Factors affecting variability in PM2.5 exposure concentrations in a metro system

Highlights: • Door-side PM_2.5 concentrations typically increase when train doors open. • Train in-cabin PM_2.5 concentrations are lower than ambient PM_2.5 concentration. • Underground metro train in-cabin PM_2.5 concentrations are higher than above-ground. • Ambient PM_2.5 infiltration into cabins is higher for above-ground than underground. • Non-ambient sources contributed around half of PM_2.5 inside the underground metro. The objectives of this study were to: (1) evaluate PM_2.5 inflow to metro train cabins when doors open at stations; (2) assess the spatial and temporal variability in PM_2.5 exposure concentration; and (3) quantify the relationship between in-cabin concentration versus outdoor and non-ambient PM_2.5. We measured in-cabin PM_2.5 concentrations using portable monitors at the door-side and center of a train cabin simultaneously on a Hong Kong metro line. In addition, platform and in-cabin pollutant concentrations near a train door were simultaneously measured. Short-term spikes in PM_2.5 concentrations typically occur near train doors when doors open, related to inflow of ambient air aboveground and tunnel air underground. In-cabin PM_2.5 exposure concentrations are typically lower away from the doors when the doors open. PM_2.5 concentrations inside train cabins and on station platform operating above-ground are more influenced, compared to underground, by outdoor PM_2.5. Moreover, non-ambient sources contribute approximately 50% of train in-cabin and station platform PM_2.5 concentrations during underground operation. The results help more accurately quantify commuting PM_2.5 exposure on a metro system, and can be used to improve population-based exposure simulation models.