Origin of polycyclic aromatic hydrocarbons and other organic pollutants in the air particles of subway stations in Barcelona

Highlights: • PM_2.5 levels in subway platforms are higher than those for urban outdoor. • PAH levels at the platforms are similar to those in the outdoor ambient air. • Maintenance works and inverse ventilation do not influence the indoor air quality for PAH. • PSD and advanced ventilation systems reduce PM_2.5 concentrations in the platform. • 75% of the detected PAH concentrations in the platforms origin from outdoor air. Underground subways transport large numbers of citizens in big cities, which must breathe air with limited ventilation. These atmospheric conditions may enhance the concentration of air pollutants from both outdoor and indoor air. The influence of ventilation conditions and maintenance activities on the concentrations of air pollutants have been studied. Particulate matter with aerodynamic diameter smaller than 2.5 μm (PM_2.5) in indoor air was sampled in ten platforms of nine subway stations of the metropolitan area of Barcelona in 2015 and 2016. These particles were analyzed for polycyclic aromatic hydrocarbons (PAH) and organic tracer compounds. The concentrations of PAH were in the range of the street air levels with higher PAH values in the colder period. No influence of nighttime maintenance activities was observed on the platform air quality during daytime. Source apportionment analysis using the concentrations of hopanes, nicotine and levoglucosan as molecular tracer compounds showed that 75% of the detected PAH at the platforms have an outdoor PM origin. The modern subway stations, with advanced ventilation and platform screen doors that separate the subway system from the platform, showed lowest PAH and PM concentrations.