NASA research shows Earth's atmosphere contains an unexpectedly large amount of an ozone-depleting compound from an unknown source decades after the compound was banned worldwide.
Carbon tetrachloride (CCl4), which was once used in applications such as dry cleaning and as a fire-extinguishing agent, was regulated in 1987 under the Montreal Protocol along with other chlorofluorocarbons that destroy ozone and contribute to the ozone hole over Antarctica.
Parties to the Montreal Protocol reported zero new CCl4 emissions between 2007-2012. However, the new research shows worldwide emissions of CCl4 average 39 kilotons per year, approximately 30 percent of peak emissions prior to the international treaty going into effect.
As of 2008, CCl4 accounted for about 11 percent of chlorine available for ozone depletion, which is not enough to alter the decreasing trend of ozone-depleting substances. For nearly a decade, scientists have debated why the observed levels of CCl4 in the atmosphere have declined slower than expectations, which are based on what is known about how the compound is destroyed by solar radiation and other natural processes.
With zero CCl4 emissions reported between 2007-2012, atmospheric concentrations of the compound should have declined at an expected rate of 4 percent per year, NASA says. Observations from the ground showed atmospheric concentrations were only declining by 1 percent per year.
Model simulations of global atmospheric chemistry and the losses of CCl4 due to interactions with soil and the oceans pointed to an unidentified ongoing current source of CCl4. The results produced the first quantitative estimate of average global CCl4 emissions from 2000-2012.
In addition to unexplained sources of CCl4, the model results showed the chemical stays in the atmosphere 40 percent longer than previously thought.
"People believe the emissions of ozone-depleting substances have stopped because of the Montreal Protocol," said Paul Newman, chief scientist for atmospheres at NASA's Goddard Space Flight Center, and a co-author of the study. "Unfortunately, there is still a major source of CCl4 out in the world."
In July, NASA launched its first spacecraft dedicated to studying atmospheric carbon dioxide. The Orbiting Carbon Observatory-2 (OCO-2) soon will begin a minimum two-year mission to locate Earth's sources of and storage places for atmospheric CO2, the leading human-produced greenhouse gas responsible for climate change and a critical component of the planet's carbon cycle.
Last week, the space agency launched a new field campaign that will begin flights over the Arctic to study the effect of sea ice retreat on Arctic climate. The Arctic Radiation IceBridge Sea and Ice Experiment (ARISE) is NASA's first Arctic airborne campaign designed to take simultaneous measurements of ice, clouds and the levels of incoming and outgoing radiation, the balance of which determines the degree of climate warming.