Non-methane exhaust composition in the Sydney Harbour Tunnel: a focus on benzene and 1,3-butadiene

The concentrations of individual hydrocarbon species in the Sydney Harbour Tunnel were measured and used to estimate the average composition of emissions from moving motor vehicles in the Sydney urban area. The mean composition of non-methane hydrocarbons in the tunnel air on a weight basis was relatively constant. The mean concentrations for benzene and 1,3-butadiene were 45 and 13 ppbv, respectively, which in turn represented ~ 5.2% w/w and ~ 1.0% w/w of the total non-methane C₂-C₁₀ hydrocarbons in the tunnel air. The unit risk factor and the maximum incremental reactivity factor for 1,3-butadiene are approximately 30 times higher and 25 times higher, respectively, than the corresponding values for benzene. The concentration (μg m^-3) of benzene, however, is only about 5 times that of 1,3-butadiene. On this basis, the relative contribution to the risk associated with exposure to fresh motor vehicle emissions in Sydney would be about 6 times higher for 1,3-butadiene than for benzene. Similarly, the contribution made by 1,3-butadiene to the total hydrocarbon reactivity of the tunnel air will be about 5 limes that of benzene. Samples of three different grades of petrol (leaded, unleaded and premium unleaded) from three different brands of fuel were also analysed on two separate occasions. Compositions of leaded and standard unleaded petrol averaged across the three different brands are quite similar. The average aromatic content (% w/w) of the 3 different commercial brands were ~ 35 and 30% for leaded and unleaded petrol, respectively. However, premium unleaded petrol has a much higher aromatic content of ~ 47% w/w. Comparison of the petrol and tunnel compositions demonstrated that benzene is enriched relative to other aromatics in exhaust compared to its proportion in the petrol.