The chemical composition and complex refractive index of rural and urban influenced aerosols determined by individual particle analysis

From July 1997 to June 1998 aerosol particle samples (diameter 0.1–25 μm) were collected at Kleiner Feldberg (Taunus mountains, Germany), a rural location that is temporarily influenced by the nearby urban Rhein-Main area and/or by long-range transport from East Germany and Eastern Europe. The atmospheric concentrations of the elements sodium to lead (11⩽ Z⩽83) were determined by total reflection X-ray fluorescence analysis. Size, morphology and chemical composition of more than 27,000 individual aerosol particles were determined by high-resolution scanning electron microscopy and energy-dispersive X-ray microanalysis. Based on morphology and chemical composition the particles were classified in the particle groups: ammonium sulfates, calcium sulfates, sea salt, alumosilicates, silica, metal oxides/hydroxides, soot, biological particles, carbon/sulfate mixed particles, and remaining carbon-rich particles ( C rest). Polluted air masses at Kleiner Feldberg are characterized by high number concentrations of soot (up to 80% in the size range from 0.1 to 0.2 μm), metal oxides and sulfates. Anthropogenic and natural sources of alumosilicates, silica and metal oxides/hydroxides are easily distinguished by their morphology. From the size resolved relative abundance of the different particle groups the total and the size resolved complex refractive index (CRI) of the dry particulate matter ( λ = 550 nm ) was calculated for the different sampling days. Urban influenced air masses are characterized by high real (1.60–1.73) and imaginary parts (0.034–0.086) of the total CRI, rural air masses by lower real (1.54–1.61) and imaginary parts (0.001–0.021). The high real parts of the CRI of polluted air masses are predominantly caused by the high abundance of metal oxide/hydroxide particles, the high imaginary parts by high abundances of soot.