ارزیابی غلظت بریلیوم-7 و رابطه آن با متغیرهای جوی (مطالعه موردی: شهر تهران)

Introduction: The present study was conducted to assess the concentration of radionuclide beryllium-7 in the atmospheric air of Tehran and to investigate its relationship with key atmospheric variables including temperature, relative humidity, wind speed, and precipitation. Beryllium-7, as a natural tracer of cosmic origin, is considered a valuable tool for studying atmospheric dynamic processes and vertical exchanges in the troposphere. This study was designed and implemented in 2022 with the aim of filling the information gap in the field of atmospheric studies of natural radioactivity in the country, especially in the Tehran metropolitan area, which faces serious air pollution challenges.
Materials and methods: The study area is Tehran, with a geographical location of 34 to 36.5 degrees north latitude and 50 to 53 degrees east longitude, which is limited to the Alborz highlands from the north and the central desert of Iran from the south. Considering technical and time constraints, the sampling station was selected on the premises of the Geophysical Research Institute (located on North Kargar Street, coordinates 51.23 degrees North and 35.44 degrees east, altitude 1419 meters above sea level). Sampling of total suspended particulate matter (TSP) was carried out using a high volume sampler at an altitude of 8 meters above the ground. Sampling was carried out for 6 hours with an average flow rate of 1.5 cubic meters per minute and on fiberglass filters with dimensions of 20 by 25 centimeters and an efficiency of 99.99%. Field operations were carried out in four seasons of the year and on average five times per month (with random timing during the night and day hours). Simultaneously with sampling, environmental parameters such as temperature, relative humidity, and wind speed were measured at the station location. Beryllium-7 concentration was determined using a high-purity germanium (HPGe) coaxial gamma detector and measuring gamma photons emitted with energy of 477 keV (with an efficiency of 11%) on solid filter samples, without the need for chemical preparation.
Results and discussion: The results showed that the concentration of beryllium-7 in the Tehran atmosphere has significant seasonal fluctuations. The total average concentration of this radionuclide during the study period was calculated to be 4.4±0.87 mBq/m3. The maximum monthly average concentration of beryllium-7 was recorded in October with a value of 22.6±0.12 mBq/m3 and its minimum in February with a value of 25.2±0.42 mBq/m3, indicating an increase in the concentration in the hot and dry seasons and a decrease in the cold and humid seasons. There is a nonlinear but physically meaningful pattern between temperature and beryllium-7 concentration. In the warm months of the year (spring and summer), the increase in temperature was accompanied by increased atmospheric instability and growth in the height of the mixing layer, leading to a more efficient transport of beryllium-7 from the upper layers of the atmosphere to the ground surface and, as a result, an increase in its concentration. A clear inverse relationship was observed between relative humidity and beryllium-7 concentration. In the months with high humidity (autumn and winter), the efficiency of removal processes (especially wet deposition and hygroscopic growth of particles) increased, reducing the residence time and concentration of beryllium-7 in the atmosphere. There is a relatively weak and dual relationship between wind speed and beryllium-7 concentration. In some cases, an increase in wind speed was accompanied by an increase in concentration (due to enhanced mixing and transport from distant areas), and in some cases it led to dilution and a decrease in concentration. Although precipitation is known to be an effective factor in the removal of particulate matter, the results of this study showed that it does not have a direct and significant effect on changes in the concentration of total particulate matter and beryllium-7. The mean total particulate matter (TSP) concentration fluctuated significantly during the sampling period; its maximum was recorded in October (177.6±10.2 mBq/m3) and its minimum in May (110.6±10.6 mBq/m3). Pearson correlation analysis between total particulate matter concentration and beryllium-7 concentration indicated a positive correlation of moderate strength (r=0.55), which was significant at the 90% confidence level (p=0.066), although not at the 95% confidence level. This positive relationship is physically attributed to the preferential binding of beryllium-7 to fine aerosol particles, but the lack of complete statistical significance and the scatter of the data suggest that more dynamic atmospheric processes (such as vertical mixing and removal efficiency) play a more decisive role in controlling the seasonal fluctuations of beryllium-7.
Conclusion: The present study clearly showed that the concentration of beryllium-7 in Tehran air is influenced by a combination of meteorological factors and its seasonal variations are mainly controlled by atmospheric dynamic processes such as vertical mixing, atmospheric stability, and wet deposition efficiency. Although there is a moderate positive correlation between beryllium-7 concentration and particulate matter, beryllium-7 cannot be considered as a function of particulate matter mass alone. Local climatic conditions (such as temperature inversion in winter) play an inhibitory role in the vertical transport of beryllium-7 and lead to a decrease in its concentration at the ground surface. This study also emphasizes the importance and necessity of conducting local research in the field of atmospheric radioactivity, taking into account the technical and instrumental limitations available in the country, and shows that accurate knowledge of beryllium-7 behavior can be used as an efficient tool to better understand the processes of transport and removal of pollutants in the urban atmosphere of Tehran.