Airborne particles in city bus: concentrations, sources and simulated pulmonary solubility

PM10 and PM2.5 concentrations in Ljubljana city bus were monitored during entire shift, and individual particles were morphologically and chemically characterised in order to determine PM concentration variability, particle sources, solubility in simulated pulmonary environment and effects on human health. PM measurements revealed high mean PM10 (82.8 μg/m3) and PM2.5 (47 μg/m3), which were highest and most variable during rush hours with fluid traffic and lowest during traffic jams with standing vehicles. Individual particle analysis showed that airborne particles were dominated by metal-bearing phases, particularly small-sized (Cr,Mn,Zn)-bearing Fe-oxyhydroxides and Al-/Fe-Al-oxides, large (Fe,Cr,Ni)- and (Cu,Zn,Ni)-alloys, and small-sized Sb-sulphide and Ba-sulphate. Non-metallic phases were represented by (Ca,Mg)-carbonates, Al-silicates, Na-chloride and Ca-sulphate. Comparison with possible source materials (vehicle exhaust emissions, brake disc dust and road sediment) showed that primary sources of these metal-bearing phases were wear of brake discs, brake pads and tyres, and also wear of engine components and catalytic converters. Most non-metallic phases originated from resuspension of road sediment, containing road sanding materials, but also from emissions of burned fuel and lubricating oil (Ca-sulphate). Assessment of effects on human health indicated that mean PM concentrations, which significantly exceeded daily limit values, increased mortality (by 2–3%) and morbidity (by 7–8%) risk for bus drivers. Simplified PHREEQC calculations of airborne metal-bearing phase solubility in aqueous solutions simulating pulmonary environment showed that metallic Fe, Ba-sulphate, Sb-sulphide and Al-oxide, partly also Cu-bearing metal alloys, were soluble under reducing and oxidising conditions, but released metals were removed from solution by precipitation of stable secondary metal-bearing phases.