Measurements of the number and size distribution of the atmospheric aerosol were made at ground level at Meppen, W. Germany using two optical particle counters. The aerosol was measured infrequently from July, 1977, through March, 1978, over the particle radius range 0.21-5.9 micrometers using a Royco model 220 particle counter. A Particle Measuring System ASAS 300B aerosol particle counter measured the aerosol over a similar radius range every hour and, often, every half-hour from 14 March through 9 May, 1978. This report discusses the behavior and characteristics of the Meppen aerosol distribution compared to an extensive set of airborne measurements of the aerosol in the lower troposphere at nine sites in Europe, including Meppen, and the use of local meteorological observations to estimate the local particle size distribution. Volume distribution plots and the results of a model fit using log-normal distributions show good agreement between the airborne and ground data sets. Both data sets show three volume modes, each well fit by a log-normal distribution. The behavior and values of the log-normal fit parameters for the accumulation and middle modes are in good agreement between the two data sets. The greater frequency of occurrence, and, generally, bigger size of the ground coarse modes compared to the airborne coarse modes indicates a larger average particle number concentration for the ground measured modes. The data show that each of the three modes is not equally affected by changes in relative humidity. The middle mode is the most sensitive and the coarse mode is the least sensitive. It appears that the relative humidity sensitive middle mode must be present for the formation of a mist or fog. (Author).