An October influx of submicron particles into the lower stratosphere

Although the concentration of particles in the stratosphere above the sulfate layer has been found to be typically 2–8 cm−3 STP, on three occasions over the United States and four over Australia, concentrations greater by at least two orders of magnitude have been encountered. The particles have modal diameters of about 0.05 μm but are often joined together to form chains. In appearance and size distribution they closely resemble the particles obtained on some rocket flights at levels above 60 km. Phenomena that may be related and have been observed to occur at the same time of year are stratospheric aerosol layers, which appear optically dense when they are viewed horizontally looking toward the sun, and some twilight striations. The origin of the particles has not been determined.