Airborne contamination of forest soils by carbonaceous particles from industrial coal processing

In the German Ruhr-area industrial coal processing emitted large amounts of carbonaceous particles for a century until 1970. Our objectives were to detect the presence of airborne carbonaceous particles and assess their impact on the chemical structure of soil organic matter in two forest soils (Podzols) with potential sources of carbonaceous particles approximately 10 to 30 km away. Contamination was not visible macroscopically. Organic matter was characterized in bulk soils and in particle-size separates by elemental analysis, magnetic susceptibility measurement, reflected light microscopy, and ¹³C solid-state nuclear magnetic resonance (NMR) spectroscopy. Organic and mineral horizons contained carbonaceous particles including char, coke, and bituminous coal from coal combustion, coking, coal processing, and steel production. In the organic horizons of both soils we observed a material high in magnetic susceptibility (max. 109 × 10^-8 m³kg^-1), whereas in the mineral horizons only the Podzol with an intense intermixing modern-type humus had high magnetic susceptibility. This Aeh horizon was extremely rich in organic carbon (139.4 g organic C kg^-1), concentrated in the 20 to 2000 μm size separates. In the second Podzol, like in many natural soils, C concentrations were largest in the <20 μm separates. Bloch decay ¹³C magic angle spinning (MAS) NMR spectroscopy revealed a highly aromatic structure of the carbonaceous particles. Airborne carbonaceous particles formed a macroscopically indistinguishable mixture with natural soil organic matter and could be present in many soils neighboring industrialized areas.