Influence of Asian outflow on Rishiri Island, northernmost Japan: Application of radon as a tracer for characterizing fetch regions and evaluating a global 3D model

Atmospheric 222Rn was monitored from December 2008 to November 2010 on Rishiri Island (45°07′N, 141°12′E), northernmost Japan. Seasonal 222Rn variation was characterized by high concentrations from November to February and low concentrations from May to July, caused by the alternation of continental and maritime fetch regions. 222Rn tracer and back trajectory cluster analyses indicated that the predominant continental fetch region was southeastern Siberia and northeastern China. 222Rn emitted from China and South Korea, whose economies are growing rapidly, did not significantly affect the Rishiri site. The major maritime fetch region was the Sea of Okhotsk and the Bering Sea. A global three-dimensional model (NICAM-TM) accurately simulated 222Rn concentrations on Rishiri Island and in the seasonal fetch regions. The time series of 222Rn data will make it possible to evaluate the sources and sinks of atmospheric greenhouse gases being monitored at Rishiri Island, which complements other sites in the Asia–Pacific rim region, and to validate model simulations used to examine trans-boundary air pollution.