- Email: [email protected]
Estimation of inhaled airborne particle number concentration by subway users in Seoul, Korea
Environmental Pollution 231 (2017) 663e670
Contents lists available at ScienceDirect
Environmental Pollution journal homepage: www.elsevier.com/locate/envpol
Estimation of inhaled airborne particle number concentration by subway users in Seoul, Korea* Minhae Kim a, b, Sechan Park a, b, Hyeong-Gyu Namgung b, Soon-Bark Kwon a, b, * a b
Railway System Engineering, University of Science and Technology (UST), Uiwang-si 16105, South Korea Transportation Environmental Research Team, Korea Railroad Research Institute (KRRI), Uiwang-si 16105, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 March 2017 Received in revised form 29 June 2017 Accepted 19 August 2017
Exposure to airborne particulate matter (PM) causes several diseases in the human body. The smaller particles, which have relatively large surface areas, are actually more harmful to the human body since they can penetrate deeper parts of the lungs or become secondary pollutants by bonding with other atmospheric pollutants, such as nitrogen oxides. The purpose of this study is to present the number of PM inhaled by subway users as a possible reference material for any analysis of the hazards to the human body arising from the inhalation of such PM. Two transfer stations in Seoul, Korea, which have the greatest number of users, were selected for this study. For 0.3e0.422 mm PM, particle number concentration (PNC) was highest outdoors but decreased as the tester moved deeper underground. On the other hand, the PNC between 1 and 10 mm increased as the tester moved deeper underground and showed a high number concentration inside the subway train as well. An analysis of the particles to which subway users are actually exposed to (inhaled particle number), using particle concentration at each measurement location, the average inhalation rate of an adult, and the average stay time at each location, all showed that particles sized 0.01e0.422 mm are mostly inhaled from the outdoor air whereas particles sized 1e10 mm are inhaled as the passengers move deeper underground. Based on these findings, we expect that the inhaled particle number of subway users can be used as reference data for an evaluation of the hazards to health caused by PM inhalation. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Particulate matter (PM) Particle size Particle number concentration (PNC) Subway user Inhaled particle number
1. Introduction With the ever increasing number of city commuters, concerns over the air quality on modes of transport has become an important issue and many studies have analyzed the exposure level of commuters to airborne particles (Cheng et al., 2009; Kingham et al., 2013; Knibbs et al., 2011; Kumar and Gupta, 2016; Ramos et al., 2015; Wang and Gao, 2011; Xu et al., 2016; Yan et al., 2015). Since subway systems carry large numbers of passengers in small, enclosed spaces compared to other modes of transportation, there is the potential for exposure to indoor pollution if the air conditioning and ventilation system of the facilities and vehicles are not effectively managed.
*
This paper has been recommended for acceptance by David Carpenter. * Corresponding author. Transportation Environmental Research Team, Korea Railroad Research Institute (KRRI), Uiwang-si 16105, South Korea; Railway System Engineering, University of Science and Technology (UST), Uiwang-si 16105, South Korea. E-mail address: [email protected] (S.-B. Kwon). http://dx.doi.org/10.1016/j.envpol.2017.08.077 0269-7491/© 2017 Elsevier Ltd. All rights reserved.
Ozgen et al. (2016) reported that the number concentration of ultrafine particles (UFP: dp < 0.1 mm) outdoors was around two times higher than that within the subway system and that the mass concentration of PM within the subway system was two to four times higher than it was outdoors. Wang and Gao (2011) measured the number concentration of particles (5 nme3 mm) and the mass concentration of PM of less than 2.5 mm (PM2.5) in New York subway stations. They reported that while the number concentration of particles outdoors was higher (at 60,629 #/cm3) than on the platform (at 37,657 #/cm3) the PM2.5 mass concentration on the platform was higher (at 68.29 mg/m3) than it was outdoors (at 29.60 mg/m3). Lonati et al. (2011) measured the mass concentration of PM in the size ranges of 0.3e0.5 mm and 2.5e10 mm and found that the mass concentrations of 0.3e0.5 mm PM outdoors and on the platform were similar while the mass concentration of 2.5e10 mm PM in the subway was much higher than it was outdoors. A study that compared the mass concentrations of PM10 and PM2.5 at different passenger locations in a subway station in Seoul, Korea, showed that the PM10 concentrations were 155 mg/m3 outdoors, 312 mg/m3 on the concourse and 359 mg/m3 on the platform,
664
M. Kim et al. / Environmental Pollution 231 (2017) 663e670
while the PM2.5 concentrations were 102 mg/m3, 126 mg/m3, and 129 mg/m3 for each location, respectively. The concentration of larger particles was more than 3 times higher within the subway station than it was outdoors, while the difference was significantly lower for smaller particles (Kim et al., 2008). As reported in the preceding studies, the difference in PM concentration as measured outdoors and within the subway stations varied by PM size and that the specific particles which people were exposed to differed by location. As such, many recent studies have now focused on the actual inhalation dose of mg per individual instead of simply examining concentration level. Ramos et al. (2015) compared the mass concentrations of PM10 and PM2.5 in various public transit systems and presented PM inhalation dose/ km using the average inhalation rate of adults, moving distance, and stay time. Yu et al. (2012) calculated the PM1 inhalation dose using the stay time in public transit system, PM1 mass concentration and inhalation rate. Lei et al. (2016) measured the PM2.5 mass concentration in indoor spaces, outdoors and on public transit systems, and presented the exposed PM inhalation dose per person at each location for a period of one day. Summarizing these studies, a person using the subway is exposed to a greater extent to smaller PM outdoors and to larger PM when in underground subway stations. Therefore, mass concentration is commonly used as a reference value for a person's exposure to PM and many governments also use mass concentration as a reference for the regulation of PM in the atmosphere and indoor environments. However, this approach has its short comings. This is because smaller particles contribute little to the findings when converted to total mass concentration but these particles are actually more harmful to the human body since they can become secondary pollutants by bonding with other atmospheric pollutants, such as nitrogen oxides (NOx), which have relatively large surface areas (Nel et al., 2006). Moreover, while larger particles can be filtered out of inhaled air by nasal mucosa and the upper airways, particles of 1 mm or less have been found all the way down to the bronchioles and alveoli (Londahl et al., 2006; Oberdorster et al., 2005). To reflect this phenomenon, Koehler and Peters (2015) proposed an analysis using number concentration instead of mass concentration to better reflect the concentration of UFP when evaluating the relationship between PM and health. Furthermore, Penttinen et al. (2001) argued for the consideration of number concentration at the same time as mass concentration when evaluating indoor air quality. This study measured the number concentration of PM, according to particle size, to which subway users are exposed to whilst traversing along actual movement paths. Subways were chosen as they have the largest transport share - at 39% compared to 27% for
buses and 22.8% for passenger cars - among means of transportation in Seoul (Statistics of Seoul, 2014). This study will present the number of PM inhaled by subway users as reference material for analysis of the hazards to the human body arising from the inhalation of PM. 2. Methods Two transfer stations in Seoul that handle the largest daily number of passengers were selected in order to measure PNC according to particle size and to observe subway users' exposure to PM. Two measurements were conducted on Line 1 in the afternoons of October 7e9, 2015 and on Line 2 in the afternoons of December 14e15, 2015. The measuring devices used in this study were a Nanoscan SMPS (TSI, Model 3910, USA) and an optical particle sizer (OPS; TSI, Model 3330, USA). The Nanoscan SMPS can measure particles sized between 0.01 and 0.420 mm while the OPS can measure particles sized between 0.3 and 10 mm. For the measurement of PM number concentration with the Nanoscan SMPS, particles were grouped into 13 sizes of 11.5, 15.4, 20.5, 27.4, 36.5, 48.7, 64.9, 86.6, 115.5, 154, 205.4, 273.8 and 365.2 nm. Since the Nanoscan SMPS requires around 1 min of time to measure UFP particles, it was fixed at three locations: outdoors (A, near the access to the station); the concourse (B, pathway between the outdoors and the platform); and the platform (C, passenger waiting area for the train). The sampling flow rate of Nanoscan SMPS was 0.75 L/min, and three measurements were made repeatedly at intervals of 60 s. For measurement with the OPS, a tester put the OPS on a backpack and took measurements whilst moving. Particles were grouped into 14 sizes of 0.3, 0.316, 0.422, 0.562, 0.75, 1, 1.334, 1.778, 2.371, 3,162, 4.217, 5.623, 7.499 and 10 mm. A conductive tube connected to the sampling inlet of the OPS measuring device minimized sampling loss and measurements were taken at 1.2 m from the floor. The sampling flow rate of the OPS was 1.2 L/min and measurements were taken at 5-s intervals during a period of 30e40 min as the tester moved around. To analyze the distribution of PM inhaled by subway users, measurements taken were at four locations: outdoors (A); on the concourse (B); on the platform (C); and whilst getting on the train, as shown in Fig. 1. The continuous measurement of PM took place for approximately 40 min and included both the stated measurement points as well as the time spent waiting for the subway train, transferring lines and ascending/descending the stairs. The PM10 concentration in the outdoor air during the measurement period refers to that provided by the Korea Environment Corporation (KECO; www. airkorea.or.kr) while the outdoor temperature and relative humidity refer to that provided by the Korea Metrological
Fig. 1. Moving path of subway users as reproduced in this study.
M. Kim et al. / Environmental Pollution 231 (2017) 663e670
665
Table 1 Outdoor temperature, relative humidity and PM10 on the days of measurements. Location
Date
Time
Temperature ( C)
Relative Humidity (%)
PM10 (mg/m3)
Route 1
Oct. 7, 2015
1:00 pm 2:00 pm 3:00 pm 1:00 pm 2:00 pm 3:00 pm Average 1:00 pm 2:00 pm 3:00 pm 1:00 pm 2:00 pm 3:00 pm Average
25.3 25.1 24.4 25.6 25.9 26.1 25.4 6.1 5.9 5.7 7.5 8.1 6.6 6.7
43 50 50 34 28 34 39.8 79 85 87 54 51 55 68.5
57 67 64 47 58 45 56.3 40 37 42 59 57 77 52.0
Oct. 8, 2015
Route 2
Dec. 14, 2015
Dec. 15, 2015
Administration (KMA; http://web.kma.go.kr/eng) as shown in Table 1. Route 1 is the path taken by users of Subway Line 1, while Route 2 is the path taken by the users of Subway Line 2. During the afternoons in the fall when the PM on Route 1 was measured, the average outdoor temperature was 25.4 C while humidity was 39.8%. During the afternoons in the winter when the PM on Route 2 was measured, the average outdoor temperature was 6.7 C while humidity was 68.5%, indicating some seasonal difference. However, the PM concentrations in the outdoor air was similar, with 56.3 mg/ m3 in the fall and 52 mg/m3 in the winter. 3. Results & discussion 3.1. Characteristics of particle size distribution Fig. 2 shows the size distribution of particles between 0.01 mm and 10 mm, as measured with the OPS and Nanoscan SMPS at each measurement location. As can be seen in the figure, the solid red line, dashed green line and long-dashed blue line refer to the PM size distribution in the outdoor air, on the concourse and on the platform, respectively. Although the PM size distributions at each site were similar, particles sized 0.15 mm or less showed a difference in concentration between the three locations. Particles sized approximately between 0.15 mm and 0.7 mm showed a high
concentration in the outdoor air and a similar or lower concentration on the concourse and platform. Particles sized around 0.7 mm or larger showed their highest concentration on the platform, followed by outdoors and on the concourse. For a more specific analysis of size distribution of smaller particles, those particles sized 0.3 mm or less (as measured with the Nanoscan SMPS) are shown on a linear-scale y-axis graph in Fig. 2. The graph shows a peak particle size at around 0.04 mm in the outdoor air and on the concourse. On the other hand, the number concentrations of particles between 0.04 and 0.15 mm on the platform were almost constant compared to those in the outdoor air and on the concourse. Given that the peak of 0.04 mm coincides with the peak value of PM size distributions occurring during the combustion of diesel (Kittelson et al., 1998) and the peak value of the size distribution of particles on the roadside (AQEG, 2005), the outdoor PM measured in this study can be attributed mostly to on-road emissions. The linear graph of number concentration of particles sized between 0.3 mm and 1 mm (as measured with the OPS) shows a peak value at around 0.3 mm in the outdoor air, on the concourse and the platform. The size distributions at the three locations were similar, showing the highest concentration outdoors, followed by on the concourse and the platform. However, the concentration of PMs between 0.8 mm and 3 mm was higher on the platform than it was outdoors.
Fig. 2. PM size distribution in the outdoor air, concourse and platform in log and linear scale.
666
M. Kim et al. / Environmental Pollution 231 (2017) 663e670
3.2. Change in PM number concentration along the movement path Fig. 3 shows the change in number concentration of particles sized 0.3e10 mm along the path taken by subway users on Route 1. The size ranges measured with the OPS were separated into three groups of 0.3e0.422 mm, 0.422e1 mm and 1e10 mm showing similar changes, and the aggregated concentration of each group is presented along the path taken by subway users. A, B, C and D in the graph represent the outdoors, on the concourse, on the platform and on the train, respectively. The figures measured on the transfer path taken by subways users in another subway station (Transfer in Fig. 3) was excluded from the analysis. For PM sized 0.3e0.422 mm (Fig. 3(a)), the number
concentration was highest outdoors (A) and decreased at the measurement positions located deeper underground (B/C/D). Likewise, the number concentration increased the closer the measurement location was to the exit from the platform. The changes to number concentration of particles sized 0.422e1 mm, as shown in Fig. 3(b), show a somewhat different trend from particles of between 0.3 and 0.422 mm in size, but a similar trend to particles between 1 and 10 mm in size. The changes to number concentration of particles sized 1e10 mm, as shown in Fig. 3(c), show a different trend than those in Fig. 3(a) and thus indicate a considerably higher concentration on the platform and a lower concentration outdoors. Table 2 shows the average value, standard deviation, maximum value, and minimum value to analyze quantitatively the number concentration value at each location. The average number concentration of particles sized 0.3e0.422 mm along Route 1 was the highest outdoors at 395.4 #/cm3. The number concentrations on the concourse, on the platform and in the train were 311.4, 268.5 and 220.4 #/cm3, respectively. On the other hand, the number concentrations of particles sized 1e10 mm on the concourse were 2.85 #/cm3, 4.48 #/cm3 on the platform and 3.75 #/cm3on the train, indicating that the number concentration was lower on the concourse than on the platform and inside the train. The number concentration measured outdoors was 4.08 #/cm3, the second highest recording next to that of the platform. The findings for all measurements taken on Route 2 were similar. The number concentration of particles sized 0.3e0.422 mm was the highest 479.5 #/cm3 outdoors. The number concentrations on the concourse, on the platform and inside the train were 283.5, 322.6 and 464.4 #/cm3, respectively, indicating a lower concentration on the concourse than at the other two locations. For larger particles between 1 and 10 mm, the number concentration measured outdoors was 3.71 #/cm3, while the number concentrations on the concourse and the platform were 3.08 #/cm3 and 7.35 #/cm3, respectively, indicating that the concentration on the platform was twice that of the concourse. The number concentration in the train was 16.1 #/cm3 or about four times that found outdoors. The difference in number concentration outdoors and in the train was much smaller for smaller sized particles (0.3e0.422 mm) than for larger sized particles (1e10 mm). The studies by Wang and Gao (2011) and Ozgen et al. (2016), as mentioned earlier, which reported that the number concentration was higher in the outdoor air for smaller particles and higher on the platform for larger particles, are consistent with the findings of this study. According to a study of PNC on a subway platform in Prague, Czech Republic, the number concentration of smaller particles of around 0.6 mm or less in size was barely affected by the environment of either the platform or the train, as it was found to be most affected by PM concentration in the outdoor air (Cusack et al., 2015). 3.3. Average PM number concentration at each location
Fig. 3. Particle number concentration distribution along movement path for Route 1 for particle size ranges of (a) 0.3e0.422 mm (b) 0.422e1 mm and (c) 1e10 mm, with A referring to the outdoor air, B to the concourse, C to the platform area, and D to inside the train. “Transfer” refers to the transfer path.
Fig. 4 shows the average PM number concentrations (including results of both Nano Scan and OPS data) measured on Route 1 and Route 2, separated into four size groups of (a) 0.3 mm or less, (b) 0.3e0.422 mm, (c) 0.422e1 mm and (d) 1e10 mm. The shaded boxes show the maximum value, the minimum value and the deviation. The solid lines refer to the median values while the dotted lines refer to the average values. The number concentration of particles sized 0.3 mm or less was at its highest outdoors, at 34,532 #/cm3, followed by on the concourse, at 24,117 #/cm3, and finally the platform, at 19,643 #/cm3. The number concentration of particles sized 0.3 mm or less outdoors was about twice as high as that measured on the platform. The average size of PM was 51.4 nm outdoors and increased the deeper underground the tester travelled to 56.6 nm at the bottom of the stairs, 57.1 nm on the
M. Kim et al. / Environmental Pollution 231 (2017) 663e670
667
Table 2 Averaged particle number concentration (PNC), standard deviation, maximum and minimum values at each location on Route 1 and Route 2. Averaged Particle Number Concentration (#/cm3)
0.3e0.422 mm (A) Outdoor
(B) Concourse
(C) Platform
(D) Inside Train
(A) Outdoor
(B) Concourse
(C) Platform
(D) Inside Train
Route 1
395.4 6.86 402.9 374.2 479.5 13.5 500.9 435.8
311.4 50.7 404.8 238.5 283.5 48.3 529.0 245.7
268.5 15.8 302.4 247.0 322.6 26.5 384.8 265.6
220.4 14.1 253.5 196.0 464.4 20.8 506.6 345.1
4.08 0.34 4.49 3.23 3.71 0.59 4.77 2.52
2.85 0.69 5.82 1.46 3.08 1.23 9.79 1.18
4.48 1.69 9.50 1.85 7.35 2.31 12.7 3.24
3.75 0.54 5.71 2.87 16.1 3.41 26.4 9.25
Route 2
Mean S.D. Max Min Mean S.D. Max Min
1-10 mm
Fig. 4. PM number concentrations measured along Route 1 and Route 2: (a) <0.3 mm (b) 0.3e0.422 mm (c) 0.422e1 mm (d) 1e10 mm.
concourse, and 62.6 nm on the platform. Although particles sized 0.3e0.422 mm showed similar average number concentrations on the concourse and the platform, the concentration was higher outdoors (Fig. 4(b)). The average number concentration of particles sized 0.422e1 mm was in the range of 20e30 #/cm3 at all measurement locations, with the number concentration on the concourse the lowest (Fig. 4(c)). The number concentration of particles sized 1e10 mm was highest on the platform while measurement of the outdoor air and air on the concourse both showed similar number concentrations. Table 3 shows the coefficient of variation (CV) that represents the ratio of standard deviation to average value for each particle size group at each location. The CV of particles 0.3 mm or less was highest at 33.57% outdoors, which can presumably be attributed to the effect of a large variety of road
Table 3 Coefficient of variations for PM number concentration at each location for each size group. Coefficient of variation (CV) in PM number concentration (%)
(A) Outdoors (B) Concourse (C) Platform
<0.3 mm
0.3e0.422 mm
0.422e1 mm
1-10 mm
33.57 13.20 5.07
9.58 17.41 10.83
18.63 19.93 26.46
13.94 28.54 36.12
traffic (road pollution source) since the subway entrance is located on the side of the road (Cheng et al., 2011). The CV of particles sized 1e10 mm was highest on the platform, at 37.62%. The larger particles show the greatest variation on the platform because when the
668
M. Kim et al. / Environmental Pollution 231 (2017) 663e670
platform screen doors (PSDs) installed on the subway platform open frequently, large sized particles scattered by the train draft blow onto the platform from the tunnel side (Kwon et al., 2016). 3.4. Exposure scenario and analysis of exposure to PM for subway users As mentioned in the Introduction, the analysis of inhaled doses of PM with consideration given to inhalation rate is very important in understanding the hazards to the health of subway users. It is also necessary to study how inhaled doses of PM differ according to their respiration volume which in turn differs according to user activity patterns (standing up, walking, running etc.). For example, one study reported that the inhalation dose of UFP by those with asthma was larger than that of healthy persons because of the different respiration volumes (Chalupa et al., 2004). This study used its own measurement results and the average respiration volume of adults according to activity patterns to analyze the exposure level, which is defined as the total number of particles inhaled by a subway user each minute. The following Equation (1) calculates the inhaled PM number per minute (IPN) using the measured particle number concentration (PNC) at each location and the inhalation rate (IR):
IPNðN=minÞ ¼ PNC N=cm3 IRðL=minÞ
(1)
The average inhalation rate of an adult (aged 21 to 30) announced by the United States' Environmental Protection Agency (EPA, 2011) is 12 L/min during light intensity activity such as standing up, and 26 L/min during moderate intensity activity such as walking. Assuming that most subway users walk in from the outside to the concourse and mostly stand at the platform and on the train, the appropriate different inhalation rates were used in our analysis of exposure level. Since retention time is an important factor for analysis of exposure level and differs according to location, the inhaled PM number per minute was calculated first as shown in Table 4. The average subway user inhales around 790 million particles sized 0.3 mm or less per minute at the entrance to the subway station, around 530 million particles per minute on the concourse, and around 190 million particles per minute at the platform. The average subway user inhales 11.37 million particles sized 0.3e0.422 mm per minute at the entrance to the subway station, 7.73 million particles per minute on the concourse, 3.35 million particles per minute at the platform and 4.1 million particles per minute in the train. The exposure level to particles sized 0.422 mm or less was highest outdoors, followed by on the concourse, the platform, and finally on the train. On the other hand, exposure level to particles sized 1e10 mm was highest on the train at 11,900 particles, followed by on the concourse, at 77,000 particles, and on the platform, at 71,000 particles. Existing studies which used mass concentration showed an inhalation dose of PM1 to be 28.6 ± 25.9 mg in the subway station and 45.2 ± 30.1 mg in the outdoor air, indicating a dose that is 1.6 times higher in the outdoor air. (Yu et al., 2012). Ramos et al. (2016) reported the inhalation
dose of subway users to be 2.29 mg of PM1, 2.57 mg of PM2.5, 2.94 mg of PM4 and 3.43 mg of PM10, indicating that inhalation dose increases with particle size. The previous analysis was for the number of PM exposed per minute for assumed activities at each location. Based on this we analyzed the total number of PM to which subway users are actually exposed to. Fig. 5 shows the percentage contribution of location (outdoors, on the concourse, on the platform and inside the train) to total PM inhaled based on a survey of retention time of commuters on the public transit system in Seoul (Kim et al., 2014). This survey reported that the average resident of the capital region spent 68 min commuting on the public transit system, which includes 16 min outdoors, 11 min on the concourse, 6 min on the platform, and 35 min inside the train. The rate of inhalation dose according to location clearly differs by particle size. For particles sized 0.3 mm or less, around 64% was inhaled outdoors, 30% on the concourse and 6% on the platform. For particles sized 0.3e0.422 mm, 42% was inhaled outdoors, 20% on the concourse, 5% on the platform and 33% inside the train. For particles sized 0.422e1 mm, 33% was inhaled outdoors, 15% on the concourse and 5% on the platform, indicating that the combined rate of inhalation outdoors and on the concourse was much smaller than the above two cases. On the other hand, the rate of inhalation in the train was 48%, indicating that almost half of the total PM was inhaled inside the train. For particles sized 1e10 mm, 59% of PM was inhaled in the train while only 23% was inhaled outdoors. For particles inhaled inside the train, 33% were between 0.3 mm and 0.422 mm, 48% were between 0.422 mm and 1 mm, and 59% were 1 mm or larger, indicating that the rate of inhalation increased with particle size. On the other hand, the rate of inhalation outdoors decreased as particle size increased, since 65% of the particles were 0.3 mm or less in size, 42% of particles were between 0.3 and 0.422 mm, and 23% were between 1 and 10 mm. The rates of inhalation on the platform were similar (5e6%) for all sizes, which can be attributed to a relatively short stay time.
4. Conclusion This study measured the number concentration of PM of different sizes on an actual path taken by most subway users in Seoul, Korea, to evaluate the PNC trend at each location. Moreover, we calculated the inhaled PM number in order to investigate the exposure of each user to particles at each location. The number concentration of particles between 10 nm and 0.7 mm, to which subway users would be exposed, was highest outdoors followed by on the concourse and platform. Particles were measured in 14 size ranges by the OPC and then separated into three groups of 1) 0.3e0.422 mm, 2) 0.422e1 mm and 3) 1e10 mm, according to distribution characteristics, and measured along an actual path of movement taken by the majority of subway users. Our analysis showed trends that are clearly differentiated according to particle size. For 0.3e0.422 mm PM, PNC was highest outdoors and
Table 4 Averaged particle number concentration (PNC) observed in this study, inhalation rate (IR) by walking or standing up and calculated inhaled particle number (IPN) at each location. PNC (#/cm3)
Point
(A) (B) (C) (D)
Outdoors Concourse Platform Inside Train
<0.3 mm
0.3e0.422 mm
0.422e1 mm
1-10 mm
30,939 20,377 15,792 e
437.5 297.4 295.6 342.4
31.1 19.6 27.3 44.7
3.9 2.97 5.92 9.92
IR (L/min)
IPN (103 #/min) <0.3 mm
0.3e0.422 mm
0.422e1 mm
1-10 mm
12 12 26 26
790,218 529,802 189,504 e
11,373.8 7733.3 3546.6 4109
809 508.2 327.2 535.9
101.3 77.1 71 119
M. Kim et al. / Environmental Pollution 231 (2017) 663e670
669
Fig. 5. Rate of inhalation dose of PM in four size groups: outdoors, concourse, platform and train (a) < 0.3 mm (b) 0.3e0.422 mm (c) 0.422e1 mm (d) 1e10 mm.
decreased as the tester moved deeper underground. On the other hand, the PNC between 1 and 10 mm increased as the tester moved deeper underground and showed a high number concentration inside the subway train as well. The intermediate-sized particles of 0.422e1 mm in diameter did not show much difference in PNC at different points along the path taken by subway users. Particles of 0.3 mm or higher, which had a high number concentration in the outdoor air, also showed their widest variation in measurement since these sized particles are greatly affected by the level of road traffic. At the platform, relatively large particles (1e10 mm) showed a larger variation in PNC since they irregularly blew into the platform area from the tunnel as the PSDs opened. An analysis of the particles to which subway users are actually exposed to - using particle concentration at each measurement location, the average respiration volume of an adult, and the average stay time at each location - showed that particles sized 0.01e0.422 mm are mostly inhaled in the outdoor air but less so as users move deeper underground. On the other hand, a greater numbers of particles sized between 1 mm and 10 mm are inhaled as the passengers move deeper underground. Based on these findings, it is expected that PNC and particles of various sizes inhaled at each location on path taken by subway users can be used in the future as reference data for the evaluation of the hazards to health from PM inhalation. Acknowledgement This research was supported by a grant from R&D Program of the Korea Railroad Research Institute (KRRI), Republic of Korea. References AQEG, 2005. Particulate Matter in the UK: Summary. Defra, London.
€rster, G., Utell, M.J., Frampton, M.W., 2004. UlChalupa, D.C., Morrow, P.E., Oberdo trafine particle deposition in subjects with asthma. Environ. Health Perspect. 112 (8), 879e882. Cheng, Y.H., Liu, C.C., Lin, Y.L., 2009. Levels of ultrafine particles in the Raipei Rapid transit system. Transp. Res. Part D. 14, 479e486. Cheng, Y., Zou, S.C., Lee, S.C., Ho, K.F., Watson, J.G., Han, Y.M., Zhang, R.J., Zhang, F., Yau, P.S., Huang, Y., Bai, Y., Wu, W.J., 2011. Characteristics and source apportionment of PM1 emissions at a roadside station. J. Hazard. Mater 195, 82e91. Cusack, M., Talbot, N., Ondracek, J., Minguillon, M.C., Martins, V., Klouda, K., Schwarz, J., Zdimal, V., 2015. Variability of aerosols and chemical composition of PM10, PM2.5 and PM1 on a platform of the Prague underground metro. Atmos. Environ. 118, 176e183. EPA, 2011. Exposure Factors Handbook 2011 Edition (Final). U.S. Environmental Protection Agency, Washington, DC. EPA/600/R-09/052F. KECO. Korea Environment Corporation. www.airkorea.or.kr. Kim, K.Y., Kim, Y.S., Roh, Y.M., Lee, C.M., Kim, C.N., 2008. Spatial distribution of particulate matter (PM10 and PM2.5) in Seoul Metropolitan Subway stations. J. Hazard. Mater 154, 440e443. Kim, S.K., Ko, J.H., Lee, S.H., Jang, J.E., Jung, K.S., 2014. Evaluation of the quality of public transport commuting in Seoul. Seoul Inst. Rep. 1e109. Kingham, S., Longley, L., Salmond, J., Pattinson, W., Shrestha, K., 2013. Variations in exposure to traffic pollution while travelling by different modes in a low density, less congested city. Environ. Pollut. 181, 211e218. Kittelson, D.B., Watts, W.F., Arnold, M., 1998. Review of Diesel Particulate Matter Sampling Methods: Supplemental Report# 2, Aerosol Dynamics, Laboratory and On-road Studies. University of Minnesota. KMA. Korea Metrological Administration. http://web.kma.go.kr/eng. Knibbs, L.D., Cole-Hunter, T., Morawska, L., 2011. A review of commuter exposure to ultrafine particles and its health effects. Atmos. Environ. 45, 2611e2622. Koehler, K.A., Peters, T.M., 2015. New methods for personal exposure monitoring for airborne particles. Curr. Environ. Health Rep. 2, 399e411. Kumar, P., Gupta, N.C., 2016. Commuter exposure to inhalable, thoracic and alveolic particles in various transportation modes in Delhi. Sci. Total Environ. 541, 535e541. Kwon, S.B., Namgung, H.G., Jeong, W., Park, D., Eom, J.K., 2016. Transient variation of aerosol size distribution in an underground subway station. Environ. Monit. Assess. 188 (6), 1e11. Lei, X., Xiu, G., Li, B., Zhang, K., Zhao, M., 2016. Individual exposure of graduate students to PM2.5 and black carbon in Shanghai, China. Environ. Sci. Pollut. Res. 23, 12120e12127. Lonati, G., Ozgen, S., Ripamonti, G., Cernuschi, S., Giugliano, M., 2011. Pedestrian exposure to size-resolved particles in milan. J. Air & Waste Manage. Assoc. 61,
670
M. Kim et al. / Environmental Pollution 231 (2017) 663e670
1273e1280. Londahl, J., Pagels, J., Swietlichi, E., Zhou, J., Ketzel, M., Massling, A., Bohgard, M., 2006. A set-up for field studies of respiratory tract deposition of fine and ultrafine particles in humans. J. Aerosol Sci. 37, 1152e1163. Nel, A., Xia, T., Madler, L., Li, N., 2006. Toxic potential of materials at the nanolevel. Science 311, 622e627. Oberdorster, G., Oberdorster, E., Oberdorster, J., 2005. Nanotoxicology: an emerging discipline evolving from studies of ultrafine particles. Environ. Health Perspect. 113 (7), 823e839. Ozgen, S., Ripamonti, G., Malandrini, A., Ragettli, M.S., Lonati, G., 2016. Particle number and mass exposure concentrations by commuter transport modes in Milan. Italy. Environ. Sci. 3 (2), 168e184. Penttinen, P., Timonen, K.L., Mirme, A., Ruuskanen, J., Pekkanen, J., 2001. Ultrafine particles in urban air and respiratory health among adult asthmatics. Eur. Respir. J. 17, 428e435. Ramos, C.A., Wolterbeek, H.T., Almeida, S.M., 2016. Air pollutant exposure and
inhalation dose during urban commuting: a comparison between cycling and motorized modes. Air Qual. Atmos. hlth. 1e13. Ramos, M.J., Vasconcelos, A., Faria, M., 2015. Comparison of particulate matter inhalation for users of different transport modes in Lisbon. Transp. Res. Procedia 10, 433e442. Wang, X., Gao, O., 2011. Exposure to fine particle mass and number concentrations in urban transportation environments of New York City. Transp. Res. Part D. 16, 384e391. Xu, B., Yu, X., Gu, H., Miao, B., Wang, M., Huang, H., 2016. Commuters' exposure to PM2.5 and CO2 in metro carriages of Shanghai metro system. Transp. Res. Part D. 47, 162e170. Yan, C., Zheng, M., Yang, Q., Zhang, Q., Qiu, X., Zhang, Y., Fu, H., Li, X., Zhu, T., Zhu, Y., 2015. Commuter exposure to particulate matter and three transportation modes in Beijing, China. Environ. Pollut. 204, 199e206. Yu, Q., Lu, Y., Xiao, S., Shen, J., Li, X., Ma, W., Chen, L., 2012. Commuters' exposure to PM1 by common travel modes in Shanghai. Atmos. Environ. 59, 39e46.
Contents lists available at ScienceDirect
Environmental Pollution journal homepage: www.elsevier.com/locate/envpol
Estimation of inhaled airborne particle number concentration by subway users in Seoul, Korea* Minhae Kim a, b, Sechan Park a, b, Hyeong-Gyu Namgung b, Soon-Bark Kwon a, b, * a b
Railway System Engineering, University of Science and Technology (UST), Uiwang-si 16105, South Korea Transportation Environmental Research Team, Korea Railroad Research Institute (KRRI), Uiwang-si 16105, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 March 2017 Received in revised form 29 June 2017 Accepted 19 August 2017
Exposure to airborne particulate matter (PM) causes several diseases in the human body. The smaller particles, which have relatively large surface areas, are actually more harmful to the human body since they can penetrate deeper parts of the lungs or become secondary pollutants by bonding with other atmospheric pollutants, such as nitrogen oxides. The purpose of this study is to present the number of PM inhaled by subway users as a possible reference material for any analysis of the hazards to the human body arising from the inhalation of such PM. Two transfer stations in Seoul, Korea, which have the greatest number of users, were selected for this study. For 0.3e0.422 mm PM, particle number concentration (PNC) was highest outdoors but decreased as the tester moved deeper underground. On the other hand, the PNC between 1 and 10 mm increased as the tester moved deeper underground and showed a high number concentration inside the subway train as well. An analysis of the particles to which subway users are actually exposed to (inhaled particle number), using particle concentration at each measurement location, the average inhalation rate of an adult, and the average stay time at each location, all showed that particles sized 0.01e0.422 mm are mostly inhaled from the outdoor air whereas particles sized 1e10 mm are inhaled as the passengers move deeper underground. Based on these findings, we expect that the inhaled particle number of subway users can be used as reference data for an evaluation of the hazards to health caused by PM inhalation. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Particulate matter (PM) Particle size Particle number concentration (PNC) Subway user Inhaled particle number
1. Introduction With the ever increasing number of city commuters, concerns over the air quality on modes of transport has become an important issue and many studies have analyzed the exposure level of commuters to airborne particles (Cheng et al., 2009; Kingham et al., 2013; Knibbs et al., 2011; Kumar and Gupta, 2016; Ramos et al., 2015; Wang and Gao, 2011; Xu et al., 2016; Yan et al., 2015). Since subway systems carry large numbers of passengers in small, enclosed spaces compared to other modes of transportation, there is the potential for exposure to indoor pollution if the air conditioning and ventilation system of the facilities and vehicles are not effectively managed.
*
This paper has been recommended for acceptance by David Carpenter. * Corresponding author. Transportation Environmental Research Team, Korea Railroad Research Institute (KRRI), Uiwang-si 16105, South Korea; Railway System Engineering, University of Science and Technology (UST), Uiwang-si 16105, South Korea. E-mail address: [email protected] (S.-B. Kwon). http://dx.doi.org/10.1016/j.envpol.2017.08.077 0269-7491/© 2017 Elsevier Ltd. All rights reserved.
Ozgen et al. (2016) reported that the number concentration of ultrafine particles (UFP: dp < 0.1 mm) outdoors was around two times higher than that within the subway system and that the mass concentration of PM within the subway system was two to four times higher than it was outdoors. Wang and Gao (2011) measured the number concentration of particles (5 nme3 mm) and the mass concentration of PM of less than 2.5 mm (PM2.5) in New York subway stations. They reported that while the number concentration of particles outdoors was higher (at 60,629 #/cm3) than on the platform (at 37,657 #/cm3) the PM2.5 mass concentration on the platform was higher (at 68.29 mg/m3) than it was outdoors (at 29.60 mg/m3). Lonati et al. (2011) measured the mass concentration of PM in the size ranges of 0.3e0.5 mm and 2.5e10 mm and found that the mass concentrations of 0.3e0.5 mm PM outdoors and on the platform were similar while the mass concentration of 2.5e10 mm PM in the subway was much higher than it was outdoors. A study that compared the mass concentrations of PM10 and PM2.5 at different passenger locations in a subway station in Seoul, Korea, showed that the PM10 concentrations were 155 mg/m3 outdoors, 312 mg/m3 on the concourse and 359 mg/m3 on the platform,
664
M. Kim et al. / Environmental Pollution 231 (2017) 663e670
while the PM2.5 concentrations were 102 mg/m3, 126 mg/m3, and 129 mg/m3 for each location, respectively. The concentration of larger particles was more than 3 times higher within the subway station than it was outdoors, while the difference was significantly lower for smaller particles (Kim et al., 2008). As reported in the preceding studies, the difference in PM concentration as measured outdoors and within the subway stations varied by PM size and that the specific particles which people were exposed to differed by location. As such, many recent studies have now focused on the actual inhalation dose of mg per individual instead of simply examining concentration level. Ramos et al. (2015) compared the mass concentrations of PM10 and PM2.5 in various public transit systems and presented PM inhalation dose/ km using the average inhalation rate of adults, moving distance, and stay time. Yu et al. (2012) calculated the PM1 inhalation dose using the stay time in public transit system, PM1 mass concentration and inhalation rate. Lei et al. (2016) measured the PM2.5 mass concentration in indoor spaces, outdoors and on public transit systems, and presented the exposed PM inhalation dose per person at each location for a period of one day. Summarizing these studies, a person using the subway is exposed to a greater extent to smaller PM outdoors and to larger PM when in underground subway stations. Therefore, mass concentration is commonly used as a reference value for a person's exposure to PM and many governments also use mass concentration as a reference for the regulation of PM in the atmosphere and indoor environments. However, this approach has its short comings. This is because smaller particles contribute little to the findings when converted to total mass concentration but these particles are actually more harmful to the human body since they can become secondary pollutants by bonding with other atmospheric pollutants, such as nitrogen oxides (NOx), which have relatively large surface areas (Nel et al., 2006). Moreover, while larger particles can be filtered out of inhaled air by nasal mucosa and the upper airways, particles of 1 mm or less have been found all the way down to the bronchioles and alveoli (Londahl et al., 2006; Oberdorster et al., 2005). To reflect this phenomenon, Koehler and Peters (2015) proposed an analysis using number concentration instead of mass concentration to better reflect the concentration of UFP when evaluating the relationship between PM and health. Furthermore, Penttinen et al. (2001) argued for the consideration of number concentration at the same time as mass concentration when evaluating indoor air quality. This study measured the number concentration of PM, according to particle size, to which subway users are exposed to whilst traversing along actual movement paths. Subways were chosen as they have the largest transport share - at 39% compared to 27% for
buses and 22.8% for passenger cars - among means of transportation in Seoul (Statistics of Seoul, 2014). This study will present the number of PM inhaled by subway users as reference material for analysis of the hazards to the human body arising from the inhalation of PM. 2. Methods Two transfer stations in Seoul that handle the largest daily number of passengers were selected in order to measure PNC according to particle size and to observe subway users' exposure to PM. Two measurements were conducted on Line 1 in the afternoons of October 7e9, 2015 and on Line 2 in the afternoons of December 14e15, 2015. The measuring devices used in this study were a Nanoscan SMPS (TSI, Model 3910, USA) and an optical particle sizer (OPS; TSI, Model 3330, USA). The Nanoscan SMPS can measure particles sized between 0.01 and 0.420 mm while the OPS can measure particles sized between 0.3 and 10 mm. For the measurement of PM number concentration with the Nanoscan SMPS, particles were grouped into 13 sizes of 11.5, 15.4, 20.5, 27.4, 36.5, 48.7, 64.9, 86.6, 115.5, 154, 205.4, 273.8 and 365.2 nm. Since the Nanoscan SMPS requires around 1 min of time to measure UFP particles, it was fixed at three locations: outdoors (A, near the access to the station); the concourse (B, pathway between the outdoors and the platform); and the platform (C, passenger waiting area for the train). The sampling flow rate of Nanoscan SMPS was 0.75 L/min, and three measurements were made repeatedly at intervals of 60 s. For measurement with the OPS, a tester put the OPS on a backpack and took measurements whilst moving. Particles were grouped into 14 sizes of 0.3, 0.316, 0.422, 0.562, 0.75, 1, 1.334, 1.778, 2.371, 3,162, 4.217, 5.623, 7.499 and 10 mm. A conductive tube connected to the sampling inlet of the OPS measuring device minimized sampling loss and measurements were taken at 1.2 m from the floor. The sampling flow rate of the OPS was 1.2 L/min and measurements were taken at 5-s intervals during a period of 30e40 min as the tester moved around. To analyze the distribution of PM inhaled by subway users, measurements taken were at four locations: outdoors (A); on the concourse (B); on the platform (C); and whilst getting on the train, as shown in Fig. 1. The continuous measurement of PM took place for approximately 40 min and included both the stated measurement points as well as the time spent waiting for the subway train, transferring lines and ascending/descending the stairs. The PM10 concentration in the outdoor air during the measurement period refers to that provided by the Korea Environment Corporation (KECO; www. airkorea.or.kr) while the outdoor temperature and relative humidity refer to that provided by the Korea Metrological
Fig. 1. Moving path of subway users as reproduced in this study.
M. Kim et al. / Environmental Pollution 231 (2017) 663e670
665
Table 1 Outdoor temperature, relative humidity and PM10 on the days of measurements. Location
Date
Time
Temperature ( C)
Relative Humidity (%)
PM10 (mg/m3)
Route 1
Oct. 7, 2015
1:00 pm 2:00 pm 3:00 pm 1:00 pm 2:00 pm 3:00 pm Average 1:00 pm 2:00 pm 3:00 pm 1:00 pm 2:00 pm 3:00 pm Average
25.3 25.1 24.4 25.6 25.9 26.1 25.4 6.1 5.9 5.7 7.5 8.1 6.6 6.7
43 50 50 34 28 34 39.8 79 85 87 54 51 55 68.5
57 67 64 47 58 45 56.3 40 37 42 59 57 77 52.0
Oct. 8, 2015
Route 2
Dec. 14, 2015
Dec. 15, 2015
Administration (KMA; http://web.kma.go.kr/eng) as shown in Table 1. Route 1 is the path taken by users of Subway Line 1, while Route 2 is the path taken by the users of Subway Line 2. During the afternoons in the fall when the PM on Route 1 was measured, the average outdoor temperature was 25.4 C while humidity was 39.8%. During the afternoons in the winter when the PM on Route 2 was measured, the average outdoor temperature was 6.7 C while humidity was 68.5%, indicating some seasonal difference. However, the PM concentrations in the outdoor air was similar, with 56.3 mg/ m3 in the fall and 52 mg/m3 in the winter. 3. Results & discussion 3.1. Characteristics of particle size distribution Fig. 2 shows the size distribution of particles between 0.01 mm and 10 mm, as measured with the OPS and Nanoscan SMPS at each measurement location. As can be seen in the figure, the solid red line, dashed green line and long-dashed blue line refer to the PM size distribution in the outdoor air, on the concourse and on the platform, respectively. Although the PM size distributions at each site were similar, particles sized 0.15 mm or less showed a difference in concentration between the three locations. Particles sized approximately between 0.15 mm and 0.7 mm showed a high
concentration in the outdoor air and a similar or lower concentration on the concourse and platform. Particles sized around 0.7 mm or larger showed their highest concentration on the platform, followed by outdoors and on the concourse. For a more specific analysis of size distribution of smaller particles, those particles sized 0.3 mm or less (as measured with the Nanoscan SMPS) are shown on a linear-scale y-axis graph in Fig. 2. The graph shows a peak particle size at around 0.04 mm in the outdoor air and on the concourse. On the other hand, the number concentrations of particles between 0.04 and 0.15 mm on the platform were almost constant compared to those in the outdoor air and on the concourse. Given that the peak of 0.04 mm coincides with the peak value of PM size distributions occurring during the combustion of diesel (Kittelson et al., 1998) and the peak value of the size distribution of particles on the roadside (AQEG, 2005), the outdoor PM measured in this study can be attributed mostly to on-road emissions. The linear graph of number concentration of particles sized between 0.3 mm and 1 mm (as measured with the OPS) shows a peak value at around 0.3 mm in the outdoor air, on the concourse and the platform. The size distributions at the three locations were similar, showing the highest concentration outdoors, followed by on the concourse and the platform. However, the concentration of PMs between 0.8 mm and 3 mm was higher on the platform than it was outdoors.
Fig. 2. PM size distribution in the outdoor air, concourse and platform in log and linear scale.
666
M. Kim et al. / Environmental Pollution 231 (2017) 663e670
3.2. Change in PM number concentration along the movement path Fig. 3 shows the change in number concentration of particles sized 0.3e10 mm along the path taken by subway users on Route 1. The size ranges measured with the OPS were separated into three groups of 0.3e0.422 mm, 0.422e1 mm and 1e10 mm showing similar changes, and the aggregated concentration of each group is presented along the path taken by subway users. A, B, C and D in the graph represent the outdoors, on the concourse, on the platform and on the train, respectively. The figures measured on the transfer path taken by subways users in another subway station (Transfer in Fig. 3) was excluded from the analysis. For PM sized 0.3e0.422 mm (Fig. 3(a)), the number
concentration was highest outdoors (A) and decreased at the measurement positions located deeper underground (B/C/D). Likewise, the number concentration increased the closer the measurement location was to the exit from the platform. The changes to number concentration of particles sized 0.422e1 mm, as shown in Fig. 3(b), show a somewhat different trend from particles of between 0.3 and 0.422 mm in size, but a similar trend to particles between 1 and 10 mm in size. The changes to number concentration of particles sized 1e10 mm, as shown in Fig. 3(c), show a different trend than those in Fig. 3(a) and thus indicate a considerably higher concentration on the platform and a lower concentration outdoors. Table 2 shows the average value, standard deviation, maximum value, and minimum value to analyze quantitatively the number concentration value at each location. The average number concentration of particles sized 0.3e0.422 mm along Route 1 was the highest outdoors at 395.4 #/cm3. The number concentrations on the concourse, on the platform and in the train were 311.4, 268.5 and 220.4 #/cm3, respectively. On the other hand, the number concentrations of particles sized 1e10 mm on the concourse were 2.85 #/cm3, 4.48 #/cm3 on the platform and 3.75 #/cm3on the train, indicating that the number concentration was lower on the concourse than on the platform and inside the train. The number concentration measured outdoors was 4.08 #/cm3, the second highest recording next to that of the platform. The findings for all measurements taken on Route 2 were similar. The number concentration of particles sized 0.3e0.422 mm was the highest 479.5 #/cm3 outdoors. The number concentrations on the concourse, on the platform and inside the train were 283.5, 322.6 and 464.4 #/cm3, respectively, indicating a lower concentration on the concourse than at the other two locations. For larger particles between 1 and 10 mm, the number concentration measured outdoors was 3.71 #/cm3, while the number concentrations on the concourse and the platform were 3.08 #/cm3 and 7.35 #/cm3, respectively, indicating that the concentration on the platform was twice that of the concourse. The number concentration in the train was 16.1 #/cm3 or about four times that found outdoors. The difference in number concentration outdoors and in the train was much smaller for smaller sized particles (0.3e0.422 mm) than for larger sized particles (1e10 mm). The studies by Wang and Gao (2011) and Ozgen et al. (2016), as mentioned earlier, which reported that the number concentration was higher in the outdoor air for smaller particles and higher on the platform for larger particles, are consistent with the findings of this study. According to a study of PNC on a subway platform in Prague, Czech Republic, the number concentration of smaller particles of around 0.6 mm or less in size was barely affected by the environment of either the platform or the train, as it was found to be most affected by PM concentration in the outdoor air (Cusack et al., 2015). 3.3. Average PM number concentration at each location
Fig. 3. Particle number concentration distribution along movement path for Route 1 for particle size ranges of (a) 0.3e0.422 mm (b) 0.422e1 mm and (c) 1e10 mm, with A referring to the outdoor air, B to the concourse, C to the platform area, and D to inside the train. “Transfer” refers to the transfer path.
Fig. 4 shows the average PM number concentrations (including results of both Nano Scan and OPS data) measured on Route 1 and Route 2, separated into four size groups of (a) 0.3 mm or less, (b) 0.3e0.422 mm, (c) 0.422e1 mm and (d) 1e10 mm. The shaded boxes show the maximum value, the minimum value and the deviation. The solid lines refer to the median values while the dotted lines refer to the average values. The number concentration of particles sized 0.3 mm or less was at its highest outdoors, at 34,532 #/cm3, followed by on the concourse, at 24,117 #/cm3, and finally the platform, at 19,643 #/cm3. The number concentration of particles sized 0.3 mm or less outdoors was about twice as high as that measured on the platform. The average size of PM was 51.4 nm outdoors and increased the deeper underground the tester travelled to 56.6 nm at the bottom of the stairs, 57.1 nm on the
M. Kim et al. / Environmental Pollution 231 (2017) 663e670
667
Table 2 Averaged particle number concentration (PNC), standard deviation, maximum and minimum values at each location on Route 1 and Route 2. Averaged Particle Number Concentration (#/cm3)
0.3e0.422 mm (A) Outdoor
(B) Concourse
(C) Platform
(D) Inside Train
(A) Outdoor
(B) Concourse
(C) Platform
(D) Inside Train
Route 1
395.4 6.86 402.9 374.2 479.5 13.5 500.9 435.8
311.4 50.7 404.8 238.5 283.5 48.3 529.0 245.7
268.5 15.8 302.4 247.0 322.6 26.5 384.8 265.6
220.4 14.1 253.5 196.0 464.4 20.8 506.6 345.1
4.08 0.34 4.49 3.23 3.71 0.59 4.77 2.52
2.85 0.69 5.82 1.46 3.08 1.23 9.79 1.18
4.48 1.69 9.50 1.85 7.35 2.31 12.7 3.24
3.75 0.54 5.71 2.87 16.1 3.41 26.4 9.25
Route 2
Mean S.D. Max Min Mean S.D. Max Min
1-10 mm
Fig. 4. PM number concentrations measured along Route 1 and Route 2: (a) <0.3 mm (b) 0.3e0.422 mm (c) 0.422e1 mm (d) 1e10 mm.
concourse, and 62.6 nm on the platform. Although particles sized 0.3e0.422 mm showed similar average number concentrations on the concourse and the platform, the concentration was higher outdoors (Fig. 4(b)). The average number concentration of particles sized 0.422e1 mm was in the range of 20e30 #/cm3 at all measurement locations, with the number concentration on the concourse the lowest (Fig. 4(c)). The number concentration of particles sized 1e10 mm was highest on the platform while measurement of the outdoor air and air on the concourse both showed similar number concentrations. Table 3 shows the coefficient of variation (CV) that represents the ratio of standard deviation to average value for each particle size group at each location. The CV of particles 0.3 mm or less was highest at 33.57% outdoors, which can presumably be attributed to the effect of a large variety of road
Table 3 Coefficient of variations for PM number concentration at each location for each size group. Coefficient of variation (CV) in PM number concentration (%)
(A) Outdoors (B) Concourse (C) Platform
<0.3 mm
0.3e0.422 mm
0.422e1 mm
1-10 mm
33.57 13.20 5.07
9.58 17.41 10.83
18.63 19.93 26.46
13.94 28.54 36.12
traffic (road pollution source) since the subway entrance is located on the side of the road (Cheng et al., 2011). The CV of particles sized 1e10 mm was highest on the platform, at 37.62%. The larger particles show the greatest variation on the platform because when the
668
M. Kim et al. / Environmental Pollution 231 (2017) 663e670
platform screen doors (PSDs) installed on the subway platform open frequently, large sized particles scattered by the train draft blow onto the platform from the tunnel side (Kwon et al., 2016). 3.4. Exposure scenario and analysis of exposure to PM for subway users As mentioned in the Introduction, the analysis of inhaled doses of PM with consideration given to inhalation rate is very important in understanding the hazards to the health of subway users. It is also necessary to study how inhaled doses of PM differ according to their respiration volume which in turn differs according to user activity patterns (standing up, walking, running etc.). For example, one study reported that the inhalation dose of UFP by those with asthma was larger than that of healthy persons because of the different respiration volumes (Chalupa et al., 2004). This study used its own measurement results and the average respiration volume of adults according to activity patterns to analyze the exposure level, which is defined as the total number of particles inhaled by a subway user each minute. The following Equation (1) calculates the inhaled PM number per minute (IPN) using the measured particle number concentration (PNC) at each location and the inhalation rate (IR):
IPNðN=minÞ ¼ PNC N=cm3 IRðL=minÞ
(1)
The average inhalation rate of an adult (aged 21 to 30) announced by the United States' Environmental Protection Agency (EPA, 2011) is 12 L/min during light intensity activity such as standing up, and 26 L/min during moderate intensity activity such as walking. Assuming that most subway users walk in from the outside to the concourse and mostly stand at the platform and on the train, the appropriate different inhalation rates were used in our analysis of exposure level. Since retention time is an important factor for analysis of exposure level and differs according to location, the inhaled PM number per minute was calculated first as shown in Table 4. The average subway user inhales around 790 million particles sized 0.3 mm or less per minute at the entrance to the subway station, around 530 million particles per minute on the concourse, and around 190 million particles per minute at the platform. The average subway user inhales 11.37 million particles sized 0.3e0.422 mm per minute at the entrance to the subway station, 7.73 million particles per minute on the concourse, 3.35 million particles per minute at the platform and 4.1 million particles per minute in the train. The exposure level to particles sized 0.422 mm or less was highest outdoors, followed by on the concourse, the platform, and finally on the train. On the other hand, exposure level to particles sized 1e10 mm was highest on the train at 11,900 particles, followed by on the concourse, at 77,000 particles, and on the platform, at 71,000 particles. Existing studies which used mass concentration showed an inhalation dose of PM1 to be 28.6 ± 25.9 mg in the subway station and 45.2 ± 30.1 mg in the outdoor air, indicating a dose that is 1.6 times higher in the outdoor air. (Yu et al., 2012). Ramos et al. (2016) reported the inhalation
dose of subway users to be 2.29 mg of PM1, 2.57 mg of PM2.5, 2.94 mg of PM4 and 3.43 mg of PM10, indicating that inhalation dose increases with particle size. The previous analysis was for the number of PM exposed per minute for assumed activities at each location. Based on this we analyzed the total number of PM to which subway users are actually exposed to. Fig. 5 shows the percentage contribution of location (outdoors, on the concourse, on the platform and inside the train) to total PM inhaled based on a survey of retention time of commuters on the public transit system in Seoul (Kim et al., 2014). This survey reported that the average resident of the capital region spent 68 min commuting on the public transit system, which includes 16 min outdoors, 11 min on the concourse, 6 min on the platform, and 35 min inside the train. The rate of inhalation dose according to location clearly differs by particle size. For particles sized 0.3 mm or less, around 64% was inhaled outdoors, 30% on the concourse and 6% on the platform. For particles sized 0.3e0.422 mm, 42% was inhaled outdoors, 20% on the concourse, 5% on the platform and 33% inside the train. For particles sized 0.422e1 mm, 33% was inhaled outdoors, 15% on the concourse and 5% on the platform, indicating that the combined rate of inhalation outdoors and on the concourse was much smaller than the above two cases. On the other hand, the rate of inhalation in the train was 48%, indicating that almost half of the total PM was inhaled inside the train. For particles sized 1e10 mm, 59% of PM was inhaled in the train while only 23% was inhaled outdoors. For particles inhaled inside the train, 33% were between 0.3 mm and 0.422 mm, 48% were between 0.422 mm and 1 mm, and 59% were 1 mm or larger, indicating that the rate of inhalation increased with particle size. On the other hand, the rate of inhalation outdoors decreased as particle size increased, since 65% of the particles were 0.3 mm or less in size, 42% of particles were between 0.3 and 0.422 mm, and 23% were between 1 and 10 mm. The rates of inhalation on the platform were similar (5e6%) for all sizes, which can be attributed to a relatively short stay time.
4. Conclusion This study measured the number concentration of PM of different sizes on an actual path taken by most subway users in Seoul, Korea, to evaluate the PNC trend at each location. Moreover, we calculated the inhaled PM number in order to investigate the exposure of each user to particles at each location. The number concentration of particles between 10 nm and 0.7 mm, to which subway users would be exposed, was highest outdoors followed by on the concourse and platform. Particles were measured in 14 size ranges by the OPC and then separated into three groups of 1) 0.3e0.422 mm, 2) 0.422e1 mm and 3) 1e10 mm, according to distribution characteristics, and measured along an actual path of movement taken by the majority of subway users. Our analysis showed trends that are clearly differentiated according to particle size. For 0.3e0.422 mm PM, PNC was highest outdoors and
Table 4 Averaged particle number concentration (PNC) observed in this study, inhalation rate (IR) by walking or standing up and calculated inhaled particle number (IPN) at each location. PNC (#/cm3)
Point
(A) (B) (C) (D)
Outdoors Concourse Platform Inside Train
<0.3 mm
0.3e0.422 mm
0.422e1 mm
1-10 mm
30,939 20,377 15,792 e
437.5 297.4 295.6 342.4
31.1 19.6 27.3 44.7
3.9 2.97 5.92 9.92
IR (L/min)
IPN (103 #/min) <0.3 mm
0.3e0.422 mm
0.422e1 mm
1-10 mm
12 12 26 26
790,218 529,802 189,504 e
11,373.8 7733.3 3546.6 4109
809 508.2 327.2 535.9
101.3 77.1 71 119
M. Kim et al. / Environmental Pollution 231 (2017) 663e670
669
Fig. 5. Rate of inhalation dose of PM in four size groups: outdoors, concourse, platform and train (a) < 0.3 mm (b) 0.3e0.422 mm (c) 0.422e1 mm (d) 1e10 mm.
decreased as the tester moved deeper underground. On the other hand, the PNC between 1 and 10 mm increased as the tester moved deeper underground and showed a high number concentration inside the subway train as well. The intermediate-sized particles of 0.422e1 mm in diameter did not show much difference in PNC at different points along the path taken by subway users. Particles of 0.3 mm or higher, which had a high number concentration in the outdoor air, also showed their widest variation in measurement since these sized particles are greatly affected by the level of road traffic. At the platform, relatively large particles (1e10 mm) showed a larger variation in PNC since they irregularly blew into the platform area from the tunnel as the PSDs opened. An analysis of the particles to which subway users are actually exposed to - using particle concentration at each measurement location, the average respiration volume of an adult, and the average stay time at each location - showed that particles sized 0.01e0.422 mm are mostly inhaled in the outdoor air but less so as users move deeper underground. On the other hand, a greater numbers of particles sized between 1 mm and 10 mm are inhaled as the passengers move deeper underground. Based on these findings, it is expected that PNC and particles of various sizes inhaled at each location on path taken by subway users can be used in the future as reference data for the evaluation of the hazards to health from PM inhalation. Acknowledgement This research was supported by a grant from R&D Program of the Korea Railroad Research Institute (KRRI), Republic of Korea. References AQEG, 2005. Particulate Matter in the UK: Summary. Defra, London.
€rster, G., Utell, M.J., Frampton, M.W., 2004. UlChalupa, D.C., Morrow, P.E., Oberdo trafine particle deposition in subjects with asthma. Environ. Health Perspect. 112 (8), 879e882. Cheng, Y.H., Liu, C.C., Lin, Y.L., 2009. Levels of ultrafine particles in the Raipei Rapid transit system. Transp. Res. Part D. 14, 479e486. Cheng, Y., Zou, S.C., Lee, S.C., Ho, K.F., Watson, J.G., Han, Y.M., Zhang, R.J., Zhang, F., Yau, P.S., Huang, Y., Bai, Y., Wu, W.J., 2011. Characteristics and source apportionment of PM1 emissions at a roadside station. J. Hazard. Mater 195, 82e91. Cusack, M., Talbot, N., Ondracek, J., Minguillon, M.C., Martins, V., Klouda, K., Schwarz, J., Zdimal, V., 2015. Variability of aerosols and chemical composition of PM10, PM2.5 and PM1 on a platform of the Prague underground metro. Atmos. Environ. 118, 176e183. EPA, 2011. Exposure Factors Handbook 2011 Edition (Final). U.S. Environmental Protection Agency, Washington, DC. EPA/600/R-09/052F. KECO. Korea Environment Corporation. www.airkorea.or.kr. Kim, K.Y., Kim, Y.S., Roh, Y.M., Lee, C.M., Kim, C.N., 2008. Spatial distribution of particulate matter (PM10 and PM2.5) in Seoul Metropolitan Subway stations. J. Hazard. Mater 154, 440e443. Kim, S.K., Ko, J.H., Lee, S.H., Jang, J.E., Jung, K.S., 2014. Evaluation of the quality of public transport commuting in Seoul. Seoul Inst. Rep. 1e109. Kingham, S., Longley, L., Salmond, J., Pattinson, W., Shrestha, K., 2013. Variations in exposure to traffic pollution while travelling by different modes in a low density, less congested city. Environ. Pollut. 181, 211e218. Kittelson, D.B., Watts, W.F., Arnold, M., 1998. Review of Diesel Particulate Matter Sampling Methods: Supplemental Report# 2, Aerosol Dynamics, Laboratory and On-road Studies. University of Minnesota. KMA. Korea Metrological Administration. http://web.kma.go.kr/eng. Knibbs, L.D., Cole-Hunter, T., Morawska, L., 2011. A review of commuter exposure to ultrafine particles and its health effects. Atmos. Environ. 45, 2611e2622. Koehler, K.A., Peters, T.M., 2015. New methods for personal exposure monitoring for airborne particles. Curr. Environ. Health Rep. 2, 399e411. Kumar, P., Gupta, N.C., 2016. Commuter exposure to inhalable, thoracic and alveolic particles in various transportation modes in Delhi. Sci. Total Environ. 541, 535e541. Kwon, S.B., Namgung, H.G., Jeong, W., Park, D., Eom, J.K., 2016. Transient variation of aerosol size distribution in an underground subway station. Environ. Monit. Assess. 188 (6), 1e11. Lei, X., Xiu, G., Li, B., Zhang, K., Zhao, M., 2016. Individual exposure of graduate students to PM2.5 and black carbon in Shanghai, China. Environ. Sci. Pollut. Res. 23, 12120e12127. Lonati, G., Ozgen, S., Ripamonti, G., Cernuschi, S., Giugliano, M., 2011. Pedestrian exposure to size-resolved particles in milan. J. Air & Waste Manage. Assoc. 61,
670
M. Kim et al. / Environmental Pollution 231 (2017) 663e670
1273e1280. Londahl, J., Pagels, J., Swietlichi, E., Zhou, J., Ketzel, M., Massling, A., Bohgard, M., 2006. A set-up for field studies of respiratory tract deposition of fine and ultrafine particles in humans. J. Aerosol Sci. 37, 1152e1163. Nel, A., Xia, T., Madler, L., Li, N., 2006. Toxic potential of materials at the nanolevel. Science 311, 622e627. Oberdorster, G., Oberdorster, E., Oberdorster, J., 2005. Nanotoxicology: an emerging discipline evolving from studies of ultrafine particles. Environ. Health Perspect. 113 (7), 823e839. Ozgen, S., Ripamonti, G., Malandrini, A., Ragettli, M.S., Lonati, G., 2016. Particle number and mass exposure concentrations by commuter transport modes in Milan. Italy. Environ. Sci. 3 (2), 168e184. Penttinen, P., Timonen, K.L., Mirme, A., Ruuskanen, J., Pekkanen, J., 2001. Ultrafine particles in urban air and respiratory health among adult asthmatics. Eur. Respir. J. 17, 428e435. Ramos, C.A., Wolterbeek, H.T., Almeida, S.M., 2016. Air pollutant exposure and
inhalation dose during urban commuting: a comparison between cycling and motorized modes. Air Qual. Atmos. hlth. 1e13. Ramos, M.J., Vasconcelos, A., Faria, M., 2015. Comparison of particulate matter inhalation for users of different transport modes in Lisbon. Transp. Res. Procedia 10, 433e442. Wang, X., Gao, O., 2011. Exposure to fine particle mass and number concentrations in urban transportation environments of New York City. Transp. Res. Part D. 16, 384e391. Xu, B., Yu, X., Gu, H., Miao, B., Wang, M., Huang, H., 2016. Commuters' exposure to PM2.5 and CO2 in metro carriages of Shanghai metro system. Transp. Res. Part D. 47, 162e170. Yan, C., Zheng, M., Yang, Q., Zhang, Q., Qiu, X., Zhang, Y., Fu, H., Li, X., Zhu, T., Zhu, Y., 2015. Commuter exposure to particulate matter and three transportation modes in Beijing, China. Environ. Pollut. 204, 199e206. Yu, Q., Lu, Y., Xiao, S., Shen, J., Li, X., Ma, W., Chen, L., 2012. Commuters' exposure to PM1 by common travel modes in Shanghai. Atmos. Environ. 59, 39e46.