Particulate Matters Levels in Subway Tunnels and Cabins

This research aims to discern the characteristics of PM10 concentrations measured in subway cabins, tunnels and outdoors at the Seoul Metropolitan subways. PM10 concentrations in cabins were found between the range of 45.3 and 101.9μg/m 3 , and in tunnels the ranges were between 155.3 and 230.6μg/m 3 . PM10 concentrations in cabins and tunnels during rush hours were 16.5% and 3.8% respectively higher than non-rush hours. Fine particles were higher than that of coarse particles both in subway cabins and tunnels.


Introduction
From the time its first line began operations in 1974, the Seoul Metropolitan Subway has been expanding to connect the entire city area. In 2011 a total of 2.4 billion people had used the subway from lines 1 to 8, with about 6.66 million. The subway takes up a 35.2% share of total transportation usage. As additional subway lines are planned for other metropolitan cities, the number of passengers using subways nationwide is expected to grow significantly (Seoul, 2009).
Air quality in subway cabins is comparatively worse than in the indoor spaces of other public buildings at ground level. Therefore, passenger health becomes an increasing concern as they are exposed to such pollutants over a prolonged period of time (Kim et al., 2006). PM 10 is a major pollutant in subway stations that is known to be generated from various factors such as subway train operation, passenger movement, and the inflow of outside air through the ventilation system. Epidemiological studies show that exposure to PM 10 is highly hazardous to the body, since it rises the sensible pollution level and along with it is the possible occurrence of disease and death rates. As PM 10 concentrations measured in subways are generally higher and more toxic than those in the air (Nieuwenhuijsen et al., 2007), they play a major role in deteriorating subway air quality and as such require proper control.
In this study, we measured the PM 10 concentrations of the Seoul Metropolitan subway lines from 5 to 8 using the light scattering method in order to observe PM 10 characteristics in subway cabins and tunnels. We also compared the PM 10 concentrations of outside air with those inside the cabins and tunnels, and analyzed the significance.

Research Methods
We used the light scattering method for this study as it is easy to control and enables continuous measurement. In this research, we measured the PM 10 concentration with Sky-OPC (Grimm 1.108). For measurement, the dust spectrometer divides PM 10 in the air into 32 channels by size at about a 1.2ℓ/min flow rate. The device can store data at 6-second intervals.
The measurement was conducted outdoors, in the cabins and tunnels of the Seoul Metropolitan subway lines 5 to 8 during rush and non-rush hours. Details of the measured lines are described in table 1 (Seoul Metropolitan Rapid Transit Corporation, 2011). Line 5 has a 52.3 km in length and transported 296.5 million passenger traffic.
The measurement was carried out at the center of the subway cabins to minimize the influence of outside air. Measurement in the tunnels was conducted by putting tubes out through the windows to suction the ambient air. The sampling places for each individual measurement are shown in figure 1. SPSS ver. 13.0 was used to analyze the data.   Table 2 shows the results of PM 10 concentrations in cabins of lines 5 to 8. The average PM 10 concentration was higher during the rush hours than non-rush hours in all lines. During the rush hours, each line showed 115.8µg/m 3 , 83.1µg/m 3 , 90.7µg/m 3 , and 66.0µg/m 3 for lines 5, 6, 7 and 8, respectively. Line 5 showed the highest level and line 8 as the lowest. Most of the results were lower than 150µg/m 3 , which is the level of guidelines for public transportations in Korea with the exemptions of lines 5 and 7. In terms of hourly measurement results by line, PM 10 concentrations during rush hours were 22.2%, 35.3%, 12.4%, and 23.1%, higher than during non-rush hours, for lines 5, 6, 7 and 8, respectively. This is probably because particles carried in by the number of passengers and suspended particles by the operation of trains during rush hours affect the PM 10 concentration in cabins. Brains et al. (2005) and Fromme et al. (2007) reported that the indoor PM 10 concentration was greatly affected by the activities of the people in a room. In a passenger cabin, there is an increase in passenger activity when the passengers enter and exit the cabin. However, quantitative measures to observe passenger activities inside a cabin were not available for this study.   Tables 6 to 9 show the results of PM 10 concentration measurement in tunnels on lines 5 to 8. PM 10 concentrations were higher during the rush hours than non-rush hours for all lines. During rush hours, 355.6µg/m 3 , 206.9µg/m 3 , 242.7µg/m 3 , and 148.6µg/m 3 were for lines 5, 6, 7 and 8 respectively, with line 5 having the highest level and line 8 got the lowest. In terms of hourly measurement results by line, PM 10 concentrations during rush hours were 81.6%, 11.5%, 22.8%, and 13.9% higher than during non-rush hours for lines 5, 6, 7 and 8, respectively.  Figure 2 shows PM 10 concentrations in subway cabins and tunnels of lines 5 to 8. For all subway lines, the PM 10 concentrations were higher in tunnels than in cabins and it also shows that is higher during rush hours than non-rush hours. The concentration in cabins of line 5, which shows the highest concentration in tunnels, was also high in comparing the cabins between the other lines, while line 8, the lowest concentration in tunnels, was also low in cabins. This is probably because the PM 10 concentration in subway cabins affects PM 10 concentration in tunnels. An outdoor PM10 concentration was measured for three places, at the same time it was measured in subway cabins and tunnels.

Correlations of PM 10 Concentrations
We selected station G of line 5 to measure PM 10 concentration in tunnels, subways and outdoors, simultaneously. Though Station G is not a transfer station, it is widely used at ordinary times.  Figure 3 shows the concentration distribution of PM 10 for Station G. The concentrations of fine particles are higher than that of coarse particles in the three places (outdoors, subway tunnels and cabins). Figure 4 shows the concentration distribution of PM 10 by size at the three places. In the case of outdoors, the concentration is highest when the particle sizes are between 0.35 µm and 4.5 µm. For tunnels, the concentrations are at its highest level when the particle size is between 0.25 µm and 0.35 µm, and at 1.6µm. Subway cabins show peak concentration at 0.35, 2.5 and 6.5 µm, demonstrating that PM 10 concentrations are affected by outside air and tunnels.
According to the PMF study for subway cabins and tunnels the PM 10 consisted of 52.5% inorganic components, 10.2% anions, and 37.3% other materials including cations, organic and inorganic carbons, and other unmeasured components. The iron content was the highest, generally accounting for 10-70% of the PM 10 concentration. The fours sources identified by the PMF model were soil and road dust, railroad-related pollution sources, secondary nitrate, and Clfactor mixed with secondary sulfates and the sources accounted for 27.2%, 47.6%, 16.2%, and 9.1% of the PM 10 , respectively (Park et al., 2014;Park et al., 2012). Thus, it is required to install ventilation facilities to block the PM 10 brought in from tunnels and reduce the PM 10 carried in by the passengers.

Conclusion
In this study, we measured the PM concentrations of the Seoul metropolitan subways. Sky-OPC was used in order to characterize PM concentrations in subway cabins and tunnels. We also compared the PM concentrations of outside air with those in cabins, tunnels, and analyzed the significance.
The average PM concentration in cabins of lines 5 to 8 is significantly higher during rush hours than non-rush hours among all the lines used. In addition, the average PM concentrations in tunnels of lines 5 to 8 are also relatively higher during rush hours than non-rush hours for all lines.
Outdoor PM 10 concentration was measured for three points at the same time it was measured in cabins and tunnels. The concentrations of fine particles are higher than that of coarse particles in three places (outdoors, tunnels and cabins). Subway cabins show the peak concentrations at 0.35, 2.5 and 6.5µm, demonstrating that PM concentrations are affected by outside air and tunnels. Thus, as it has been mentioned, it is essential to install ventilation facilities to block the PM 10 brought in from tunnels and reduce the PM carried in by passengers.