Data on atmospheric 129I concentrations and 129I/137Cs ratios for suspended air particulate matter dispersed in eastern Japan just after the 2011 nuclear accident in Fukushima, Japan

Data of the atmospheric activity concentrations (in Bq/m3) of 129I dispersed into the environment as aerosol immediately after the nuclear accident at Fukushima Daiichi Nuclear Power Plant in 2011 are presented. The radioactivity of 129I was determined in suspended particulate matter (SPM) collected on filter tapes at 41 SPM monitoring sites in Fukushima and other prefectures in eastern Japan including the metropolitan area. For quantitative determination of 129I in SPM samples by accelerator mass spectrometry (AMS), 129I was chemically separated. Prior to the 129I measurement, the 137Cs activity was determined for the same SPM sample by gamma-ray spectrometry using Ge-semiconductor detectors. Combining activity concentrations of the two nuclides, an activity ratio of 129I/137Cs (in Bq/Bq) was calculated for each SPM sample. In our research project, atmospheric activity concentrations of 129I and 137Cs, and their activity ratios were obtained for 920 SPM samples. Scientific discussion related to those data was described in the research article entitled “Time-series variations of atmospheric 129I concentrations and 129I/137Cs ratios in eastern Japan just after the 2011 nuclear accident in Fukushima, Japan” (Ebihara et al. 2022), where 363 data sets were presented. The remaining 557 data sets are presented in this article, so this data article makes up for the original research article (Ebihara et al. 2022). Blank values were obtained for whole analytical procedure. In addition, those for reagents and filters (both bland-new and used filters) were analyzed for assessing the contribution of the 129I activity from these samples. Those data also are presented in this article.

for each SPM sample. In our research project, atmospheric activity concentrations of 129 I and 137 Cs, and their activity ratios were obtained for 920 SPM samples. Scientific discussion related to those data was described in the research article entitled "Time-series variations of atmospheric 129 I concentrations and 129 I/ 137 Cs ratios in eastern Japan just after the 2011 nuclear accident in Fukushima, Japan" (Ebihara et al. 2022), where 363 data sets were presented. The remaining 557 data sets are presented in this article, so this data article makes up for the original research article (Ebihara et al. 2022). Blank values were obtained for whole analytical procedure. In addition, those for reagents and filters (both bland-new and used filters) were analyzed for assessing the contribution of the 129 I activity from these samples. Those data also are presented in this article.  Table   Subject Environmental Science Specific subject area Pollution of radioactive aerosol containing fissiogenic 129 I and 137 Cs dispersed into the environment by the 2011 Fukushima nuclear power plant accident Type of data Table  How data were acquired Data were obtained by the following methods; (i) Accelerator mass spectrometry (AMS) for the 129 I radioactivity measurement, and (ii) Gamma-ray spectrometry for the 137 Cs radioactivity measurement. Data format Raw and analyzed Description of data collection Suspended particulate matter (SPM) samples analyzed in this study have been hourly collected in Fukushima and other prefectures in eastern Japan during and after the 2011 Fukushima nuclear disaster. The radioactivity of 129 I was determined by AMS using a tandem accelerator at the University of Tokyo after chemically separating 129 I in SPM. The radioactivity of 137 Cs was non-destructively determined by gamma-ray spectrometry at Tokyo Metropolitan University. Data source location For their 129

Value of the Data
• These data are useful because they can document the atmospheric concentration of 131 I [2] , which is one of the most concerned radioactive nuclides on the occasion of nuclear accidents and could not be sufficiently quantified due to its short half-life (about 8 days). As 129 I is an isotope to 131 I and has a long half-life (1.5 × 10 7 y), it can be a proxy of 129 I [2] . These data are also useful for tracing the transfer of radioiodine dispersed into the atmosphere. • These data benefit atmospheric scientists who manage to model the transportation of radioactive air masses (e.g., [3] ). The data also benefit health physicists managing to estimate the radiation exposure due to radionuclides released into the environment (e.g., [4] ). • These data can be used for estimating the degree of radiation exposure against thyroid due to inhalation of aerosol-carrying 131 I. These data can be also used for chronologically depicting how the nuclear accident was expanded since the nuclear power reactors at FD1NPP were damaged by an earthquake-triggered tsunami in March, 2011.

Data Description
This data article presents data of the atmospheric activity concentrations (in Bq/m 3 ) of 129 I dispersed into the environment as aerosol immediately after the nuclear accident at Fukushima Daiichi Nuclear Power Plant on March 11, 2011. Data were obtained by measuring quantities of 129 I in suspended particulate matter (SPM) collected on filter tapes at 41 SPM monitoring sites in Fukushima and other prefectures in eastern Japan, including the metropolitan area of Tokyo and the surrounding area. In our research project, atmospheric activity concentrations of 129 I and 137 Cs and their activity ratios were obtained for 920 SPM samples. Discussion related to some of those samples was made in the research article entitled "Time-series variations of atmospheric 129 I concentrations and 129 I/ 137 Cs ratios in eastern Japan just after the 2011 nuclear accident in Fukushima, Japan" [1] , where 363 sets were presented. The remaining 557 data are presented in this article, so this data article makes up for the original research article [1] . This article contains four tables ( Tables 1 -4) and one figure ( Fig. 1 ). Table 1 summarizes sampling  information for the SPM samples. A similar table appears as Table 1 in the research article [1] , but Table 1 of this article provides detailed information. For instance, each location is specified in terms of a set of longitude and latitude. Besides, a flow (suction) late (in L/min) and the number of SPM samples analyzed also are given for each sampling site. A flow rate is from 15 to 18 L/min, with 18 L/min being a standard rate. With one hour suction, a flow rate of 18 L/min corresponds to 1.08 m 3 /h for one SPM sample. Atmospheric SPM samples were collected with two types of filter materials (glass fiber (GF) and polytetrafluoroethylene (PTFE)). Among the 41 sampling sites where the SPM samples were collected, GF and PTFE filters were used at 31 and 10 sites, respectively. Tables 2 and 3 list hourly atmospheric radioactivity concentrations (in Bq/m 3 ) of 129 I and 137 Cs and their radioactivity ratios (in Bq/Bq) for SPM samples collected with the use of GF filter and PTFE filter, respectively. The activity concentration data of 137 Cs are from Oura et al. [5] and Tsuruta et al. [6] . Numerical values in Table 2 are given either in bold or in italics. Data in bold were judged to be free from cross contamination of radioactive nuclides and reliable enough for scientific discussion, while those in italics are suspected to be cross-contaminated and should be regarded as inaccurate values. Among the data in Table 2 for 445 SPM samples, 338 and 107 data are in bold and in italics, respectively. In Table 3 , where 112 data are presented, besides the above-mentioned two categolized data (accurate and inaccurate data in bold and italics, respectively), the third group of data are indicated in italicized bold for 129 I activity concentrations and 129 I/ 137 Cs activity ratios of such SPM samples whose 137 Cs activity data were judged to be accurate (shown in bold), being exempted from cross-contamination. In comparing 129 I activity concentration values determined from GF filter-collected SPM with those from PTFE-collected samples, it was found that PTFEcollected SPM samples were systematically lower than those from GF-collected SPM samples. Therefore, the third-categolized values in Table 3 were to be regarded as reference values [1] . In Table 4, absolute values of the 129 I radioactivity (in Bq) for four kinds of blank samples are summarized. These are reagent blanks (a), procedure blanks including reagent blanks (b) and filter blanks. For filter blanks, two kinds of filter blank values were obtained for brand-new filter (c) and used-filter (d). Used-filter samples were taken from open (marginal) space between        a Values in bold are judged to be free from cross-contamination and used in the discussion, while those in italics are suspected to be cross-contaminated and should be regarded as inaccurate values. A number of aEb denotes ax10 b .Note that the data may be revised after re-checking in the future. b Corresponding to the numbers shown in Fig. 1     a Values in bold are judged to be free from cross-contamination, while those in italics are suspected to be crosscontaminated and should be regarded as inaccurate values.Values in bold and itarics are regarded as reference values. A number of aEb denotes ax10 b .Note that the data may be revised after re-checking in the future. b Corresponding to the numbers shown in Fig. 1  two sets of consecutive 24 spots (for one day). In Table 4(a) and (b) , experimental run numbers (run #), iodine ( 127 I) mass used as carriers (in mg) and 129 I activity per unit mass of iodine (in Bq/mg I) are given. In Table 4(c) , in addition to iodine carrier mass (in mg) and 129 I activity per unit iodine mass (in Bq/mg I), blank values not only for filters used for collecting SPM but also those for collecting APM [2] are given. In Table 4(d) , SPM sampling sites, run #, sampling date, carrier mass of iodine (in mg) are given. Values of the 129 I activity (in Bq) in Table 4(c) and 4(d) include procedural blank values. Fig. 1 shows a relationship between used-filter blanks (spot

Samples
Sampling sites of the SPM samples analyzed for the 129 I activity are listed in Table 1 and illustrated in Fig. 1 of the research article [1] . An SPM sample consists of airborne particulate matter in aerosol ( < 10 μm) and a filter material (either glass fiber (GF) or polytetrafluoroethylene (PTFE) filter) on which aerosol is accumulated. Each (whole) SPM sample contains aerosol in the atmosphere of about 1 m 3 , accumulated for one hour. The SPM samples analyzed were chosen from those once used for non-destructive determination of the 137 Cs radioactivity [ 5 , 6 ]. Among them, such samples as those having relatively high 137 Cs radioactivity concentrations (about 1 Bq/m 3 and higher) were chosen for the 129 I activity measurement. A part of the whole   circler SPM spot (11 mm; 95 mm 2 ) was used for the determination of the 129 I radioactivity.
Usually, a quarter part ( ∼24 mm 2 SPM spot on ∼1 x 1 cm filter) was used. For some cases, especially when the 129 I activity was assumed to be considerably high based on the pre-measured 137 Cs activity, one eighth was used. Cutting of an SPM sample with filter was done manually with scissors. Prior to the 129 I determination, each cut piece of the SPM samples was subjected to gamma-ray spectrometry using Ge semiconductor detectors for determining the 137 Cs activity, followed by radiographical imaging using an imaging plate for identifying the location of beta-ray emitters (mainly 137 Cs).

Sample Preparation and Processing for AMS
For quantitative determination of the 129 I in SPM samples by AMS, 129 I was chemically separated. Chemical separation procedures for 129 I in SPM were modified from those developed for the determination of trace halogens (chlorine, bromine and iodine) in silicate rock samples including meteorites (e.g., [ 7 , 8 ]). The procedures are essentially the same as those described in [2] and briefly summarized as follows. A cut piece of each SPM filter (a square filter of ∼1 x ∼1 cm with a quadrant of the SPM disk (11 mm for a typical case)) was placed on the bottom of a Ni crucible, in which a known amount KI ( ∼ 1 or 2 mg of 127 I) as iodine carrier was taken in solution, alkalized with concentrated NaOH solution and dried on a hot plate, soaked with concentrated NaOH solution and fused with ∼ 1 g of NaOH pellets on a Meker burner. After heating gently for two minutes and, then, intensively for five minutes, the fusion cake formed was disaggregated with water. With centrifugation, the supernatant solution containing 129 I was separated and neutralized with 6 M HNO 3 with a certain amount of Na 2 SO 3 as reductant. By adding AgNO 3 solution drop by drop to the solution, AgI was precipitated and subjected to AMS. For each analytical run, blank samples of reagents and the whole procedure were prepared and chemically processed along with SPM samples for monitoring the blank contributions of 129 I. For the reagent blank samples, the same amounts of KI and Na 2 SO 3 as used for SPM samples were taken. Reagent blank values are summarized in Table 4(a) . For the procedure blank samples, the same chemical procedure as applied to SPM samples was performed with the use of chemicals but no filters. Procedure blank values are summarized in Table 4(b) . Note that those values include reagent blanks. In addition to reagent and whole procedure blanks, blank contributions from the filter material were monitored. There were two types of filter blanks; 129 I in brand-new filter and 129 I in used-filter. For the latter case, an open (marginal) part of the usedfilter was sampled with one cut piece per one filter roll. The size of each sample subjected to the AMS analysis was adjusted to that of an SPM sample having a quadrant of each SPM spot ( ∼1 x 1 cm as a filter size). These blanks were called spot blanks. Filter blanks for bland-new filters and used-filters are given in Table 4(c) and Table 4(d) , respectively.

Accelerator Mass Spectrometry (AMS)
The 129 I activity in each SPM sample was determined based on an isotopic ratio of 129 I/ 127 I determined by AMS and an amount of 127 I carrier added into a crucible before the alkaline fusion. AMS was performed using a tandem accelerator at the MALT facility of the University of Tokyo [9] . For obtaining reliable values of the 129 I/ 127 I isotopic ratio by the MALT AMS, an isotopic ratio of 129 I/ 127 I is desired to fall between 10 −14 and 10 −8 . With the use of 1 to 2 mg of 127 I as a carrier of iodine, most SPM samples analyzed have isotopic ratios of 129 I/ 127 I within this range and, therefore, no further procedure for adjusting the isotopic ratio of iodine was needed.

Ethics Statements
There are no matters that are ethically problematic.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships which have, or could be perceived to have, influenced the work reported in this article.