2022 Volume 100 Issue 2 Pages 343-359
Temporal variations of atmospheric radon-222 (222Rn) 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-222Rn events were extracted from 5 years to 12 years of 222Rn 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 222Rn emission inventories, but their absolute values were generally underestimated, indicating that our understanding of 222Rn emission processes in East Asia is lacking. The high-resolution model (∼ 60 km mesh) demonstrated that observed consecutive high-222Rn peaks at several-hour timescales were caused by two 222Rn streams from different regions and were not well resolved by the low-resolution model (∼ 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 222Rn values normalized to daily means was used to analyze the diurnal 222Rn cycle, allowing diurnal cycles in winter to be extracted from 222Rn data that are highly variable due to sporadic continental 222Rn outflows, which tend to obscure the diurnal variations. Normalized diurnal cycles of 222Rn 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 222Rn 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 222Rn emissions.
