Relative contributions of ambient air and internal sources to multiple air pollutants in public transportation modes

Commuters are often exposed to relatively high air pollutant concentrations in public transport microenvironments (TMEs) because of their proximity to emission sources. Previous studies have mainly focused on assessing the concentrations of air pollutants in TMEs, but few studies have distinguished between the contributions of ambient air and internal sources to the exposure of commuters to air pollutants. The main objective of this study was to quantify the contributions of ambient air and internal sources to the measured particulate matter and gaseous pollutant concentrations in selected TMEs in Hong Kong, a high-rise, high-density city in Asia. A sampling campaign was conducted to measure air pollutant concentrations in TMEs in Hong Kong in July and November 2018 using portable air quality monitors. We measured the concentrations of each pollutant in different TMEs and quantified the infiltration of particulate matter into these TMEs. The double-decker bus had the lowest particulate matter concentrations (mean PM₁, PM_2.5, and PM₁₀ concentrations of 5.1, 9.5, and 13 μg/m³, respectively), but higher concentrations of CO (0.9 ppm), NO (422 ppb), and NO₂ (100 ppb). For all the TMEs, about half of the PM_2.5 were PM₁ particles. The Mass Transit Railway (MTR) subway system had a PM_2.5/PM₁₀ ratio of about 0.90, whereas the PM_2.5/PM₁₀ ratio was about 0.60-0.70 for the other TMEs. The MTR had infiltration factor estimates <0.4 for particulate matter, lower than those of the double-decker bus and minibus. The MTR had the highest contribution from internal sources (mean PM₁, PM_2.5, and PM₁₀ concentrations of 4.6, 13.4, and 15.8 μg/m³, respectively). This study will help citizens to plan commuting routes to reduce their exposure to air pollution and help policy-makers to prioritize effective exposure reduction strategies.