Short-term variation of palladium in airborne particulate matter

Abstract

Platinum group elements (PGEs) can be naturally found only at very low concentration in the earth crust, a situation which is changing with the release of PGEs from automobile catalysts. Platinum (Pt), rhodium (Rh), and palladium (Pd) are used in motor vehicle exhaust systems to catalyze the conversion of hydrocarbons, carbon monoxide, and nitrogen oxides into the less harmful substances carbon dioxide, nitrogen, and water. With the use of automobile catalytic converters fine particulate material originating from the abrasion and the deterioration of the surfaces of the catalysts is introduced in the environment (Wei and Morrison 1994; Moldovan et al. 1999). They are emitted mostly as abraded wash coat particles with a size range from sub-micrometer to several micrometers (Gomez et al. 2001; Zereini et al. 2001) with emission rates in the ng km-1 range (Moldovan et al. 1999; Palacios et al. 2000). As a result a clear link between the use of automobile catalytic converters and increasing concentrations of PGE in the environment has been observed, especially in traffic exposed environmental samples like road dust, soils or adjacent vegetation (Schäfer et al. 1999; Jarvis et al. 2001; Zereini et al. 2001). Platinum is the element that has received the most attention among the PGEs studies, while Pd and Rh have been monitored in more recent research projects. Recently it has been demonstrated that Pd has a greater mobility in the environment than either Pt or Rh (Moldovan et al. 2001), which means that Pd undergoes environmental transformations into more reactive species which may be bioavailable (Schäfer et al. 1998; Rauch and Morrison 2000; Moldovan et al. 2001; Philippeit and Angerer 2001). Therefore it becomes more important to investigate Pd levels in the environment. Although palladium concentrations have been determined in different environmental samples such as road dust, soils or vegetation (Jarvis et al. 2001; Müller and Heumann 2000; Schäfer et al. 1999) information about the atmospheric occurrence is very sparse. Limited sample amounts combined with numerous interferences in the most sensitive analytical techniques are considered to be the major difficulties. However, several tech niques have been applied to the determination of palladium traces in airborne particulate matter including electro thermal atomic absorption spectrometry (Limbeck et al. 2003), electro thermal atomization laser excited atomic fluorescence spectrometry (Tilch et al. 2000) and inductively coupled plasma mass spectrometry (Rauch et al. 2001; Petrucci et al. 2000) in particular in combination with isotope dilution mass spectrometry (Kanitsar et al. 2003). However, in all of the mentioned studies the described results were derived from very limited sample sets disabling the determination of temporal trends. Data about long-term observations of PGE concentrations in airborne particulate matter, which are necessary to define average concentrations, are reported for Pt and Rh only. Zereini et al. (2001) observed a 46-fold enhancement in the Pt levels of air over a 10-year period (from 1988 to 1998), whereas the Rh concentration increased by a factor of 27. These results indicate a trend toward continuous increases in ambient concentrations of these metals between 1988 and 1998. Based on the growing demand for palladium in the last years (Matthey 2001) a similar increase of the palladium content in airborne dust could be expected. Schäfer et al. (1999) have shown that the short-term variation of PGE concentrations and their ratios in airborne dust were due to wind and rain. These findings were confirmed by Limbeck et al. (2004), who reported a complete annual cycle for the palladium concentration in airborne particulate matter, indicating that the seasonal variation of the palladium concentration is caused by changes in the meteorological conditions. However, these studies were based on the investigation of monthly collected aerosol samples and provide therefore no information about the daily variation of the palladium concentration in airborne particulate matter. To assess the risks associated with the emission of palladium into the environment additional data on ambient concentrations is needed; in particular investigations with shorter sampling intervals (days instead of weeks or months) are required. In this work aerosol samples collected at three different sites in Vienna, Austria were analysed to determine the short-term variation of palladium in the atmospheric aerosol. Based on the determined palladium concentrations and the results obtained for the aerosol mass the palladium content of the aerosol samples was calculated. The results obtained for the three investigated sites were compared and the differences observed between the individual sampling sites are discussed.