Measurements of aerosol size distributions, chemical composition, and optical properties were carried out at two locations in Israel. Tel Aviv was chosen as an urban region, and Mitzpe Ramon as a desert region. The measurements indicate that the size spectra between 0.15 and 12 μm can be approximated by a power law type of distribution. The spectra in Tel Aviv show higher concentrations and greater slopes than those from the desert. Diurnal variations between day and night are observed in the size distribution of the summertime measurements in Tel Aviv. These are believed to be related to the increase in aerosol size due to absorption of water vapor during high relative humidity periods. The chemical composition in Tel Aviv was found to depend strongly on the meteorological conditions. During westernly flow, which is the prevailing wind direction in Tel Aviv, the aerosols contain large amounts of NaCl particles, which are not usually found in the desert. During Sharav conditions (dry conditions and easterly flow) the aerosols in Tel Aviv are, as is expected, very similar to those found in the desert. The optical measurements also suggest strong differences between Tel Aviv and the desert. Filter samples of atmospheric particulate matter from Tel Aviv always appeared darker than those from the desert site. Measurements of imaginary refractive index in the 0.3- to 1.7-μm spectral region are presented. These measurements are consistently higher by a factor of 2--4 in Tel Aviv as compared to simultaneous samples from the desert site. Seasonal variations also have been observed, and their possible causes are discussed. The size distribution, composition, and imaginary refractive index observations are interpreted in terms of aerosol origins and prevailing local and synoptic scale meteorological conditions.