ROTATING-DISK-SAMPLER MEASUREMENT OF AEROSOLS 225 EXPERIMENTAL WORK Sampling The stainless-steel disk, 1 in. in diameter and '/ in, thick, was connected to a Heller GT-21 stirrer with electronic speed control. The speed was checked with a stroboscope. The electron-microscope grid was centered on the disk face and attached to the surface with two pieces of scotch tape. Two runs, 10 and 11, were made in the laboratory with the rotating disk at speeds of 2040 and 1980 rpm, respectively. The experiments were carried out consecutively (Sept. 11 to 13, 1963) with a brief in- terruption to replace the electron-microscope grid and to adjust the position of the rotating-disk assembly. The sampling times were approximately 13 and 25 hr for runs 10 and 11, respectively. In run 10 the disk face was positioned in a vertical plane, and in run 11 it was horizontal and faced the ceiling. The laboratory door was kept closed most of the time. A 20-in. window fan installed above the laboratory door was not operated during the sampling periods, but the fan opening permitted contact with the outside hallway. Since a draft was always present in the hallway, air exchange with the room was probable. It should be possible to reduce the sampling time in a numberof ways: (1) Since the deposition rate is directly proportional to the aerosol concentration, the sampling time would be reduced in direct propor- tion to an increase in concentration, (2) since the deposition rate is proportional to we, the speed can be increased above the 2000 rpm of these experiments with proper attention to mechanical problems and to the maintenance of a laminar flow regime; and (3) it would be posSible to run over a shorter time by examining and photographing more fields. A limitation is imposed by the amount of background material present on the fresh grid. Preparation of Electron-microscope Grids The grids were made of 400-mesh copper screen with a diameter of ¥, in. and a thickness of approximately 0.001 in. They contained about 600 square openings, each having dimensions on each side of about 25 uw. The grid substrate was a carbon film prepared by evaporating carbon onto freshly cleaved mica, Details of the process are given in Pasceri’s dissertation.! The films obtained were estimated to be between 100 and 200 in thickness. The carbon films were stripped from the mica by floating them onto the surface of demineralized, double-filtered (0.3- Millipore filters) water. Beneath the surface of the water, several grids were laid on a clean Monel screen. The screen was gently lifted from the