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Trace ammonia in laboratory air reacts easily with sulfuric acid aerosol samples to form crystalline ammonium sulfate. Using argon atmospheres, however, we have succeeded in protecting sampling surfaces from ammonia contamination. We find that atmospheric aerosols treated in this way contain only sulfuric acid. After an hour exposed to laboratory air, these same samples convert to ammonium sulfate. We are continuing to collect aerosol particles, using argon control, to determine if the absence of crystalline sulfate is common. But so far there is no evidence that aerosols are being neutralized by ammonia in the stratosphere.

Samples from the stratosphere obtained by U-2 aircraft after the first three major eruptions of Mount St. Helens contained large globules of liquid acid and ash. Because of their large size, these globules had disappeared from the lower stratosphere by late June 1980, leaving behind only smaller acid droplets. Particle-size distributions and mineralogy of the stratospheric ash grains demonstrate in-homogeneity in the eruption clouds.

A successful flight of a recoverable payload designed to collect particles from a noctilucent cloud was made on August 1, 1968, at 0410 local daylight time (0910 UT), from Fort Churchill, Canada. Photographs obtained by project observers at Thompson, Canada, 400 km southwest of Churchill, showed that the noctilucent cloud was over the launch site at the time of flight. Electron microscope examinations of collecting surfaces exposed above 60 km showed about 6000 particles/cm • of a certain type on surfaces facing the flight direction. Few of these particles were found on nonflight control surfaces. Most of the particles range in size from 0.1-0.2 • and do not exhibit features suggesting that any liquid was associated with them. The shape of these particles is generally equant, often rod-like, subrounded to rounded; the particles are moderately dense to electrons, show internal structure, and have occasional protuberances. Considerable variation of particle concentration was observed from one flight surface to the next. Flight surfaces show some damage from aerodynamic heating. Thus, caution is necessary in applying these findings pending confirmatory sounding rocket flights through noctilucent clouds. Evidence from this flight, from previous rocket and satellite flights, and theoretical studies suggests that the noctilucent cloud particles are uplifted from lower altitudes and could have a terrestrial as well as cosmic origin. Intensive investigations of noctilucent clouds have been made since their first recorded discovery as an unusual phenomenon by Backhouse [1885]. These tenuous clouds form at an altitude of 80 km during the summer seasons above latitudes of 60 ø in both hemispheres. Fogle and Haurwitz [1966] have summarized the historical and observational data that they and numerous other investigators have obtained. Despite extensive study, the origin and composition of noctilucent clouds remain in doubt. Helmholtz [1889] and later StSrmer [1933] observed and confirmed that the clouds were composed of particles made visible by scattered sunlight. Visual observations by Vestine [1934], Astapovich [1939], Ludlam [1957], and Witt [1957] led these investigators to predict a particle size less than i /•. Spectra obtained by Grishin [1956] were later interpreted by Deirmendjian and Vestine [1959], who concluded that dielectric spheres smaller than 0.8 /• could cause the observed scattering. Polarization measurements were interpreted by Witt [1960a, b] and Villmann [1962] using Mie scattering

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