Bizarre particles in stratospheric clouds could make arctic ozone more vulnerable to the climate change expected this century, researchers report in the 9 February issue of Science. The unexpectedly large particles could help destroy enough ozone during the spring to create a bona fide ozone hole over the Arctic, much like the one above Antarctica.
The discovery came during a January 2000 research flight nearly to the North Pole, report atmospheric chemist David Fahey of the National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colorado, and colleagues. An instrument on NASA's highflying ER-2 plane designed to measure nitrogen-rich gases instead detected humongous particles containing nitric acid. The polar stratospheric cloud (PSC) particles that form in the extreme cold of polar winter generally run a few tenths of a micrometer to a micrometer in diameter. But these oversized particles, more than 3000 times the usual mass, ranged up to 20 micrometers. "These things are rocks compared to" the usual PSC particles, Fahey says.
The discovery is catching researchers' attention because the particles can play an indirect role in ozone destruction. Ozone is protected by the nitrogen atoms in nitric acid, because they can chemically tie up ozone-destroying chlorine and bromine in inactive, harmless forms. So when nitrogen is locked up in PSCs, more chlorine and bromine can remain active to destroy ozone. And PSC rocks fall far faster than feathery-light PSC particles, so the rocks can efficiently ferry nitrogen out of some layers of the stratosphere, leaving active chlorine and bromine behind to have their way with ozone.
The discovery of PSC rocks is "the first time we've clearly seen reactive nitrogen being stripped from the polar arctic stratosphere," says James Anderson of Harvard University. He adds that "the most important question" is how this system will respond as greenhouse gases build. If these rocks remove enough ozone-protecting nitrogen, ozone destruction over the Arctic might continue even after PSCs disappear in the spring. As greenhouse gases alter stratospheric climate so as to encourage PSC rock formation, Fahey and his colleagues say, the big particles could delay recovery of arctic ozone.