Scientists have taken a giant step forward in better understanding the atmospheric cleansing process that removes many pollutants and gases involved in ozone depletion and climate change. This new insight will help scientists more accurately predict the future state of the atmosphere.

In a paper that appears in the April 21 issue of Science, scientists from government and university laboratories say that a dramatic decline in human releases of the industrial chemical methyl chloroform in recent years has provided an unprecedented opportunity to gain a much clearer picture of the process by which the atmosphere cleanses itself of this gas and many other trace gases.

Methyl chloroform is a human-made chemical that was used extensively in the past as a solvent and degreasing agent. However, it contributes significantly to ozone-depletion in the stratosphere. As a result, its production was severely limited by international regulations and voluntary reductions by companies in the mid-1990s, causing emissions to decline very rapidly from 1996 to 1999. In the absence of strong emissions, insights into the processes by which gases are removed from the atmosphere are now possible.

According to Stephen Montzka, a researcher at NOAA's Climate Monitoring and Diagnostics Laboratory in Boulder, Colo., and lead author of the paper, "Now we have more accurate estimates of the persistence in the atmosphere of many chemicals emitted from human activities and natural processes, such as methane, methyl bromide, hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs). Thus, we can use this information to gauge their environmental effects and to develop more accurate models of the atmosphere."

The environmental impacts of gases are generally related to their persistence in the atmosphere. "Previous estimates of the persistence or ‘atmospheric lifetimes' of trace gases were based on studies containing substantial uncertainties," Montzka said.

Until now, an understanding of the environmental effects of trace gases in the atmosphere relied heavily upon accurate measurements of atmospheric concentrations and releases of methyl chloroform to the atmosphere. But because emissions of methyl chloroform are now quite small, the conclusions in the present study hinge primarily on accurately measuring the rate of change of this gas in the atmosphere. This task, says Montzka, is much easier than determining the true concentration of methyl chloroform in the global atmosphere, and knowing exactly how much of this chemical is released to the atmosphere each year from human activity.

Methyl chloroform is removed from the atmosphere primarily by chemical reaction with the hydroxyl radical, one of the most important and potent atmospheric cleansing agents or oxidants. Because many gases involved in regulating global climate and ozone are also eliminated from the atmosphere by the hydroxyl radical, these results provide an improved understanding of the behavior of a wide range of pollutants in the atmosphere.

The authors from NOAA, the University of Colorado, and Harvard University came to their conclusions by analyzing measurements they made of methyl chloroform in air samples over the past 8 years. These samples were collected at 10 remote sites across the globe to provide a picture of how the concentration and distribution of methyl chloroform changed as human releases diminished.

They also found that average concentrations of the hydroxyl radical are higher in the Southern Hemisphere than in the Northern Hemisphere. This result is opposite of what is calculated by many atmospheric models in use today. These models generally suggest that higher levels of this important oxidant should be found in the Northern Hemisphere as a result of more pollution there.

Montzka says that the insights provided from this work will "provide needed constraints to atmospheric models. This should result in a better understanding of the present atmosphere and an improved accuracy in predicting the future state of the atmosphere."