DEPTH-STRUCTURE OF AIRBORNE MICROPARTICLES SAMPLED DOWNWIND FROM THE CITY OF KARLSRUHE IN THE RIVER RHINE VALLEY

Particulate material from outdoor air collected north of Karlsruhe was analysed with secondary mass spectrometry (SNMS and SIMS) especially to examine the depth distribution within the grains. Two size modes were found, submicron particles in the range of 0.2-0.8 mu m and coarse particles with diameters of 2-5 mu m. For both types a shell structure was found. The core of the coarse particles contained geogenic material, which was covered by a double layer structure. The topmost layer (60 nm) contained nitrates and ammonium sulfate, while a deeper layer of 400 nm primarily consisted of carbonate and organic carbon compounds. Soot from traffic sources seemed to be the origin of the submicron particles with a core dominated by graphitic and organic carbon, nearly completely covered by a 15 nm thick layer of ammonium sulfate. The latter is of importance for the removal of the soot from the atmosphere since the conversion of the soot surface from originally hydrophobic into hydrophilic behaviour should ease wet precipitation.