other ideas in the works to explain the situation in the tropics, including one which involves sea salt.

"Sodium chloride from sea spray releases chlorine which may go through a series of steps, similar to the steps of bromine, to deplete ozone," Jacob says. "It's all theoretical work here [at Harvard], but experimentalists at [the University of Virginia] and MIT have observed ozone-eating chlorine gases over the ocean."

Other atmospheric chemists are skeptical. California State University at Fullerton Professor of Chemistry Barbara Finlayson-Pitts, who is currently researching the chlorine hypothesis, says chlorine is more likely to create ozone than destroy it.

According to Finlayson-Pitts, organic molecules such as hydrocarbons participate in a string of reactions involving hydroxide (OH) to eventually form nitrogen dioxide and ozone.

The only ozone-creating molecule generated by humans is nitric oxide, but the molecule must be first converted to nitrogen dioxide before it can form ozone.

Finlayson-Pitts and her colleagues are studying the possibility that chlorine can serve as a substitute for hydroxide, reacting with organics to hasten the formation of ozone. And, she says, the chlorine reaction might take place instantaneously, while the hydroxide reaction can take up to two weeks.

But Finlayson-Pitts concedes that chlorine can react to destroy ozone as well as to create it.

"It depends on the conditions," says Finlayson-Pitts. "If there are enough organics, the chlorine will react with them quickly to produce ozone rather than destroying it."

The Role of Tropospheric Ozone

Despite its beneficial effects, tropospheric ozone has gained far more publicity for its role as a major contributor to air pollution and smog in Los Angeles and other large cities.

Ozone in the troposphere and the stratosphere play very different roles, which initially seem unconnected. Jacob says this is not the case. "Ozone is a very complicated molecule, just in terms of its environmental effects," he says.

One of ozone's chief tasks is to clean up tons of junk thrown up into the air by human consumption.

"We release on the order of a billion tons of material into the atmosphere," Jacob says. "The atmosphere can't hold that much, and what goes up must come down."

Many pollutants fall back to earth on their own. Carbon dioxide, for example, is absorbed by trees and by the oceans to be used in photosynthesis. Dust particles fall because of their weight.

But another class of pollutants, including methane and other "greenhouse gases," is not scavenged back to the surface. Instead, these need to be broken down chemically into forms that fall like dust, or are washed out of the atmosphere by rain or are similarly cleansed from the atmosphere.

The reactions which these molecules must undergo, called oxidation, need ozone for to work.

"Most of the oxidation takes place in the troposphere," said Jacob. "There are only a few strong radical oxidants that can perform the reactions. They ordinates from ozone."

One negative effect of a loss of tropospheric ozone can be illustrated by the present trend to phase out chloroflorocarbons (CFCs), present in aerosols and Styrofoam, and replace them with hydrochloroflorocarbons, (HCFCs).

"CFCs, which destroy stratospheric ozone, are going out, and HCFCs are coming in," Jacob says. "The HCFCs won't break down the ozone in the stratosphere, because they can be oxidized in the troposphere."

If levels of tropospheric ozone remain high enough, HCFCs will be broken down and scavenged without reaching the stratosphere. But if tropospheric ozone levels drop, HCFCs will reach the stratosphere and cause just as much damage as their predecessors.

"We want to boost the oxidizing power of the atmosphere. If tropospheric ozone levels had not risen over the past 100 years, we'd in trouble," Jacob says. "But at the same time we dump pollutants that need to be destroyed, we dump more ozone to destroy them."

And in a statement which seems to contradict atmospheric chemists, Jacob jokes. "Maybe pollution isn't so nasty after all."