Aerosol particles with diameters between 0.1 and 25 mu m were collected on the island of Helgoland (Germany) in October of 1996 (NORDEX 96 field campaign). The chemical bulk composition was determined for four different size fractions by total reflection X-ray fluorescence analysis. In addition, size, morphology, and chemical composition of approximately 3000 individual particles were determined by high-resolution scanning electron microscopy and electron probe microanalysis. Due to the use of a field emission source electron microscope the size range studied is extended down to 10 nm. The high lateral resolution of this instrument facilitates the detection of biological particles and soot, two components of North Sea aerosols not recognized by previous workers. The chemical bulk composition and the relative abundance of different particle groups depend on the air mass trajectories. Variations of the chemical bulk composition between all samples can be explained by assuming the mixing, in different proportions, of a marine and a continental component. The marine component is characterized by a high abundance of sea-salt, aged sea-salt and a mixture of sea-salt and alumosilicates. Air masses that crossed the continent prior to sampling on Helgoland have higher abundances of alumosilicates, Si-rich particles, Fe-rich particles, calcium sulphates, soot and carbonaceous material. The anthropogenic Fraction is about 8-33% (derived from the particle number concentrations) for marine air masses from the North Atlantic, and 14-69% for air masses that crossed the European continent. (C) 2000 Elsevier Science Ltd. All rights reserved.