Size distribution of airborne magnetic ‘back spherules’ in the 1–5 μ range

Summary Airborne particulate matter was sampled at Mt. Cimone, Italy, to determine the size distribution of black spherules in the 1–5 diameter range. The magnetic fraction of airborne particulate matter was separated by forcing air through a strong magnetic sampling device, where ferromagnetic particles accumulated on microscope slides.Sizes of the black spherules were determined by counting under a microscope. Samples were dissolved and analyzed for iron content. Distribution was found to be in agreement with that of deep-sea sediment and satellite high atmosphere samples. The rate of terrestrial accretion of cosmic matter composed of black spherules appears to be approximately 10^–15 g cm^–2 sec^–1.