Inhalable particulate matter (size <2.5µm: PM 2.5) 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 µg/m 3) and elemental carbon (mostly 3-6 µg/m 3 ; OC/EC=3.4), followed by SO 4 2 , Fe, Ca, K, Al 2 O 3 , 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 open-top double decker tourist buses than in the lower floor. Overall, major element concentrations inside the buses are typically 2-8 times higher than 24haveraged 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 (Al 2 O 3 1.3 µg/m 3 vs. summer average of 0.3 µg/m 3), with summer enrichment in Na, Mg, P, V, Ni and SO 4 2being 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 2-Methylpentane (14-36 µg/m 3), Toluene (10-30 µg/m 3), Xylene isomers (10-28 µg/m 3 , with m->> o-> p-Xylene) and n-Pentane (5-15 µg/m 3). ƩBTEX concentrations were <70 µg/m 3 , with Toluene being commonest, followed by m-Xylene, with p-Xylene, o-Xylene and Ethylbenzene each below 7 µg/m 3 and Benzene concentrations always less than the EU limit value of 5 µg/m 3. 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 Ʃ(alkane+alkene+aromatic) indoor concentrations exceeded 800 µg/m 3 , with ƩBTEX ten times higher than normal.

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Public bus system worldwide transports daily more than 34 million people; therefore, air quality inside buses is an important urban air pollution issue. Airborne microbiological composition and abundance was examined inside five different fuel type buses (diesel (EURO IV and EURO V), hybrid diesel (HD), compressed natural gas (CNG), hybrid natural gas (HCNG) and electric vehicle (EV)) from Barcelona bus system, inside and outside the tourist bus and different periods of campaigns (summer: May-September and winter: November-March). Quantitative PCR was used to quantify total bacteria, Penicillium/Aspergillus and Cladosporium sp. and Kinetic Chromogenic Limulus Amebocyte Lysate (LAL) Assay for endotoxins. The targeted 16S rRNA genes, were amplified and sequenced to asses bacterial community composition and biodiversity. Regarding the targeted microorganisms the results showed low bioaerosol concentrations, being fungi and endotoxins below the detection limit, although bacterial airborne has a geometric average of 10 3 equivalent E. Coli genomes/m 3 . Airborne bacterial community biodiversity showed that Actinobacteria and Proteobacteria were the most abundant Phylum in the different sampled buses and in the two seasons (summer: May-September and winter: November-March). While regarding to genera taxonomic category, the different fuel type buses presented a high overlap degree, being Cutibacterium the most abundant and Paracoccus the one only detected inside. The different seasons showed that the different taxonomic genera decreas from summer: May-September to winter: November-March and in this case Paracoccus was the most abundant. Overall, biodiversity inside buses is greatly affected by the presence of humans inside, but also by the outdoor sources at the moment of the sampling, and the seasonal period, more than the type of bus.