The DCFs were then obtained from the correlation between count rate outside the car and absorbed dose rate in air calculated from software using the 22 × 22 response matrix method [ 18 ] ( Table 1 ). The DCF (nGy h -1 /cps) was evaluated as 0.16. In the method for determining the ambient dose rate from count rate, the dispersion of dose conversion coefficient is affected by the abundance ratio of K-40, U-238, Th-232, Cs-134 and Cs-137. Thus, this effect resulted in a negligible change in this study. The obtained R 2 from measurements ranged from 0.728 to 0.876. The dispersion of DCF is affected by the abundance ratios of K-40, U-238 series, Th-232 series [ 26 ], Cs-134 and Cs-137. Lower R 2 values were exhibited in the Tokyo metropolitan area measurements compared to those for other areas in Japan [ 23 ].

The SFs values for each measurement year were obtained to calculate absorbed dose rates in air as shown in Table 1 . The SFs ranged from 1.35 to 1.56. The SF is influenced by the type of car used in a survey, the number of passengers and the scintillation spectrometer position inside the car. In previous reports, SFs have ranged from 1.1 to 1.9 [ 5 – 7 , 17 , 19 , 21 – 25 ], and the presently obtained SFs were in this range. The coefficient of determination (R 2 ) from measurement correlations ranged from 0.661 to 0.774 and the calculated R 2 for metropolitan Tokyo had lower values compared to those measured in another Japanese report (R 2 = 0.967, n = 35) [ 23 ]. In the measurement of count rates outside the car, a scintillation spectrometer ideally should be placed in an open space at a distance of 10–20 m from the car and nearby artificial structures to eliminate the impact on count rates outside the car from these structures. The population density of metropolitan Tokyo is ranked first in the world and there are many artificial structures. Therefore, measurements at such an ideal place were impossible, and that resulted in mid-level correlations being exhibited by the metropolitan Tokyo measurements.

Changes of distribution of absorbed dose rates in air in metropolitan Tokyo

The absorbed dose rates in air (nGy h-1) outside the car 1 m above the ground surface were calculated using both SF and DCF (Eq 1). The changes of absorbed dose rates in air measured in 2014 [5], 2015, 2016, 2017 and 2018 are shown in Fig 3. The outliers were defined as: < lower quartile– 1.5 × distance from upper quartile to lower quartile (IQD) or > upper quartile + 1.5 × IQD (KaleidaGraph, Synergy Software, USA). The average absorbed dose rates in air (ranges) in metropolitan Tokyo were 60 ± 11 nGy h-1 (23–142 nGy h-1; n = 4,018) for 2014 [5], 59 ± 10 nGy h-1 (24–118 nGy h-1; n = 4,018) for 2015, 59 ± 9 nGy h-1 (28–106 nGy h-1; n = 4,346) for 2016, 58 ± 8 nGy h-1 (26–97 nGy h-1; n = 4,717) for 2017 and 59 ± 9 nGy h-1 (28–105 nGy h-1; n = 5,138) for 2018. The detailed absorbed dose rates in air in all municipalities in Tokyo are shown in S1 Table. According to the Tokyo Metropolitan Government, the average absorbed dose rate measured at 100 locations in June 2011 was 61 nGy h-1 (30–200 nGy h-1) [5]. The average absorbed dose rate in metropolitan Tokyo has not significantly changed in the past seven years but the number of high outliers (i.e., higher dose rates) has decreased yearly (Fig 3).

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larger image TIFF original image Download: Fig 3. Calculated absorbed dose rates in air from natural and artificial radionuclides measured in 2014 [ Calculated absorbed dose rates in air from natural and artificial radionuclides measured in 2014 [ 5 ]– 2018 in metropolitan Tokyo based on the measurements by the car-borne survey technique. The measurement was done on the same route (red line in Fig 2) using the same 3-in × 3-in NaI(Tl) scintillation spectrometer. https://doi.org/10.1371/journal.pone.0224449.g003

Fig 4 shows average absorbed dose rate in air from natural radionuclides measured at 61 locations (Fig 2). Those dose rates were calculated using the 22 × 22 response matrix method [18]. Those dose rates were increased in the last 10 years compared to the measured dose rate in 2003 for Tokyo (49 ± 6 nGy h-1) [6], especially in A1 area. The construction of buildings and hotels has increased sharply since 2015 as Tokyo prepares to host the 2020 Olympics, and that has resulted in the increased dose rate because many natural radionuclides are contained in the building materials. In fact, the respective numbers of newly completed units for skyscrapers (i.e., more than twenty-story building) in metropolitan Tokyo in 2014 and 2015 were 5620 and 14738 according to the statistical data published by the Ministry of Land, Infrastructure, Transport and Tourism of Japan [27]. Additionally, absorbed dose rate in air from natural radionuclides changes depending on environmental conditions such as soil moisture and radon concentration. Thus, it is difficult to make a simple comparison on dose rates before and after the F1-NPP accident.

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larger image TIFF original image Download: Fig 4. Changes of absorbed dose rate in air from natural radionuclides in the eastern (A1) and western (A2) ends of Tokyo in 2014 [5]– 2018. The gamma-ray pulse height distributions were measured outside the car for 10 min, at 61 locations (Fig 2). The gamma-ray pulse height distributions were then unfolded using the 22 × 22 response matrix method, and separated as natural radionuclides (K-40, U-238 series and Th-232 series). https://doi.org/10.1371/journal.pone.0224449.g004

Fig 5 shows distribution maps of absorbed dose rate in air measured in 2015–2018 in metropolitan Tokyo. Those maps were drawn with the same magnification and altitude color gradation scale using GMT [15] and interpolated measured dose rates using a minimum curvature algorithm because the measurements of dose rate could not be performed at some areas. While absorbed dose rate in air in all municipalities in Tokyo are shown in S1 Table, there is limitation to the details that can be shown on the dose distribution map, especially in the mountain area at the western end of Tokyo. Additionally, changes of absorbed dose rate in air from natural radionuclides need to be considered to compare dose distribution maps as shown in Fig 4. In reported measurements of 2014 that were done in a car-borne survey on the same route and used the same NaI(Tl) scintillation spectrometer [5], higher dose rates exceeding 100 nGy h-1 were observed in Katsushika Ward (#22 in Fig 1) and Okutama Town (#53 in Fig 1), and their heterogeneous distributions were shown to be due to the presence of artificial radionuclides. The maps in Fig 5, however, showed the differences in dose rates yearly became smaller on the eastern and western ends of metropolitan Tokyo. The average absorbed dose rates in A1 and A2 in Fig 1 measured in 2014 were 60 ± 12 nGy h-1 (23–142 nGy h-1; n = 2,010) and 61 ± 10 nGy h-1 (32–102 nGy h-1; n = 2,255), respectively. After four years, those values measured in 2018 became 60 ± 9 nGy h-1 (28–105 nGy h-1; n = 2,216) and 58 ± 8 nGy h-1 (34–100 nGy h-1; n = 2,922), respectively, and those ranges of dose rates became smaller.

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larger image TIFF original image Download: Fig 5. The distribution maps of absorbed dose rates in air in metropolitan Tokyo measured in 2015 (A), 2016 (B), 2017 (C) and 2018 (D). A minimum curvature algorithm was used for the data interpolation using the GMT [15]. Those maps were drawn using 4,018 data for 2015, 4,346 data for 2016, 4,717 data for 2017 and 5,138 data for 2018. https://doi.org/10.1371/journal.pone.0224449.g005

To allow more detailed discussion on changes of absorbed dose rates in air in metropolitan Tokyo, Fig 6 shows distribution maps of absorbed dose rates in air from two artificial radionuclides (Cs-134 + Cs-137). Those dose rates were calculated using the 22 × 22 response matrix method [18]. The average dose rates from artificial radionuclides for A1 and A2 areas are shown in Fig 7, and these values decreased yearly. The percent reductions for A1 area in the years 2014–2015, 2015–2016, 2016–2017 and 2017–2018 were 49%, 21%, 18% and 16%, and those percent reductions for A2 were 26%, 18%, 6% and 3%, respectively. The differences of percent reduction between A1 and A2 areas might be explained from the differences of the environment around measurement points. The percentages of road area [28] and green space [29] to total area of the administrative district are 16.5% and 3.8–23.1% for A1 area whereas those for A2 area are 6.7% and 30–97%. The deposited radionuclides on sealed surfaces such as asphalt or concrete pavements can be easily washed away by rainfall compared to bare ground or lichen-covered areas [5, 17, 30, 31]. Thus, a different reduction of dose rate between A1 and A2 areas was observed that was related to the two environment extremes. Additionally, the reduction ratios in 2014–2015 for both areas were the highest compared to other time periods (i.e., after 2016), and percent reductions then became lower yearly. Thus, it seems that the reduction of absorbed dose rate in air in A1 area was related to ecological effects such as weathering that occurred during the early term after the F1-NPP accident. These findings corresponded to those of the previous report for the F1-NPP accident [32]. In the Chernobyl accident, reduction ratios of Cs-137 contamination on street pavements were faster than that on a reference surface (a cut lawn) [33], and the same tendency was observed after the Fukushima accident as well [34].

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larger image TIFF original image Download: Fig 6. The distribution maps of absorbed dose rates in air from artificial radionuclides in 2015 (A), 2016 (B), 2017 (C) and 2018 (D). The gamma-ray pulse height distributions were measured for 10 min, at 61 locations (Fig 2). The gamma-ray pulse height distributions were then unfolded using the 22 × 22 response matrix method, and separated as artificial radionuclides (Cs-134 and Cs-137). https://doi.org/10.1371/journal.pone.0224449.g006

For a more detailed evaluation, the reduction ratio of absorbed dose rate in air related with type of asphalt was analyzed using measured data from the Joto district (#6 –#8 and #21 –#23 in Fig 1) which is a highly contaminated area in metropolitan Tokyo compared to that in other nearby wards. Fig 8 shows the transition of absorbed dose rate in air from artificial radionuclides measured at 1 m above porous asphalt (n = 3, #21 –#22 in Fig 1) and standard asphalt (n = 5, #6 –#8 and #23 in Fig 1) surfaces. The percent reductions of dose rate in the years 2014–2015, 2015–2016, 2016–2017 and 2017–2018 were 21%, 18%, 7% and 7% for porous asphalt, and those ratios for standard asphalt were 21%, 37%, 18% and 21%, respectively. Therefore, the reduction of dose rates measured on standard asphalt was occurring faster than that on porous asphalt. This can be explained from the structure difference of both asphalt types. The coarse aggregate diameters of the two are different (Fig 9). The porous asphalt material consists of coarse aggregates with a diameter of more than 2.36 mm, and the drainage function is high, resulting in its wide use recently for highways and main roads. However, it can be quickly clogged by dust depending on the amount of traffic, and deposited radiocesium that became attached to the dust particles has the property of binding strongly with the dust particles and not being easily washed away by rainfall. [31]. On the other hand, standard asphalt is low porosity asphalt consisting of fine aggregates having diameters of 0.075–2.36 mm and it is utilized for local roads and public parking areas. This type of asphalt has a water repellency effect and the deposited radiocesium and dust particles are easily washed out by rainfall compared to high porosity asphalt. Thus, the dose rate measured on standard asphalt decreased more quickly compared with that on porous asphalt. When the changing dose rates are locally evaluated, it seems that the dose rates would not be homogeneously decreased due to the asphalt type dependency.