Supercold cirrus clouds aren't as frosty as they should be, and nitric acid may be to blame, says a team of researchers. In extremely cold cirrus clouds, nitric acid seems to interfere with ice formation, causing more water to remain as vapor than would be expected. The find puts a kink in climate models and hints that nitric acid from human pollution might be tweaking the cloud cover.
Cirrus are among the highest, coldest clouds, forming 8 to 16 kilometers up. These feathery wisps of ice cover huge areas of the planet, and no climate model would be complete without them. They reflect away solar energy that would otherwise warm the Earth, and they absorb and then reemit heat from Earth back to the ground. But climate modelers may be missing important information about cirrus clouds, according to a paper in the 23 January issue of Science.
David Fahey, an atmospheric scientist at the National Oceanic and Atmospheric Administration in Boulder, Colorado, and his colleagues flew on a plane high into the troposphere, 16 kilometers above southern Florida. They collected samples from cirrus clouds and analyzed them onboard. Back on the ground they noticed something peculiar in their data: Cirrus clouds warmer than -71°C were roughly half water vapor, half ice; but clouds colder than -71°C had much more vapor than expected. That suggested the clouds contained some sort of antifreeze. Fahey and his colleagues suspected it could be nitric acid.
At -71°C, nitric acid forms crystals similar to those of water ice. Fahey and colleagues speculate that these nitric acid crystals interfere with water molecules trying to glom onto ice crystals. It's not clear how much of this high-altitude nitric acid comes from pollution says Fahey. Nitric acid has many sources, from soil bacteria to lightening, but cars and trucks spew significant amounts.
Stephen Wood, a planetary scientist at the University of Washington, Seattle, says the research is exciting and hopes that climate modelers will take notice.