Measurements of ²¹⁰Pb and ⁷Be in China and their analysis accompanied with global model calculations of ²¹⁰Pb

Abstract

Previous global models generally simulated high concentrations of 210 Pb in the surface air over China. Measured data of 210 Pb are needed in order to examine the accuracy and performance of the models. We collected the first‐ever set of surface air radionuclide measurements of 210 Pb and 7 Be at Guiyang (26.57°N, 106.72°E, 1080 m above sea level) on the east of the Himalayas in Guizhou Province and at Mt. Waliguan (36.29°N, 100.90°E, 3816 m above sea level), a Global Atmosphere Watch station for the World Meteorological Organization, on the Qinghai‐Tibetan plateau extending from the Himalayas in Qinghai Province, China. These measurements were carefully analyzed for data assurance and quality control and were used for validating models. The patterns for the variations in both measured 210 Pb and 7 Be at the Guiyang site have been shown to be consistent. In this case the increase of 7 Be concentration was the result of the weak subsidence of air developed at the periphery of the high‐pressure system. Because of the weak subsidence the boundary layer turbulence could mix the upward transport of 210 Pb, thus increasing 210 Pb concentration. The ratio of 7 Be/ 210 Pb indicated a seasonal variability with rough monthly oscillation. The ratio peaks were in spring and summer when the convective activities were significant. The 210 Pb levels measured at Mt. Waliguan, located at the peak of the mountain, could be attributed to the long‐range transport of 222 Rn that decays to 210 Pb, whereas the 7 Be concentration varied directly with the local airflow passing over the site. We found that it was not appropriate to use the ratio of 7 Be/ 210 Pb for analysis of vertical transport exchange processes at the high‐altitude Mt. Waliguan site because of the long‐range transport of 210 Pb that occurred at the site. For the purpose of model validation we have simulated the global transport of 210 Pb for the year 2002 by using a comprehensive multiscale three‐dimensional global chemical transport model developed by Météo France. It was found by comparing model results with measurements that the model generally performed poorly for the calculations of 210 Pb concentrations in China. The underestimates of model results could be the result of the unidentified high radon sources over China. However, the model was able to reproduce the variations of the 210 Pb concentrations.