Urban Contrasts in Traffic-Related PM2.5 Resolved Through High-Time-Resolution Analysis of Exhaust and Non-Exhaust Emissions.

This study characterized the spatial and temporal variability of traffic-related fine particulate matter (PM 2.5 ) in two Canadian urban environments: a downtown near-road Toronto site and a heavy-traffic corridor Vancouver site. Using high-time-resolution chemical speciation measurements and a two-step Positive Matrix Factorization (PMF) approach, the analysis resolved distinct primary and secondary sources. The constrained PMF analysis effectively separated traffic emissions into exhaust and multiple non-exhaust components (e.g., road dust, brake wear, Zn-rich factor representing upper-bound estimates of tire wear), revealing strong diurnal and weekday/weekend patterns and location-dependent variations in source contributions. Exhaust emissions were closely aligned with ambient pollutants (e.g., nitrogen oxides and ultrafine particles), while non-exhaust emissions showed stronger sensitivity to meteorological conditions. The two sites exhibited markedly different traffic-related PM 2.5 regimes, shaped by local fleet composition and roadway activity: the Vancouver site was influenced predominantly by exhaust emissions from heavy-duty diesel transport, whereas the Toronto site showed greater contributions from mechanically generated non-exhaust sources such as road dust and brake wear. This disparity between combustion-derived and mechanically generated emissions demonstrates that traffic-related PM 2.5 is highly location-dependent and governed primarily by fleet composition, roadway function, and local meteorology. Specifically, these results highlight that while the chemical markers used to identify traffic emissions remain consistent, the relative contributions of exhaust and non-exhaust sources vary systematically across urban environments. This underscores the need for site-specific air quality management that integrates diesel exhaust controls with non-exhaust emissions reduction measures to achieve meaningful reductions in urban traffic-related PM 2.5 exposure.