Chemistry and sources of PM_(2.5) and volatile organic compounds breathed inside urban commuting and tourist buses

摘要

Inhalable particulate matter (size <2.5 pm: PM2.5) inside commuting and tourist buses moving through the city of Barcelona, Spain, was chemically analysed. The analyses show PM dominated by organic carbon (mostly 10-20 pg/m3) and elemental carbon (mostly 3-6 pg/m3; OC/EC = 3.4), followed by Sat Fe, Ca, K, A1203, Mg, and Na, with calculated mineral content being around one third that of total carbon. Elemental carbon levels are higher inside diesel buses than those powered by natural gas or electricity, and higher in the upper floor of opentop double decker tourist buses than in the lower floor. Overall, major element concentrations inside the buses are typically 2-8 times higher than 24 h-averaged urban background levels, although some metallic trace elements, notably Cu and Sb, are exceptionally enriched due to the presence of brake particles, especially on routes involving higher gradients and therefore more brake use. In contrast, Cu and Sb concentrations in electric buses are unexceptional, presumably because these buses rely more on regenerative braking and are hermetically sealed when moving. Seasonal differences reveal PM to be more mineral in winter (A12031.3 pg/m3 vs. summer average of 0.3 pg/m3), with summer enrichment in Na, Mg, P, V, Ni and SO4- being attributed to marine aerosols contaminated by port emissions. Source apportionment calculations identify 6 main factors: road dust resuspension, metalliferous (brake wear and metallurgy), local urban dust, secondary sulphate and shipping (6%), vehicle exhaust (19%), and an indoor source (46%) interpreted as likely related to the textile fibres and skin flakes of bus occupants. Volatile Organic Compounds measured inside all buses except one were dominated by 2Methylpentane (14-36 pg/m3), Toluene (10-30 pg/m3), Xylene isomers (10-28 pg/m3, with m- o- > p-Xylene) and n-Pentane (5-15 pg/m3). EBTEX concentrations were <70 pg/m3, with Toluene being commonest, followed by m-Xylene, with p-Xylene, o-Xylene and Ethylbenzene each below 7 pg/m3 and Benzene concentrations always less than the EU limit value of 5 pg/m3. The VOCs mixture is similar to that recently reported from inside Barcelona taxis (although inside the larger volume bus VOC concentrations are lower than in the taxis) and is interpreted as providing a chemical fingerprint characterising traffic-contaminated ambient air in the city road environment. The notable exception to the VOC content was a brand new hybrid diesel bus still offgassing volatiles to such an extent that E(alkane + alkene + aromatic) indoor concentrations exceeded 800 pg/m3, with EBTEX ten times higher than normal.