The uncontrolled growth of some cities around the world has produced an increasing concentration of atmospheric aerosols. They have effects on the air quality in those geographical areas, affecting public health, visibility, and buildings. A better knowledge of the aerosol characteristics is required. A comparison of the sources, sampling procedures, analytical use of particle induced X-ray emission (PIXE), and composition of atmospheric aerosols in several urban areas around the world, having more than four million inhabitants, is presented. The need for standard procedures and analysis, as well as the use of other techniques in addition to PIXE, is pointed out. It is shown how local sources (traffic, industry, soil), geographical characteristics, or external sources, influence the aerosol composition.