Emission factors of inorganic ions from road traffic: A case study from the city of Naples (Italy)

PM10 samples were collected in the urban tunnel of Naples (southern Italy) during a monitoring campaign on March 2015. Two sets of samples were collected at both sides of the tunnel, each set representing the daily cycle at a 1 h time resolution. Distance-based mass per kilometer emission factors (EFs) were calculated using mass concentrations, traffic flow rates and wind speed as a function of fleet composition. Samples were analysed for mass and water-soluble inorganic ions (Na+,NH4+,K+,Ca2+,Mg2+,Cl-,NO3- and SO42-) with the aim of investigating the influence of road traffic on the contribution of these species to PM levels. Road traffic directly emits inorganic ions, both from the exhaust and non-exhaust components. Analysis of ionic composition highlighted the increase in calcium concentration, which may derive from non-exhaust sources (road dust, wear of brake pads, clutches, tires) and calcium sulfonates, phenates or salicylates, often added to motor oils. Sulphate, added to lubricant oils, is also directly emitted at a rate higher than the gaseous sulphur dioxide emission. According to our analysis, nearly 10% of PM mass is composed by water-soluble inorganic ions, most of which directly emitted by automobiles. This suggests that an important contribution to PM emissions may derive from the inorganic component and more efforts should be devoted to constrain these emissions if PM concentration had to effectively comply with air quality standards.