Pollution May Act As Antifreeze in Clouds

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Those wispy cirrus clouds that float high in the sky may be thinning out due to nitric acid pollution, a change that scientists say could affect climate. Airborne measurements of the high clouds taken in the summer of
2002 showed increased humidity in the clouds and found nitric oxide, which is a pollutant that comes from jet exhaust, combustion on the ground and other sources, according to a paper in today’s issue of the journal Science.

Clouds are a major factor in climate, said Ru-shan Gao, lead author of the study, said the impact of changes in these clouds, drifting 9 miles above the ground, still must be assessed. The clouds will be thinner, that could mean more sunlight is allowed in, warming the Earth. It also could mean more infrared radiation from the ground escapes into space, resulting in a cooling.

The nitric acid appears to act a bit like antifreeze, preventing the ice crystals that it coats from growing to their full size by absorbing water vapor from the air. That results in smaller ice crystals in the clouds and higher humidity. The authors found the effect at temperatures lower than 96 degrees below zero Fahrenheit. Further sampling of high cirrus clouds is planned to confirm the findings, Gao said.

Thomas P. Ackerman, chief scientist in atmospheric radiation at the Energy Department’s Pacific Northwest National Laboratory, welcomed the report as “the beginning of the discussion.” He noted, however, that high relative humidity has also been reported in cirrus clouds at somewhat warmer temperatures — minus 70 degrees or so — which he said cannot be explained by the same method the researchers propose for their colder readings. “It’s an interesting study, but it has to be extended to higher temperatures to explain the observations,” he said. Clouds are constantly changing and a lot more needs to be known about how ice crystals grow and shrink, added Ackerman, who was not part of the research group.

Reference: Evidence That Nitric Acid Increases Relative Humidity in Low-Temperature Cirrus Clouds R. S. Gao, et al.: Science,Volume 303, Number 5657, Issue of 23 Jan 2004, pp. 516-520. The American Association for the Advancement of Science.

Abstract In situ measurements of the relative humidity with respect to ice (RHi) and of nitric acid (HNO3) were made in both natural and contrail cirrus clouds in the upper troposphere. At temperatures lower than 202 kelvin, RHi values show a sharp increase to average values of over 130% in both cloud types. These enhanced RHi values are attributed to the presence of a new class of HNO3-containing ice particles (-ice). We propose that surface HNO3 molecules prevent the ice/vapor system from reaching equilibrium by a mechanism similar to that of freezing point depression by antifreeze proteins. -ice represents a new link between global climate and natural and anthropogenic nitrogen oxide emissions. Including -ice in climate models will alter simulated cirrus properties and the distribution of


upper tropospheric water vapor.