FOR IMMEDIATE RELEASE
Contact: Barbara McGehan
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.
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."