Particulate matter concentration and composition in the New York City subway system

In this study we investigated the concentration and composition of particulate matter (PM_2.5) in the New York City subway system. Realtime measurements, at a 1-s cadence, and gravimetric measurements were performed inside train cars along 300 km of nine subway lines, as well as on 333 platforms on 287 subway stations. The mean (±SD) PM_2.5 concentration on the underground platforms was 142 ± 69 μg/m³ versus 29 ± 20 μg/m³ for aboveground stations. The average concentrations inside train cars were 88 ± 14 μg/m³ when traveling through underground tunnels and platforms and 29 ± 31 μg/m³ while on aboveground tracks. The particle composition analysis of filtered samples was done using X-ray fluorescence (XRF), revealing that iron made up approximately 43% of the total PM_2.5 mass on station platforms, approximately 126 times higher than the outdoor ambient iron concentration. Other trace elements include silicon, sulfur, copper, nickel, aluminum, calcium, barium, and manganese. Considering the very high iron content, the comparative analysis of the measured concentration versus the standards set by the Environmental Protection Agency (US EPA) is not appropriate since those limits are largely based on particulate matter from fossil fuel combustion. Health impact analysis of inhalation of iron-based particles is needed to contextualize the results presented here.