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Subway PM 2.5 can be substantially sourced from the operation of the system itself. Improvements in subway air quality may be possible by examining the potential to reduce these emissions. To this end, PM 2.5 was measured on the trains and station platforms of the Toronto subway system. A comparison with previously published data for this system reveals significant changes in below ground platform PM 2.5 . A reduction of nearly one-third (ratio (95% CI): 0.69 (0.63, 0.75)) in PM 2.5 from 2011 to 2018 appears to have resulted from a complete modernization of the rolling stock on one subway line. In contrast, below ground platform PM 2.5 for another line increased by a factor of 1.48 (95% CI; 1.42, 1.56). This increase may be related to an increase in emergency brake applications, the resolution of which coincided with a large decrease in PM 2.5 concentrations on that line. Finally, platform PM 2.5 in two newly opened stations attained, within one year of operation, typical concentrations of the neighboring platforms installed in 1963. Combined, these findings suggest that the production of platform PM 2.5 is localized and hence largely freshly emitted. Further, PM 2.5 changed across this subway system due to changes in its operation and rolling stock. Thus, similar interventions applied intentionally may prove to be equally effective in reducing PM 2.5 . Moreover, establishing a network of platform PM 2.5 monitors is recommended to monitor ongoing improvements and identify impacts of future system changes on subway air quality. This would result in a better understanding of the relationship between the operations and air quality of subways.

Impacts of Subway System Modifications on Air Quality in Subway Platforms and Trains