Understanding Temporal Variations of Atmospheric Radon-222 around Japan Using Model Simulations

Temporal variations of atmospheric radon-222 (Rn-222) observed at four Japan Meteorological Agency stations in Japan by the Meteorological Research Institute were analyzed using an online Global Spectral Atmosphere Model-Transport Model (GSAM-TM). Monthly and diurnal variations and a series of synoptic high-Rn-222 events were extracted from 5 years to 12 years of Rn-222 observations during 2007-2019. Observed seasonal patterns of winter maxima and summer minima, driven mainly by monsoons, were well reproduced by the GSAM-TM based on existing Rn-222 emission inventories, but their absolute values were generally underestimated, indicating that our understanding of Rn-222 emission processes in East Asia is lacking. The high-resolution model (- 60 km mesh) demonstrated that observed consecutive high-Rn-222 peaks at several-hour timescales were caused by two Rn-222 streams from different regions and were not well resolved by the low-resolution model (similar to 200 km mesh). GSAM-TM simulations indicate that such cold-front-driven events are sometimes accompanied by complicated three-dimensional atmospheric structures such as stratospheric intrusion over the front, significantly affecting distributions of atmospheric components. A new calculation approach using hourly Rn-222 values normalized to daily means was used to analyze the diurnal Rn-222 cycle, allowing diurnal cycles in winter to be extracted from Rn-222 data that are highly variable due to sporadic continental Rn-222 outflows, which tend to obscure the diurnal variations. Normalized diurnal cycles of Rn-222 in winter are consistent between observations and model simulations, and seem to be driven mainly by diurnal variations of planetary boundary layer height (PBLH). These results indicate that Rn-222 in the near-surface atmosphere, transported from remote source regions, could vary diurnally by up to 10 % of the daily mean mainly owing to local PBLH variations, even without significant local Rn-222 emissions.