Comparative assessment of black carbon levels in public transit systems across three cities

This research investigates users’ exposure to black carbon (BC) in subways, buses, and BRTs. The study was conducted in Mexico City and Puebla (Mexico) and Montreal (Canada) to identify city-specific variations across modes and factors associated with levels of BC. Over 1500 min were collected, and regression model analyses were used to identify the impacts of mode and other factors. Among all measured transit modes, subway systems exhibited the highest BC concentrations (median: 10.83 μ g / m 3 ; MAD: ±6.97 μ g / m 3 ). In contrast, BRT systems showed the lowest exposure levels (3.7228 ± 2.27 μ g / m 3 ), with buses demonstrating comparable concentrations (4.00 ± 2.22 μ g / m 3 ). Notably, high concentrations in subways are primarily observed in segments where the subway operates in underground tunnels. Both surface transit modes measured slightly above ambient outdoor levels (2.82 ±1.66 μ g / m 3 ), suggesting minimal additional exposure during above-ground commute. • 42% of the observations in Montreal values above the accepted limit of 3.5 μ g m − 3 . • In Mexico, 60% of observations in BRTs and 84% in subways surpass the threshold. • In Puebla, 78% of BRT’s observations and 59% of buses surpass the 3.5 μ g m − 3 limit. • Excessive BC levels in subway trains are linked with tunnels along the route. • Engine technology plays a fundamental role in buses and BRTs.