aerosol layer exists, the small particles of debris rapidly coalesce with the larger sulfate particles, and that their subsequent fate is determined by the behavior of the sulfate aerosol particles, While the detailed history of particulate matter in the stratosphere is still not known, the High Altitude Sampling Program” has provided some knowledge of the mean stratospheric residence times involved. Observations of nuclear debris from Soviet tests indicate a fallout rate from the lower polar stratosphere commensurate with a residence half-time of about six months depending on seasonal effects. While some of the particles of the stratospheric aerosol may have appreciable settling velocities (greater than 0.1 cm/sec), it seems reasonable that most of the particles have settling velocities small enough to permit air motions to govern their fate, Once the particles leave the stratosphere (by any of several possible processes) and enter the troposphere, they are rather quickly removed by rain and deposited on the surface of the earth. Comments on Rocket Sampling of the Upper Atmosphere Although the above discussion of the behavior of particles in the upper atmosphere is quite speculative, it can be seen that knowledge of the physical behavior of the particles is vital in understanding the processes of air movements, For instance, if particles of about one micron radius were placed in the upper atmosphere (say at about 200, 000 feet) their fall rates would be quite appreciable (initially about 10 cm/sec) until they reached the lower stratosphere (about 80,000 feet), Measurements of mass or radioactivity of aerosol samples, collected by rockets in the upper atmosphere, without regard to the particle size might lead to erroneous conclusions since the measured distributions would have been governed by both gravitational effects and air motions, It thus seems that any program which involves sampling of the upper atmosphere must include determinations of particle size-concentration distributions along with those of composition, radioactivity, and the like. References DASA 532B, "Special Report on High Altitude Sampling Program,'' Maj. Albert K. Stebbins, III Ed. WD Feely, H. W., Science, 131, 645 (1960). WwW Ranz, W. E, and Wong, V. B., Ind. Eng. Chem. 44, FF Kalkstein, M. I., Bull. Amer. Phys. Soc., 6, 229 (1961). oO (June 1960), Junge, C. E., Chagnon, C. W., and Manson, J. E., Meteor.,18, 81 (1961). a 1. Feely, H. W. and Spar, J., Nature, 188, 1062 (1960). 68 1371 (1952).