Radiation Protection Dosimetry Advance Access published May 1, 2015 Radiation Protection Dosimetry (2015), pp. 1–5

doi:10.1093/rpd/ncv238

AN EXPERIMENTAL METHOD FOR QUANTITATIVELY EVALUATING THE ELEMENTAL PROCESSES OF INDOOR RADIOACTIVE AEROSOL BEHAVIOR

*Corresponding author: [email protected] An experimental method for quantitatively evaluating the elemental processes governing the indoor behaviour of naturally occurring radioactive aerosols was proposed. This method utilises transient response of aerosol concentrations to an artificial change in aerosol removal rate by turning on and off an air purifier. It was shown that the indoor–outdoor exchange rate and the indoor deposition rate could be estimated by a continuous measurement of outdoor and indoor aerosol number concentration measurements and by the method proposed in this study. Although the scatter of the estimated parameters is relatively large, both the methods gave consistent results. It was also found that the size distribution of radioactive aerosol particles and hence activity median aerodynamic diameter remained not largely affected by the operation of the air purifier, implying the predominance of the exchange and deposition processes over other processes causing change in the size distribution such as the size growth by coagulation and the size dependence of deposition.

INTRODUCTION

METHODS

Concentration and size distribution of radioactive aerosol particles are the most fundamental factors in determining the internal dose due to inhalation of radon decay products(1). From the lessons learnt from the Fukushima Daiichi Nuclear Plant accident, indoor behaviours of radionuclide-attached aerosol particles must be known to evaluate the sheltering effects of dwellings and buildings. They include indoor–outdoor exchange, formation of unattached fraction by decay of precursors, attachment to aerosol particles, plate-out or deposition to surfaces and loss by decay(2). The purpose of this study is to develop an experimental method for quantitatively evaluating the elemental processes determining the indoor behaviour of radioactive aerosols. Particle size distributions can be obtained with a low-pressure cascade impactor combined with the authors’ imaging plate method for alpha particle counting(3, 4). Although the concentrations strongly reflect the aforementioned processes, it is not simple to quantitatively deduce the magnitude of each process. The idea used in the present study is to analyse the transitional changes in the concentrations and the particle size distributions after artificial changes in the conditions governing the processes. In this study, an air purifier was used. This idea can be implemented by using a relatively simple working model that numerically describes the key processes to calculate the concentrations. In this article, experiments and analysis on the indoor–outdoor exchange process and the deposition process are mainly discussed.

Indoor–outdoor exchange Concentrations of aerosol particles in the size ranges of 0.3–0.5, 0.5–1.0, 1.0–3.0 and 3.0– 5.0 mm were measured continuously every 4 min in a room on the fifth floor (about 15 m above the ground level) of a six-story building of Nagoya University and in the outdoor air just outside of the closed window of the room. The room had a floor area of 7.5`  7.5 m2 and a ceiling height of 3.2 m and was used as an experimental space for electronic devices (referred to as Room A). The ventilation and air conditioning were kept turned off during the measurement. The measurement was made during the period of 3– 5 December 2013 with a laser particle counter (Kanomax Model 3886). In addition to the measurement described earlier, a long-term continuous measurement was conducted with the same instrument at the same locations for a 1-year period from June 2012 except for a few months for equipment maintenance and intensified measurements. Indoor processes Each run of the measurement intended for the indoor process analysis was typically about 9 h long, during which aerosol particle concentrations for the aforementioned size ranges, radon concentration, temperature and humidity were continuously measured. The air purifier (Sharp, KC-C100) was operated during the period from 1 to 4.5 h after the start of the run at a flow rate of 0.085 m3 s21. Radioactive aerosol

# The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]

Downloaded from http://rpd.oxfordjournals.org/ at Washington University School of Medicine Library on November 15, 2015

H. Yamazawa*, S. Yamada, Y. Xu, S. Hirao and J. Moriizumi Department of Energy Engineering and Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan

H. YAMAZAWA ET AL.

Working model Temporal changes in number concentration of aerosol particles in air in a room can be expressed as dC ¼ S þ ke ðCo  CÞ  kd C  ka C dt

ð1Þ

where S is the source of aerosol except for the net gain by exchange with the outdoor, which is expressed by the second term on the right hand side, and Co is the outdoor concentration. The coefficients ke, kd and ka are the rate constants of indoor–outdoor exchange, deposition and artificial removal by the air purifier, respectively. Since the High Efficiency Particulate Air filter of the air purifier removes virtually all aerosol particles, the rate constant of removal by air purifier can be expressed as ka ¼ f =V, where f ¼ 0.085 m3 s21 is the flow rate of the air purifier, and V is the volume of the room.

the square sum of the difference between the calculated and measured indoor concentrations became minimal. An example of calculation result is shown in Figure 1. The calculation result well reproduced the large concentration changes observed. Similar good results were obtained for the other particle size ranges. These results are consistent with the findings of the preceding studies(5, 6). The rate constants obtained by applying the model to the measured results are summarised in Table 1. Although the exchange rate was found to be larger for the larger particles, reasons for this result are thus far unknown. The values are fairly close to the measured ventilation rate of (0.24+0.04) ` 10 –3 s21, which was determined for the room as the average of 36 experimental results of carbon dioxide release experiments. No significant correlation was found between the exchange rate and meteorological parameters such as wind speed. The deposition rate also shows clear size dependence within the measured size range with larger values for larger particles. This size dependence corresponds to the lower indoor-to-outdoor concentration ratio mentioned earlier. Indoor processes Non-radioactive aerosol An example of the time series of concentration of aerosol particles is shown in Figure 2. The concentration smoothly decreased within about an hour to a relatively constant value during the air purifier

RESULTS AND DISCUSSION Indoor–outdoor exchange The indoor concentration closely followed the outdoor concentration even during the rain periods when the outdoor concentration dropped very sharply. According to a statistical analysis, the time lag of the indoor behind outdoor concentration change is about 40 min, and the correlation coefficients are 0.94, 0.91, 0.90 and 0.85 for the size ranges of 0.3 –0.5, 0.5 –1.0, 1.0–3.0 and 3.0– 5.0 mm, respectively. The indoor-to-outdoor concentration ratio was found, as averages for the whole measurement period, to be 0.86, 0.75, 0.55 and 0.49, respectively, for the aforementioned size ranges. The working model was applied to the measured results to calculate the indoor concentration from the outdoor concentration. The rate constants of exchange and the deposition were determined so that

Figure 1. Measured and calculated aerosol concentration.

Table 1. The exchange and deposition rates of nonradioactive aerosol determined from the measured data. Diameter (mm) 0.3– 0.5 0.5– 1.0 1.0– 3.0 3.0– 5.0

Page 2 of 5

ke (1023 s21)

kd (1023 s21)

0.26 0.33 0.39 0.44

0.078 0.114 0.172 0.244

Downloaded from http://rpd.oxfordjournals.org/ at Washington University School of Medicine Library on November 15, 2015

particle size distribution was measured with the combination of a low-pressure cascade impactor (Tokyo Direc, LP-20RPS47) and an imaging plate system(3). The impactor sampling was carried out for 1 h periods typically starting from 0, 2, 3.5, 4.5 and 6 h after the start of the run. The first period was intended to capture the condition before the air purifier operation, the following two periods for the air purifier operation and the last two periods for recovery from the extremely low concentration made by the air purifier. Eight runs were carried out during the period from July to October 2013 in Room A and six runs during the period from October 2013 to January 2014 in Room B, which had dimensions of 4.7`  3.5 m2 horizontally and 2.6 m in height and which was used as an office.

ELEMENTAL PROCESSES OF INDOOR RADIOACTIVE AEROSOL BEHAVIOR

Figure 3. Temporal change in a counts in the particle size ranges (solid squares).

operation while it increased and levelled off after turning off the air purifier. Although similar trends were measured in all the runs, about a half of the runs had increasing or decreasing trends in the level-off periods, which were probably caused by the variations in the outdoor concentrations as shown in the previous section. If the measured data are as simple as shown in this figure, it is easy to determine the deposition rate by applying the working model described by Equation (1). By letting the left hand side be zero, one can obtain the equilibrium concentration

Table 2. The exchange and deposition rates or radioactive aerosol determined from the measured data.

Con=off

S þ ke Co ¼ ke þ kd þ ka

ð2Þ

where the subscript off corresponds to ka ¼ 0. By analytically integrating Equation (1), one can obtain a functional form of concentration variations, which shows asymptotic approaches to equilibrium levels both in the air purifier ON and OFF periods. For example, for the OFF period, the following equation holds C ¼ ðCon  Coff Þeðke þkd Þt þ Coff

ð3Þ

The concentrations calculated by the analytical solutions are depicted by dashed curves in Figure 2. The values of S þ ke Co and ke þ kd needed in the calculation with Equation (3) can be determined in a trialand-error way with eye fitting or a least square sum of deviation method.

Room

Deposition rate þ exchange rate (1023 s21) Radioactive ,0.11

A B

0.11–0.28

0.28– 1.0

0.51+0.25 0.52+0.24 0.33+0.14 0.57+0.19 0.54+0.16 0.50+0.18

Nonradioactive 0.3–0.5 mm 0.57+0.30 0.63+0.15

regarded as values nearly in proportion with it. The 12 stages of the impactor were grouped into three diameter ranges to reduce statistical errors in the counts. Results similar to Figure 3 were obtained in the other runs. Common features are as follows. The count rate decreased from its initial level before the air purifier operation to a fairly low level within 1.5 h. The low count remained unchanged during the air purifier operation. After the air purifier turned off, the count recovered rather slowly when compared with the number concentration aerosol particles. The slow recovery can be attributed to the time needed for the radon progenies to build up. A similar approach of analysis was taken for the aerosol particles associated with radon progenies. Equation (2) or (3) can be applied to find the parameters describing the indoor processes. By using the parameters determined, the working model was successful in reproducing the temporal variations for all the diameter ranges (Figure 3).

Radioactive aerosol An example of temporal change in a counts in the size ranges is shown in Figure 3. The time origin corresponds to the start of the air purifier operation. Although the a counts cannot be simply related to radon progeny concentration in air, they can be

Indoor deposition rate The deposition rate determined by the analysis described earlier is summarised in Table 2. The values in the table are the sum of the exchange rate and the deposition rate. In the discussion earlier, the exchange

Page 3 of 5

Downloaded from http://rpd.oxfordjournals.org/ at Washington University School of Medicine Library on November 15, 2015

Figure 2. Temporal variation in the concentration of aerosol particles. Dots are measured data and dashed line represents model calculation results.

H. YAMAZAWA ET AL.

Size distribution Size distribution of the radon progeny-attached aerosol particles was found not to change significantly by the operation of the air purifier. An example of size distribution is shown in Figure 4, which shows relative size distributions obtained before, during and after the operation. There is no significant difference in the distribution, while the absolute values of a count are substantially lower than those before and after air purifier operation as shown in Figure 3. This invariability in the relative size distribution was found, to some extent, in all the runs. The activity median aerodynamic diameter (AMAD) values were also not strongly affected by the purifier operation as shown in Figure 5. Before the air purifier operation, the AMAD of each run had a value raging widely from about 50 to 360 nm with a geometrical mean of 103 nm. These values are comparable or slightly smaller than the values obtained in the same and nearby rooms(7). During and after the operation, the AMAD value remained relatively unchanged or slightly increased as shown by geometrical mean values of 123, 120, 113 and 121 nm for the four following time points shown in Figure 5. The invariability of the size distribution and hence that of the AMAD, which are relatively strong enough to hold against the operation of the air purifier, can be interpreted as follows. It was found in the previous discussion that the number concentrations of radioactive and non-radioactive aerosol particles can reasonably be expressed by Equation (2). The rate constants determined in this study are corresponding to characteristic time scales of about 1 h, implying that the processes explicitly expressed in the working model fairly quickly attain their equilibrium states when compared with the time interval of the impactor measurement. The invariability can, therefore, be attributed mainly to the quick response of the system, and the size distribution is primarily determined by the nature of source including the size distribution in the outdoor environment. The latter point is indirectly supported by the results shown in Figure 1. The processes that are not included in the present working model are growth in size mainly caused by coagulation and size-dependent deposition efficiency. As

Figure 4. Measured particle size frequency distribution.

Figure 5. AMAD obtained in all runs. The lines connect the data obtained in the same run. The solid squares depict geometrical means at each sampling time.

far as the present results of the size distribution are concerned, these two factors seem not so substantial. The results shown in Table 2 support this consideration.

CONCLUSIONS It was pointed out that by analysing the transient changes in concentrations caused by air purifier operation, the indoor deposition rate could be estimated with a help of a simple working model. Although the scatter of the estimated deposition rates is relatively large, the values are consistent between radioactive and non-radioactive aerosols. The indoor–outdoor exchange rate was also found to be consistent with the value determined by a conventional method. It was also found that the size distribution and hence AMAD were not strongly affected by the air purifier operation, implying a predominant effects of source characteristics on the particle size distribution. REFERENCES 1. ICRP. Human Respiratory Tract Model for Radiological Protection. ICRP Pub. 66 (1993).

Page 4 of 5

Downloaded from http://rpd.oxfordjournals.org/ at Washington University School of Medicine Library on November 15, 2015

rate of Room A was analysed to be about 0.3`  1023 s21 for the 0.3–1.0 mm diameter range of non-radioactive aerosol and to have no significant dependence on meteorological parameters. Therefore, the deposition rate in the room is estimated to be about 0.2`  1023 s21 or less. Although the scatter of the data is relatively large, the deposition rate is in fair agreement with the results in Table 1. Size dependence is not clear due to the scatter, although the deposition rate of the 0.28 –1.0 mm size range seems a little smaller than the other ranges.

ELEMENTAL PROCESSES OF INDOOR RADIOACTIVE AEROSOL BEHAVIOR 5. Mostafa, A. M.A., Tamaki, K., Moriizumi, J., Yamazawa, H. and Iida, T. The weather dependence of particle size distribution of indoor radioactive aerosol associated with radon decay products. Radiat. Prot. Dosim. 146(1– 3), 19–22 (2011). 6. Papastefanou, C. Radon decay product aerosols in ambient air. Aerosol Air Qual. Res. 9, 385–393 (2009). 7. Moriizumi, J., Yamada, S., Xu, Y., Matsuki, S., Hirao, S. and Yamazawa, H. Indoor/outdoor radon decay products associated aerosol particle-size distributions and their relation to total number concentrations. Radiat. Prot. Dosim. 160(1– 3), 196–201 (2014).

Page 5 of 5

Downloaded from http://rpd.oxfordjournals.org/ at Washington University School of Medicine Library on November 15, 2015

2. Porstendo¨rfer, J., Zock, C. and Reineking, A. Aerosol size distribution of the radon progeny in outdoor air. J. Environ. Radioact. 51, 37– 48 (2000). 3. Rahman, N. M., Iida, T., Saito, F., Koarashi, J., Yamasaki, K., Yamazawa, H. and Moriizumi, J. Evaluation of aerosol sizing characteristic of an impactor using imaging plate technique. Radiat. Prot. Dosim. 123(2), 171–181 (2006). 4. Rahman, N. M., Iida, T., Yamazawa, H., Yokoyama, S., Moriizumi, J., Saito, F. and Ito, T. The study of activity median aerodynamic diameter using imaging plate technique for assessment of effective dose from radon and its decay products. Radiat. Prot. Dosim. 124(4), 360–371 (2007).

An experimental method for quantitatively evaluating the elemental processes of indoor radioactive aerosol behavior.

An experimental method for quantitatively evaluating the elemental processes governing the indoor behaviour of naturally occurring radioactive aerosol...
222KB Sizes 0 Downloads 4 Views