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Environment

U.S. carbon tetrachloride emissions are up to two orders of magnitude higher than expected

Measurements and modeling point to unaccounted-for industrial activity as the source of the restricted-use chemical

by Sarah Everts
March 7, 2016 | APPEARED IN VOLUME 94, ISSUE 10

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Credit: Lei Hu
Unexpectedly high emissions of ozone-depleting CCl4 were found hovering over Denver, the San Francisco Bay Area, and the Gulf Coast.
Credit: Lei Hu
Unexpectedly high emissions of ozone-depleting CCl4 were found hovering over Denver, the San Francisco Bay Area, and the Gulf Coast.

Atmospheric emissions of carbon tetrachloride—a restricted ozone-depleting greenhouse gas—are on average 4 gigagrams per year in the U.S., as much as two orders of magnitude higher than expected on the basis of emission levels reported to EPA by companies that produce and use the chemical, according to a new report (Proc. Natl. Acad. Sci. USA 2016, DOI: 10.1073/pnas.1522284113). Signatories to the Montreal Protocol on Substances That Deplete the Ozone Layer agreed to ban CCl4 in dispersive applications in which it is released into the air. But industry is still allowed to use the chemical in nondispersive applications, for example as a processing agent or feedstock for producing other chemicals. Analyzing land-based and airplane-acquired air samples from across the U.S., researchers led by Lei Hu of the University of Colorado, Boulder, and Stephen A. Montzka of the National Oceanic & Atmospheric Administration found that CCl4 levels across the continental U.S. far exceed reported figures, particularly over Denver, the San Francisco Bay Area, and the states of Louisiana and Texas. The data analysis suggests that the emissions are coming primarily from industrial activity, as opposed to other candidate sources, such as uncapped landfills.

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