The output of volatile chlorine during a major volcanic event can greatly exceed the annual manmade emissions of chlorine to the atmosphere. The fate of volcanic chlorine is particularly important since it has been argued that volcanoes may actually control the chlorine budget of the stratosphere, and hence contribute to ozone depletion. Large volcanic eruptions are capable of episodically injecting huge quantities of chlorine into the stratosphere. For instance, the 1982 eruption of El Chichon in Mexico and the 1991 eruption of Mt. Pinatubo in the Philippines created global clouds of volcanic dust and sulfuric acid. These could be seen clearly from the ground in the form of beautiful sunsets, as well as from satelite observations. However, measurements taken following these eruptions showed that neither eruption increased stratospheric chlorine by more than 5%. Therefore, these powerful, relatively rare volcanic events were equivalent to less than a single year's worth of manmade chlorine input.
Researchers at NASA Ames Research Center are seeking the reason why volcanic emissions of chlorine are so limited. The figure shows that the effective removal of chlorine from eruption columns arises because volcanic plumes contain roughly 1000 times more water than chlorine. The water condenses as the plume lofts and cools, and chlorine is readily scavenged by liquid water. Less soluble gases such as sulfur dioxide remain in the volcanic plumes. These rise and disperse into the stratosphere, and produce the relatively chlorine-free volcanic clouds which are observed at the ground.
Our research explains why stratospheric chlorine levels were not observed to be significantly effected by the El Chichon or the Mt. Pintubo eruptions, even though significant amounts of chlorine were present in their volcanic plumes. Also, the results of our study contradict assertions by some that the chlorine present in the stratosphere is provided in large part by volcanic eruptions and not by industrially manufactured chlorofluorocarbons (CFCs). The formation of the Antarctic ozone hole, Arctic ozone loss, and global ozone depletion are caused primarily by the gradual transport of manmade chlorine-containing compounds to the stratosphere, which overwhelm the effect of volcanic chlorine in these processes.
COLLABORATORS: Richard Turco, University of California, Los Angeles.
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