Urban air quality comparison for bus, tram, subway and pedestrian commutes in Barcelona

Access to detailed comparisons in air quality variations encountered when commuting through a city offers the urban traveller more informed choice on how to minimise personal exposure to inhalable pollutants. In this study we report on an experiment designed to compare atmospheric contaminants inhaled during bus, subway train, tram and walking journeys through the city of Barcelona. Average number concentrations of particles 10-300 nm in size, N, are lowest in the commute using subway trains (N<2.5 ×10⁴part.cm^-3), higher during tram travel and suburban walking (2.5 ×10⁴cm^-3<N<5.0×10⁴cm^-3), and highest in diesel bus or walking in the city centre (N>5.0×10⁴cm^-3), with extreme transient peaks at busy traffic crossings commonly exceeding 1.0×10⁵cm^-3 and accompanied by peaks in Black Carbon and CO. Subway particles are coarser (mode 90nm) than in buses, trams or outdoors (<70nm), and concentrations of fine particulate matter (PM<inf>2.5</inf>) and Black Carbon are lower in the tram when compared to both bus and subway. CO<inf>2</inf> levels in public transport reflect passenger numbers, more than tripling from outdoor levels to >1200ppm in crowded buses and trains. There are also striking differences in inhalable particle chemistry depending on the route chosen, ranging from aluminosiliceous at roadsides and near pavement works, ferruginous with enhanced Mn, Co, Zn, Sr and Ba in the subway environment, and higher levels of Sb and Cu inside the bus. We graphically display such chemical variations using a ternary diagram to emphasise how "air quality" in the city involves a consideration of both physical and chemical parameters, and is not simply a question of measuring particle number or mass.