A field study of CO2 and particulate matter characteristics during the transition season in the subway system in Tianjin, China

The daily passenger flow and ride times for subway systems are increasing in many large cities. Understanding the characteristics of carbon dioxide (CO2) and particulate matter (PM) in subway stations and train cars is essential in reducing passenger exposure to these pollutants. This study conducted field sampling on two subway lines in Tianjin, China (an old line, L1, and a new line, L6), during the transition season for a period of three weeks. During the first week, the heating, ventilation, and air conditioning (HVAC) systems in subway cars were turned off, while during the following two weeks, the systems were turned on. Measurements were conducted in subway cars, in stations (6 typical above-ground and underground stations), and in the ambient environment simultaneously. The CO2 concentrations in the cars were linearly related to passenger density (p