Characteristics of PM₂.₅, CO₂ and particle-number concentration in mass transit railway carriages in Hong Kong

Fine particulate matter (PM₂.₅) levels, carbon dioxide (CO₂) levels and particle-number concentrations (PNC) were monitored in train carriages on seven routes of the mass transit railway in Hong Kong between March and May 2014, using real-time monitoring instruments. The 8-h average PM₂.₅ levels in carriages on the seven routes ranged from 24.1 to 49.8 µg/m³, higher than levels in Finland and similar to those in New York, and in most cases exceeding the standard set by the World Health Organisation (25 µg/m³). The CO₂ concentration ranged from 714 to 1801 ppm on four of the routes, generally exceeding indoor air quality guidelines (1000 ppm over 8 h) and reaching levels as high as those in Beijing. PNC ranged from 1506 to 11,570 particles/cm3, lower than readings in Sydney and higher than readings in Taipei. Correlation analysis indicated that the number of passengers in a given carriage did not affect the PM₂.₅ concentration or PNC in the carriage. However, a significant positive correlation (p < 0.001, R² = 0.834) was observed between passenger numbers and CO₂ levels, with each passenger contributing approximately 7.7–9.8 ppm of CO₂. The real-time measurements of PM₂.₅ and PNC varied considerably, rising when carriage doors opened on arrival at a station and when passengers inside the carriage were more active. This suggests that air pollutants outside the train and passenger movements may contribute to PM₂.₅ levels and PNC. Assessment of the risk associated with PM₂.₅ exposure revealed that children are most severely affected by PM₂.₅ pollution, followed in order by juveniles, adults and the elderly. In addition, females were found to be more vulnerable to PM₂.₅ pollution than males (p < 0.001), and different subway lines were associated with different levels of risk. Copyright © 2016 Springer Science+Business Media Dordrecht.