Scientists from the National Oceanic and Atmospheric Administration (NOAA) say that new, possibly biological, oceanic mechanisms are removing more methyl bromide from the atmosphere than has been previously estimated. Their research consequently suggests that the time the chemical stays in the atmosphere is shorter than was previously thought.

The research, conducted by Shari Yvon-Lewis and James Butler, with NOAA's Climate Monitoring and Diagnostics Laboratory in Boulder, CO, was published in the May 1997 issue of Geophysical Research Letters.

Yvon-Lewis and Butler calculate the lifetime of methyl bromide in the atmosphere to be about 0.7 years. This is a significant reduction from an estimated 2.0 years in 1992 which was based upon loss due to atmospheric chemistry alone. They arrived at the new, low figure after analyzing 40 years of wind speeds and ocean temperatures across the globe as well as recent measurements of methyl bromide concentrations from research cruises and laboratory studies. They also included recently published results for losses to soils.

"It now appears that loss of atmospheric methyl bromide to the ocean is just about as fast as oxidation in the atmosphere, which has been considered the primary mechanism for removing the chemical," Butler says.

Bromine, a chemical derived from methyl bromide, is thought to be about 50 times more effective in depleting stratospheric ozone than chlorine, the principal chemical that destroys ozone.

"Unlike other ozone depletors, such as chlorofluorocarbons and halons which are entirely manmade, methyl bromide comes from both natural and manmade sources," Butler reports. "Major manmade sources include biomass-burning and fumigating soils, produce, and buildings."

Fumigation emissions may constitute 20 to 30 percent of all methyl bromide produced, but further research is being done to refine this estimate. Butler says that although there is still some uncertainty, studies suggest that manmade methyl bromide could be responsible for 3 to 10 percent of global stratospheric ozone destruction.

Earlier this year NOAA scientists completed a study which showed that the waters near Antarctica remove methyl bromide from the atmosphere. It had previously been suggested that these waters, unlike most of the open ocean, could be a large source of methyl bromide. NOAA research indicated that some other mechanisms were also responsible for degrading methyl bromide in seawater.

"We know that in the ocean, dissolved methyl bromide is further broken down by hydrolysis and chloride substitution," Butler reports. "But recently it has been shown that methyl bromide also undergoes biological degradation, perhaps as a result of bacterial uptake. Field studies, showing large undersaturations in polar and subpolar waters, also suggest an additional sink mechanism."

Heretofore, he says, calculations of methyl bromide loss to the ocean have been based only on chemical degradation mechanisms. Including the biological terms increases the calculated strength of the oceanic sinks, leading to a shorter atmospheric lifetime.