Sources of subway PM2.5: Investigation of a system with limited mechanical ventilation

Identifying subway PM sources is essential to improving subway air quality. To date, no source apportionment studies exist for systems with limited mechanical ventilation. These systems often have high concentrations of PM2.5. This study investigated PM2.5 sources in the Toronto subway system using three analytical approaches. Positive matrix factorization identified three subway sources and no outdoor sources. A two-source Chemical Mass Balance (CMB) model apportioned 92% and 55% of PM2.5 to iron-rich components (wheels, rails, and contact rails & shoes) and 8% and 45% to brake pads on line 1 and 2, respectively. A simple mechanistic model combined with the CMB results revealed wear of wheels, rails, and brake pads during braking to be the main source of PM2.5 in this subway. These results indicate that below grade subways with minimal mechanical ventilation are dominated by system-sourced PM2.5 emitted during deceleration. This knowledge should help identify strategies to improve air quality in the subway systems.