Diurnal, monthly, and seasonal variations of indoor radon concentrations concerning meteorological parameters

As reported by the Turkish Atomic Energy Agency (formerly TAEK, newly TENMAK), Izmir province has higher indoor radon concentrations compared to other cities in Turkey. Since modern people spend 92% of their daily time indoors, it is important to know indoor radon levels and long-term variation. However, our knowledge of indoor radon levels of Izmir and its surrounding are limited. Moreover, there is no information about this area’s large-term variation of indoor radon. In this study, which was carried out with this motivation, indoor radon concentrations and meteorological parameters were measured in an office of the teaching staff in a university building. Data were collected hourly over 25 months (762 days). Raw data, diurnal, monthly, and seasonal variations of parameters were investigated separately. The results show that the average indoor radon concentration (18 Bq m−3) is relatively lower than national and international reference values. Indoor radon concentrations showed an increasing and decreasing trend throughout the day. Radon concentrations are slightly higher in the morning (downtime and early hours of the day) and then reduced in the afternoon. This can be related to the daily routine usage of the office, which is affected by ventilation of the room, air temperature variations, etc.


Introduction
As an odorless, tasteless, colorless, radioactive, and noble gas, radon ( 222 Rn) is formed by the decay of 226 Ra, a member of the 238 U decay chain. Radon is produced naturally within the earth's crust, moves to soil pores, and then migrates to the atmosphere from the earth's crust. This migration will be completed with the penetration of radon into the building. Radon moves into dwellings through micro-cracks, voids, poorly isolated wall joints, and air-pressure differences (Barca et al., 2021;Günay et al., 2018;Hatungimana et al., 2020;Park et al., 2018;Semwal et al., 2019;Tabar et al., 2013;Ülküm et al., 2018;Yarar et al., 2014). The average radon concentrations Abstract As reported by the Turkish Atomic Energy Agency (formerly TAEK, newly TENMAK), Izmir province has higher indoor radon concentrations compared to other cities in Turkey. Since modern people spend 92% of their daily time indoors, it is important to know indoor radon levels and longterm variation. However, our knowledge of indoor radon levels of Izmir and its surrounding are limited. Moreover, there is no information about this area's large-term variation of indoor radon. In this study, which was carried out with this motivation, indoor radon concentrations and meteorological parameters were measured in an office of the teaching staff in a university building. Data were collected hourly over 25 months (762 days). Raw data, diurnal, monthly, and seasonal variations of parameters were investigated separately. The results show that the average 1 3 Vol:. (1234567890) worldwide in outdoor and soil airs are 10 and 10,000 Bq m −3 , respectively (Podstawczyńska, 2015).
It is a fact that radon is one of the natural ionizing radiation sources, and by inhalation, it enters the human lungs and can lead to severe diseases such as lung cancer. Many studies have reported the relationship between indoor radon exposure and lung cancer, and it is estimated that exposure to high radon levels is at least 10% of lung cancer cases in the USA. IARC has defined radon gas as a human carcinogen because its progeny emits radioactive alpha particles capable of causing lung cancer (Bräuner et al., 2012;Şen et al., 2013;Yarar et al., 2014). It is classified as the second cause of lung cancer after smoking (Baltrenas et al., 2020). In addition, radon has been implicated as the primary cause of lung cancer in never-smokers (Di Carlo et al., 2019). Therefore, indoor radon levels become more important since modern people spend 92% of their daily time indoors. However, there are limited studies on indoor radon levels in Turkey (Celik et al., 2008;Degerlier & Celebi, 2008;Kapdan & Altinsoy, 2014;Sogukpinar et al., 2014;Tabar et al., 2018). One of these studies about the radon levels of Turkey is a technical report prepared by TENMAK (formerly TAEK) that shows İzmir and its surroundings have relatively higher indoor radon concentrations compared to other provinces (TAEK, 2014). In this report, indoor radon concentrations ranged from 31 to 280 Bq m −3 with an arithmetic mean of 114 Bq m −3 . Similarly, a few studies on this subject around İzmir (Özbay & Karadeniz, 2016;Özbay et al., 2021;Şen et al., 2013;Yarar et al., 2014) and these works focused on the evaluation of indoor radon levels in terms of received radiation doses. Şen et al. (2013) showed the highest indoor radon level as 707.8 Bq m −3 . In another work, indoor concentrations were measured between 39.3 and 405 Bq m −3 for houses (Yarar et al., 2014). Although this area (İzmir and its surroundings) has relatively high radon concentrations, studies on the variation of long-term variation of indoor 222 Rn concentrations are non-existent for İzmir and its surroundings.
Its noble gas character and radioactive decay properties make radon a unique tracer for monitoring temporary and periodic variations of natural processes (Steinitz et al., 2013). It is reported that indoor radon concentrations exhibit significant spatial and time-scale variations. Environmental factors (geological structure, radon concentrations of soil pores, permeability and porosity of soil, meteorological parameters) and factors related to buildings (building construction, ventilation system, occupants' activities-number of windows, periodicity of window, door opening/closing activities, etc.) have influence on radon variation characteristics (Antignani et al., 2021;Baltrenas et al., 2020;Ivanova et al., 2019;Murty et al., 2010;Podstawczyńska, 2015). Air exchange between outdoors and indoors is a significant factor for indoor radon variations (Smetanová et al., 2017). Long-term indoor air Rn-222 concentration measurements have become very popular and well documented in recent decades. Although the study area has relatively high radon concentrations, there is no study conducted to monitor the changes in radon concentrations in the longterm period (diurnal, monthly, and seasonal variations). Since studies on indoor radon levels in the study area were very few so far, it was logically felt that there was a need for a detailed study aimed at establishing baseline data on long-term variation of indoor 222 Rn levels. Therefore, a study on these aspects was initiated. This paper reports the initial results of the variability of hourly indoor air radon (Rn-222) concentration in indoor air in İzmir, which is the third most populous city in Turkey, after Istanbul and Ankara. For this purpose, an office in the two-story university building in Bornova-İzmir/Turkey was studied between January 2019 and February 2021, considering diurnal, monthly, and seasonal variations of radon time series.

Study area
The measurements were performed in the Bornova distinct of Izmir. Bornova is located at the western end of Anatolia which is under the control of north-south extension, east-west movement, parallel grabens, intervening horsts, and associated normal faults. The Anatolian plate moves approximately 20 mm/year in the east-west direction, and this movement turns into a north-south extension in Western Anatolia (Aktuǧ & Kiliçoǧlu, 2006;Ocakoglu & Demirbag, 2005). Therefore, the study area has very active tectonics causing hundreds of moderate-and high-magnitude earthquakes.

Description of the building
The two-storey university building where we studied the indoor radon concentrations is in Bornova ( Fig. 1), the third most populated metropolitan district of Izmir. The total population of Bornova district is almost 450,000, while it is 4.5 M for Izmir city (ABPRS, 2018;İçhedef, 2019). The district's climate is entirely different from the İzmir city center because it is far from the seacoast and surrounded by mountains. The weather is dry and hot in the summer months, while the winters are harsh in Bornova.
The Institute of Nuclear Science was established in 1983 and moved to its new building in 2012. The building is a reinforced concrete construction, and it has two floors with a basement (Fig. 2). There is 11 storage on the basement floor, and 32 rooms on the first floor (27 offices, 3 classrooms, 3 meeting rooms, and 2 student rooms), and 37 rooms on the second floor (27 laboratories, 3 offices, 1 waiting room, and 1 canteen). An office of teaching staff located on the second floor was selected for indoor radon measurements. The cubic volume of the office is about 72 m 3 (24 m 2 floor area and 3 m height). It has three windows opened at least once a day to ventilate the room. The frequency of opening the windows varies depending on the season.

Measurements of indoor radon and meteorological parameters
We performed 25 months (762 days) of indoor radon concentration measurements, which began in January 2019 (01/01/2019) and ended in February 2021 (02/01/2021) and took place in the office of the teaching staff of the Institute of Nuclear Sciences, Ege University. Two people occupied the office through the measurement period. The radon concentrations of the office were measured using a commercially available radon monitor (Airthings Corentium Plus). The detection limit of the radon monitor is between 6.5 and 50 kBq/m 3 (Curado et al., 2019;Galli et al., 2019;Noverques et al., 2020;Özden & Aközcan 2022). The radon monitor was situated at 1 m away from windows and doors remote from the direct impact of external air. The measurements were taken on an hourly basis.

Statistical analysis
The workflow in Fig. 3 summarizes the statistical analysis stages of the obtained data. In the first step, Fig. 1 Measurement location, Institute of Nuclear Sciences, Ege University Campus, Bornova, İzmir (is indicated by the red arrow) raw data of radon and meteorological parameters were collected. Then, data were analyzed in three steps: diurnal, monthly, and diurnal variations.
The statistical analysis was conducted in R and Rstudio (Team R. C., 2020;RStudio Team, 2019) and figures were produced using the package ggplot2 (Wickham et al., 2019). Descriptive statistics for parameters (radon, indoor air temperature, indoor air pressure, and humidity) are given in Table 1. The missing data corresponds to measurements below the detection limit.

Results and discussion
The frequency distribution of the indoor radon concentrations is given in Fig. 4. It is fitting to a lognormal distribution.
The present data analysis showed that the indoor radon activity concentration varied from 6.5 to 151.1 Bq m −3 with a mean value of 18 Bq m −3 during the study period (Table 1). UNSCEAR, 2000 has reported 40 Bq m −3 as the arithmetic means for the distribution of indoor radon concentration. The European Commission proposed that the action level is 200 Bq m −3 for new buildings (Clouvas et al., 2011). Turkey Atomic Energy Authority published a directive on radiation safety which includes the action level of 400 Bq m −3 and 1000 Bq m −3 for houses and workplaces, respectively (TAEK, 2000). It is seen that the radon data is lower than the national and international reference values. On the other hand, the results obtained from this study are considerably lower than the radon levels stated in the technical report published by TAEK (2014). These results can be attributed to the fact that the building is newly constructed, the room is well ventilated during the day, and many more. It should be noted that the room was selected because it is an office of teaching staff which frequently is used by staff and students. Consequently, a natural air exchange occurs with the frequent opening of the door. It is well known that the primary factors that affect indoor radon concentrations are ventilation, season, height, building, age, and building material (Nazaroff et al., 1989). The concentrations are expected to be relatively low since the office where the measurements are made is located on the second floor.

Diurnal indoor radon variations
This section considers discussions on the diurnal variations of indoor radon concentration at the teaching  Fig. 5, a violin chart is plotted to show the variation of hourly radon concentrations. It is seen in the graph that the extreme values increase due to the emergence of outliers, especially in the morning hours. It was noteworthy that indoor radon concentrations changed at different times of the day. Additionally, concentrations are measured slightly higher in the morning (downtime and early hours of the day) and then decreased in the afternoon. On the other hand, it can be seen from the graph that the distributions differ significantly at different times of the day. The shape of the violin becomes thinner and longer between 3 and 9 am, while at other hours, the shape of the violin becomes more rounded. This difference may be due to office hours. The office remains closed at night and midnight without any regular ventilation, and radon concentrations are likely to be measured over a broader range. Pant et al. (2016) reported an increase in radon concentration from evening to early morning until it reaches its maximum value. Murty et al. (2010) mentioned that radon concentrations are higher in the early morning hours and decrease after that and reach the minimum value during the early afternoon hours. Moreover, they pointed out that radon is strongly dependent on atmospheric pressure, relative humidity, and air temperature.
The median and mean values were not equal for most of the hours. The distribution spreads with the increase of outliers, especially in the morning hours. The slow variation of indoor radon (higher in downtime, and early hours of the day, and then decreased in the afternoon) throughout the measured period shows similarity to the results of some researchers (Chen et al., 2016;Murty et al., 2010;Pal et al., 2015;Pant et al., 2016;Xie et al., 2017). This variation may be affected by the ventilation differences and the gap between indoor and outdoor temperatures.

Monthly variations of indoor radon and meteorological parameters
The monthly averages of indoor radon concentrations, air temperature, relative humidity, and air pressure obtained within the 25 months are presented in Table 2. The monthly variations of each parameter were calculated by calculating the average of total monthly data. As can be seen from Table 2, indoor radon concentrations decrease in the spring and summer months of both years (2019 and 2020). This decline is apparent in June and July. It is thought that air circulation in the room increases with air conditioners in the office with the increasing temperatures. This view is consistent with the fact that there is no Fig. 5 Violin plots of hourly indoor radon concentrations significant increase in office temperatures as in outdoor temperatures. Likewise, the humidity values measured inside the room during these months also increase.
On the other hand, radon concentrations are mostly higher in October, November, and December compared to other months. During these periods, the lack of air conditioners, the fact that the office's central heating starts to work, and the doors and windows are less open could be the reason for the high levels. Researchers suggest that the seasonal patterns of radon might change depending on the location of the measurements and their surroundings (Hirsikko et al., 2007;Victor et al., 2019).

Seasonal variations of indoor radon and meteorological parameters
Since the study began in January, the 2019 winter season was calculated by taking the average of January and February. Similarly, the winter season of 2021 is calculated by taking the average of December 2020 and January 2021. Therefore, 2-month data were made for winter season calculations for 2019 and 2021. The other seasons were calculated using the three months' data that formed those seasons. The seasonal variation of indoor radon concentrations is figured in a ridgeline chart (Fig. 6) which allows for studying the distribution of a numeric variable for several groups.
The ridgeline chart shows that indoor radon concentrations vary in the broader range in autumn and winter than in other seasons. Maximum radon levels in spring and summer do not exceed 100 Bq m −3 . It can be seen from the graph that the seasonal distributions are almost similar, but the distributions do not fit the normal distribution. In addition, the distributions from a fluctuating graph consist of several peaks and different distributions overlapping. An exciting outcome of this chart is a comparison of the consecutive years. Although the concentration ranges and the shapes of the distributions are similar, radon concentrations showed sudden increases and decreases in 2020. Therefore, in 2020, it was observed that the distribution did not have a single peak but more than 4 small peaks. This dynamism continued in the winter season of 2021 as well. An interesting difference is seen when comparing the years. For most months of 2020, radon levels are higher than in 2019. Here, it is useful to examine the meteorological parameters. For example, contrary to radon, it is seen that atmospheric pressure values in most months in 2019 are higher than in 2020. The increase in atmospheric pressure prevents the vertical movement of radon gas that enters the building.

Conclusion
As reported by the Turkish Atomic Energy Authority (formerly TAEK, new TENMAK), the province of Izmir has higher indoor radon concentrations compared to other cities in Turkey. Since modern humans spend 92% of their daily time indoors, we need to know indoor radon levels. For this reason, in this study, İzmir and its surroundings, where we have limited information about indoor radon levels, were selected. The tectonic features of the study area are another reason that makes this study interesting. Many earthquakes occur in this region each year and this must be a reason for the relatively higher radon emissions. So far, the studies carried out in this region are on the determination of radon levels and the calculation of the radiation dose received by people. The current study reports the results of indoor radon level dependencies on various indoor parameters for an office of a two-storey university building. We focused on interpreting diurnal, monthly, and seasonal variations of a radon time series. Diurnal variations of indoor radon concentrations are measured slightly higher in the morning (downtime and early hours of the day) and then decreased in the afternoon. Monthly average radon concentrations declined in the June and July of both years (2019 and 2020). The opposite results were obtained in October, November, and December. Seasonal indoor radon concentrations varied more extensively in autumn and winter than in other seasons. It is also noted that seasonal distributions are almost similar, but they do not fit the normal distribution. The study is preliminary work on clarifying indoor radon variations and meteorological parameters. Results revealed that it is essential to assess variations of indoor radon and the effect of environmental factors. Since the current study was carried out in a single office, it was not possible to compare different offices on the same floor and offices on different floors. We plan to take long-term measurements in more offices and different floors, and compare and evaluate the outputs obtained from this study. Also, a comparison should be done between new and older buildings. This type of comparison will be uncovered influences of differences in the ages of buildings and more interpretations can be possible. In a conclusion, this study reported initial results on diurnal, monthly, and seasonal variations of indoor radon levels. The outputs are very significant for understanding indoor radon variation around the very active tectonic area.