In-cabin PM2.5 and black carbon exposure in Mumbai, India: Intermodal differences and impact of traffic conditions

Abstract

In-cabin air pollution varies across transportation modes, yet the influence of evolving urban traffic characteristics (such as average speeds, peak hours, and congestion intensity) on commuter exposure remains understudied. Increasing traffic volumes and shifting travel behaviours have altered these mobility parameters, particularly in rapidly growing cities. This study quantifies fine particulate matter (PM₂₅) and black carbon (BC) concentrations across seven common commute modes—auto-rickshaws, buses (with and without air conditioning), passenger cars, metro rail, suburban trains, and motorbikes—in a major Indian megacity. Data were collected over 194 fixed-route trips (96 in winter, 98 in summer), capturing seasonal variation and a wide range of traffic conditions. Metro rail exhibited the lowest mean PM₂.₅ (54 ± 10 μg/m³) and BC (8 ± 3 μg/m³) levels, while auto-rickshaws showed the highest BC concentrations (52 ± 11 μg/m³), indicating substantial inter-modal differences in pollutant infiltration. PM₂.₅ levels declined with increasing vehicle speed, whereas both speed and ambient traffic density influenced BC concentrations. Congestion significantly exacerbated in-cabin exposure: a ∼20 % increase in bus trip duration was associated with a 1.6–2.4-fold rise in PM₂.₅ and a 1.2–1.8-fold increase in BC levels. These findings underscore the role of traffic conditions in modulating commuter exposure and highlight the need for targeted interventions to mitigate in-transit air pollution in urban environments.