Large Particles in the Tropical Lower Stratosphere, In Situ Size Distributions
Abstract
Size resolved aerosol concentration profiles in the tropics have been completed on 5 occasions using in situ instruments deployed on balloons. Two profiles were completed from Baru, Brazil, 23°S in November 1997, one profile from near Darwin, Australia, 13°S, in November 2005, and two profiles from Niamey, Niger, 13°N, in July, August 2006. In addition to providing tropical profiles of the size distribution of stratospheric aerosol, these measurements indicate the presence of surprisingly large (> 1.0 um radius) particles several kilometers above the tropical tropopause. The presence of these particles is also surprisingly common. They were observed at all three locations: on one of two flights from Brazil, on the one flight from Australia, and on one of two flights from Niger. They also appear to be confined to the tropics. Such particles have never been observed, except following recent volcanic eruptions, in a long series of measurements from the mid latitudes. The particle layers are also surprisingly local. From Australia they were observed on the descent profile, but not on ascent, as they were in Niger. In both cases the large particles extended up to 2 km above the tropopause and were part of a 4-6 km thick layer of aerosol spanning the tropopause. Their concentrations are similar to near surface observations on the same profile. In Brazil, the large particles (2.0 um) appeared at 4 km above the tropopause, and were isolated from any larger aerosol layer. Their concentrations were similar to mid troposphere observations on the same flight. On the first flight from Niger the ascent profile indicated that the lower stratosphere was filled with particles up to 3.0 um. The particles extended to 25 km, 8 km above the tropopause. These particles appear to be the result of a recent tropical volcanic eruption; however, they were not observed on the descent profile, as evidenced by the strikingly different size distributions in the aerosol layers between the ascent and descent profiles. The origin of the large particles has not been established. Temperatures are too warm for them to be ice, or nitric acid trihydrate, and the size distribution does not conform to other measurements within cirrus or polar stratospheric clouds. The Australian and Nigerian profiles were completed in the morning, while the Brazilian fight was at night. Of all the flights, only the first Niger flight was completed near convection. Based on the size distributions, and comparisons with particle size distributions in the troposphere, we believe these measurements capture the aerosol remnants of overshooting thunderstorms which must penetrate well into the stratosphere. The sparse nature of these profiles, yet the high frequency of observations of large stratospheric particles, suggests that the transport of large particles from the lower troposphere to well into the stratosphere is a common feature of the tropical upper troposphere lower stratosphere.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.A13G..05D
- Keywords:
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- 0305 Aerosols and particles (0345;
- 4801;
- 4906);
- 0340 Middle atmosphere: composition and chemistry;
- 0341 Middle atmosphere: constituent transport and chemistry (3334)