Platinum emissions and levels in motorway dust samples: influence of traffic characteristics

An increase in the environmental levels of platinum can in part be attributed to the introduction of motor vehicle catalytic converters. Reported platinum levels in roadside dust samples range from 0.6 ng/g to 62.8+/-10.5 microg/g (dry weight, d.w.), with the highest levels in samples taken from inside road tunnels. Carriageway dust samples were collected from nine junctions (at sites of rush hour 'stop-start' activity or slip road on/off sections) of the London Orbital (M25) motorway and analysed for total Pt by inductively coupled plasma mass spectrometry (ICP-MS). These motorway junctions have mean daily traffic densities of 85,000 to 145,000 vehicles. Dust samples were fractionated (<75, 75-125 and 125-250 microm), microwave acid digested (aqua rega/HF) and analysed using (195)Pt(+) with polyatomic ion correction of (179)Hf(16)O(+). Quality control validation was performed with replicate analysis of two certified reference materials, namely, NIST 2556 (Used Autocatalyst-Pellet) and 2557 (Used Autocatalyst-Monolith). Total platinum levels (sum total of all particle fractions) ranged from 101.6 to 764.2 ng/g (dry weight) in dust samples from all sampling sites. The highest Pt levels occur in the <75 and 75-125 microm particle fractions, with the fractionated samples ranging from 4.6 to 356.2 ng/g Pt (dry weight). Statistical analysis (Student t-test) confirmed that the significantly higher Pt levels (at the 95% confidence interval) occurred in dust samples taken from carriageway sites that had the vehicle slowing/stop-start driving conditions rather than for the accelerating speed zones. Correlation analysis showed that dust Pt levels, irrespective of fraction particle size, did not directly relate to traffic density, which is contrary to other published studies. The highest Pt levels occurred at the M25/A3 (Portsmouth/Guildford to London highway) intersection suggesting that traffic flow conditions, such as erratic stop-start flows, have a greater influence on Pt particle release and accumulation in the adjacent motorway environment than traffic density.