Linear relations among atmospheric radon concentration, air temperature, and net radiation in nighttime, and their implications

In this presentation, we compare atmospheric radon concentration with meteorological conditions such as air temperature and net radiation in nighttime to clarify their quantitative relations. The monitoring was carried out on bottom of valley (with depth of about 200 m) near coastal line of the Oshika Peninsula, northeastern Japan. The detectors were a diffusion-type ionization chamber for radon concentration, a Pt100 sensor for air temperature, and thermopile-type (copper-constantan) net radiometer for net radiation, all of which were mounted on the roof top (4 m above the ground) of the monitoring station. The radon concentration was sampled every an hour and air temperature and net radiation were every ten minutes.

We analyzed the data during a nocturnal period defined as one when negative net radiation was observed. The result shows that atmospheric radon concentration increases linearly following the change of air temperature from sun set. In addition, the nocturnal increase is found to be negatively correlated with net radiation. These linear relations being realized simultaneously suggest that the nocturnal trend of radon concentration is evaluated from a parameter, air temperature difference divided by net radiation.

In nighttime, stable layer is formed and the atmospheric condition is stabilized. The height of the stable layer is characterized by total amount of net radiation divided by air temperature difference. According the previous study by Italian researcher, the height is also positively correlated with effective mixing height determined from radon concentration and exhalation rate. This means that effective mixing height is related to air temperature difference and net radiation, both of which in turn characterize the nocturnal trend of radon concentration. Based on these discussions, the slopes of the linear trends mentioned above are considered to depend on radon exhalation rate.

In summary, atmospheric radon concentration increases following the formation of nocturnal stable layer and is evaluated from air temperature difference and net radiation.