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

• Published in: Science (American Association for the Advancement of Science) 2004-01, Vol.303 (5657), p.516-520
• Main Authors: Gao, R. S., Popp, P. J., Fahey, D. W., Marcy, T. P., Herman, R. L., Weinstock, E. M., Baumgardner, D. G., Garrett, T. J., Rosenlof, K. H., Thompson, T. L., Bui, P. T., Ridley, B. A., Wofsy, S. C., Toon, O. B., Tolbert, M. A., Kärcher, B., Th. Peter, Hudson, P. K., Weinheimer, A. J., Heymsfield, A. J.
• Format: Article
• Language: English
• Subjects: Accuracy; Acids; Atmospherics; Chemical composition and interactions. Ionic interactions and processes; Chemical properties; Cirrus clouds; Climate; Climate models; Cloud physics; Clouds; Contrails; Earth, ocean, space; Exact sciences and technology; External geophysics; Greenhouse effect; Humidity; Ice; Meteorology; Molecules; Nitric acid; Principals; Properties; Scientific Concepts; Surface areas; Temperature; Temperature dependence; Troposphere; Water; Water vapor
• ISSN: 0036-8075; 1095-9203
• DOI: 10.1126/science.1091255

In situ measurements of the relative humidity with respect to ice ($RH_i$) and of nitric acid (HNO3) were made in both natural and contrail cirrus clouds in the upper troposphere. At temperatures lower than 202 kelvin,$RH_i$values show a sharp increase to average values of over 130% in both cloud types. These enhanced$RH_i$values are attributed to the presence of a new class of$HNO_3-containing$ice particles ($\Delta-ice$). We propose that surface HNO3molecules 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.