160 DREVINSKY AND PECCI is designed to collect air at a flowrate of 0.7 m°/min ambient (capacity of 28 m° ambient volume per sample). It is basically a scaled-up version of the impactor air sampler used by Chagnon and Junge’? in studies of particle sizes of natural aerosols. Figure 1 shows an exploded view of one impactor—filter sampler with its cylindrical housing of stainless steel. A manifold connects the FILTER (FOR <0.02-y-RADIUS PARTICLES) STAGE 2 (FOR 0.02 TO 0.15-p-RADIUS PARTICLES) STAGE 1 (FOR >0.15-p-RADIUS PARTICLES) Fig. 1—Exploded view of a single impactor—filter sampler. pumping system to four of these samplers, which are individually programed to operate successively over the altitude intervals of 9 to 15, 15 to 21, 21 to 27, and 27 to 30 km. A single balloon flight, therefore, provides a four-point vertical profile. The balloon ascent rate was about 0.15 km/min, and flights were concluded with a float at maximum altitude for about 30 min. Over each altitude interval, the operating sampler is designed to collect particles larger than 0.15 u in radius on stage 1, particles in the range of 0.02 u to 0.15 in radius on stage 2, and the smallest particles on either a 25-cm-diameter (9 to 15-km altitude interval) or a 15-cm-diameter (all other altitude intervals) polystyrene backup filter. At these particle cutoff sizes, the collection efficiency is 50% on the bases of a particle density of 2 g/cm? and the design flow rate. The impactor jets were individually designed and calibrated with homogeneous aerosols at midaltitude for each sampling interval. Jet dimensions were increased on samplers operating at higher altitudes to compensate for the decrease in air density, according to theoretical considerations, so that constant cutoffs could be maintained regardless of altitude. The collection surfaces on stages 1 and 2 consist of a 3.8- by 11.8- by 0.3-cm glass slide covered with pressure-sensitive cellophane tape which is coated with a thin film of high-vacuum silicone grease. Radiochemical Analyses Samples from stages 1 and 2 were removed from the slides by stripping off the tape, which was then mounted on Lucite semicylinders