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Science 23 January 2004:
Vol. 303. no. 5657, pp. 516 - 520
DOI: 10.1126/science.1091255
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Reports

Evidence That Nitric Acid Increases Relative Humidity in Low-Temperature Cirrus Clouds

R. S. Gao,1* P. J. Popp,1,2 D. W. Fahey,1,2 T. P. Marcy,1,2 R. L. Herman,5 E. M. Weinstock,6 D. G. Baumgardner,8 T. J. Garrett,9 K. H. Rosenlof,1 T. L. Thompson,1 P. T. Bui,10 B. A. Ridley,11 S. C. Wofsy,7 O. B. Toon,3 M. A. Tolbert,2,4 B. Kärcher,12 Th. Peter,13 P. K. Hudson,1,2 A. J. Weinheimer,11 A. J. Heymsfield11

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 ({Delta}-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. {Delta}-ice represents a new link between global climate and natural and anthropogenic nitrogen oxide emissions. Including {Delta}-ice in climate models will alter simulated cirrus properties and the distribution of upper tropospheric water vapor.

1 Aeronomy Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO 80305, USA.
2 Cooperative Institute for Research in Environmental Sciences; University of Colorado, Boulder, CO 80309, USA.
3 Laboratory for Atmospheric and Space Physics, Program in Atmospheric and Oceanic Sciences; University of Colorado, Boulder, CO 80309, USA.
4 Department of Chemistry and Biochemistry; University of Colorado, Boulder, CO 80309, USA.
5 NASA Jet Propulsion Laboratory, Pasadena, CA 91109, USA.
6 Atmospheric Research Project, Harvard University, Cambridge, MA 02138, USA.
7 Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA.
8 Universidad Nacional Autonoma de Mexico, Centro de Ciencias de la Atmosfera, Ciudad Universitaria, 04510 Mexico DF, Mexico.
9 Department of Meteorology, University of Utah, Salt Lake City, UT 84112, USA.
10 NASA Ames Research Center, Moffett Field, CA 94035, USA.
11 Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO 80307, USA.
12 Institut für Physik der Atmosphäre, Deutsches Zentrum für Luftund Raumfahrt, Wessling, Germany.
13 Laboratorium für Atmosphärenphysik, ETH-Zurich, CH-8093 Zurich, Switzerland.

* To whom correspondence should be addressed. E-mail: rgao{at}al.noaa.gov

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Science. ISSN 0036-8075 (print), 1095-9203 (online)