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. 0.1 μm in diameter), and significantly less abundant (<10-4 cm-3 vs. 10cm-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.
All Science Journal Classification (ASJC) codes
- Atmospheric Science