Mercury drop trend in urban soils in Beijing, China, since 1987

Mercury (Hg) concentration and its change trend in soils was investigated at different periods in Beijing. An Hg-enriched area with 1000ng·g−1 Hg concentration was delineated within the Third Ring Road of the city, where the capital of the Ming Dynasty was situated approximately 500years ago. The Hg concentration was attributed to the historical use of Hg and coal burning. The geometric mean of Hg concentration in Beijing was 714ng·g−1 in 1987, 375ng·g−1 in 2000, 294ng·g−1 in 2005, and 251ng·g−1 in 2009, indicating a drop in Hg concentration in the soil since 1987. The results show that the environmental quality of the soil Hg in Beijing had been significantly improving since 1987. The average topsoil Hg density (0cm to 20cm) decreased from 411 mg·m−2 in 1987 to 178.0 mg·m−2 in 2005, with an annual decrease of 4661 kg and an annual average drop rate of 3.15%. The sharp decrease in soil Hg content is attributed to tight emission controls implemented by the Beijing Government since 2000 and Hg emission from topsoil. However, the amount of Hg emission from urban soils is comparable with the value of Hg emission from fuel coal, oil, and natural gas in Beijing, implying that natural Hg emissions from the soil are another important atmospheric Hg source expected for emissions from fuel combustion.