MEASUREMENTS OF THE PARTICLE SIZE DISTRIBUTION OF THE TROPOSPHERIC AEROSOL WITH A ROTATING-DISK SAMPLER S. K. FRIEDLANDER* and RALPH E. PASCERIT Johns Hopkins University, Baltimore, Maryland ABSTRACT Measurements were made of the particle size distribution of the aero- sol at Baltimore, Md. Particles smaller than a few tenths of a micron in diameter were sampled by diffusion from room air to a rotating disk with an electron-microscope grid attached. The rotating disk provides a known convective field with a number of desirable characteristics, Particle size distributions were calculated from the distribution measured on the disk grid. INTRODUCTION As part of a study of the dynamics of the atmospheric aerosol, measurements have been made of the size distribution of the aerosol at Baltimore over the range between 70 A and 140 » in diameter.’ The experimental data are reported in terms of asize-distribution function, n(r), and the cumulative distribution function, N(r), defined as follows: The quantity n(r) dr is the number of particles per unit volume with radii between r and r+ dr. Then N(r), the total number of particles per unit volume with radii greater than or equal to r, is given by *Present address: W. M. Keck Engineering Laboratories, California Institute of Technology, Pasadena, Calif. 7+Present address: Allegany Ballistics Laboratory, Cumberland, Md. 222