The observation of nitric acid-containing particles in the tropical lower stratosphere

• Main Authors: 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
• Format: Article
• Language: English
• Published: Copernicus Publications 2006-01-01
• Series: Atmospheric Chemistry and Physics
• Online Access: http://www.atmos-chem-phys.net/6/601/2006/acp-6-601-2006.pdf

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&plusmn;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 &micro;m vs.&nbsp;0.1&micro;m in diameter), and significantly less abundant (<10^-4 cm^-3 vs.&nbsp;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.