Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 6 3 2006 10.5194/acp-6-601-2006 http://www.atmos-chem-phys.net/6/601/2006/ http://www.atmos-chem-phys.net/6/601/2006/acp-6-601-2006.html http://www.atmos-chem-phys.net/6/601/2006/acp-6-601-2006.pdf 601 611 2006-02-27 The observation of nitric acid-containing particles in the tropical lower stratosphere P. J. Popp T. P. Marcy E. J. Jensen B. Kärcher D. W. Fahey R. S. Gao T. L. Thompson K. H. Rosenlof E. C. Richard R. L. Herman E. M. Weinstock J. B. Smith R. D. May H. Vömel J. C. Wilson A. J. Heymsfield M. J. Mahoney A. M. Thompson Chemical Sciences Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO 80305, USA Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309, USA NASA Ames Research Center, Moffett Field, CA 94035, USA Institut für Physik der Atmosphäre, Deutsches Zentrum für Luft- und Raumfahrt, Oberpfaffenhofen, Germany Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA Atmospheric Research Project, Harvard University, Cambridge, MA 02138, USA MayComm Instruments, San Dimas, CA 91773, USA Global Monitoring Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO 80305, USA Department of Engineering, University of Denver, Denver, CO 80208, USA Mesoscale and Microscale Meteorology Division, National Center for Atmospheric Research, Boulder, CO 80301, USA Department of Meteorology, Pennsylvania State University, University Park, PA 16802, USA now at: Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80303, USA Airborne in situ measurements over the eastern Pacific Ocean in January 2004 have revealed a new category of nitric acid (HNO₃)-containing particles in the tropical lower stratosphere. These particles are most likely composed of nitric acid trihydrate (NAT). They were intermittently observed in a narrow layer above the tropopause (18±0.1 km) and over a broad geographic extent (>1100 km). In contrast to the background liquid sulfate aerosol, these particles are solid, much larger (1.7-4.7 µm vs. 0.1µm in diameter), and significantly less abundant (<10^-4 cm^-3 vs. 10 cm^-3). Microphysical trajectory models suggest that the NAT particles grow over a 6-14 day period in supersaturated air that remains close to the tropical tropopause and might be a common feature in the tropics. The small number density of these particles implies a highly selective or slow nucleation process. Understanding the formation of solid NAT particles in the tropics could improve our understanding of stratospheric nucleation processes and, therefore, dehydration and denitrification.