Mercury (Hg) is the only toxic heavy metal that can exist in gaseous form and can be transported long-range in the atmosphere. By far, mercury is the only heavy metal identified as a persistent bio-accumulative pollutant. Thus the Minamata convention was adopted by the United Nations Environment Program (UNEP) in 2013, in order to promote an international priority a unified prevention and control of mercury on a global scale. As the world's largest producer of mercury emissions, China is now faced with potential atmospheric mercury pollution. In the past years, the major urban regions such as Beijing, Tianjin and Hebei have encountered frequent and severe haze events, indicating a dramatic increase in the proportion of particulate mercury in the atmosphere, which would not only alter the biogeochemical behavior of atmospheric mercury, but also aggravate the environmental health risk of air pollution regionally. Nowadays, there are still some scientific puzzles in the migration and transformation of haze particulate mercury, for example, the processes and mechanism of complex gas-particle distribution and chemical transformation regarding mercury in the atmosphere remain unclear. The emerging mercury isotope approach could eventually provide useful information for this issue.
Mercury bound to fine aerosols (PM2.5-Hg) may undergo a photochemical reaction that causes isotopic fractionation of Hg isotopes and obscures the initial isotopic signatures. In this study, professor Jiubin CHEN and his team from the Institute of Surface-Earth System Science of Tianjin University systematically studied Hg isotopic compositions for 56 PM2.5 samples collected between Sept. 15th and Oct. 16th, 2015 from Beijing, China, among which 26 were collected during the daytime (between 8:00 a.m. and 6:30 p.m.) and 30 during night (between 7:00 p.m. and 7:30 a.m.). Geochemical characteristics of the samples and the air mass backward trajectories (PM2.5 source related) suggest that diel variation in Δ199Hg values might result primarily from the photochemical reduction of divalent PM2.5-Hg, rather than variations in emission sources. Their results provide isotopic evidence that local, daily photochemical reduction of divalent Hg is of critical importance to the fate of PM2.5-Hg in urban atmospheres and that, in addition to variation in sources, photochemical reduction appears to be an important process that transfers particle-bound Hg into its gaseous form, and affects both the particle mass-specific abundance and isotopic composition of PM2.5-Hg, which should be investigated further and be taken into account when deciphering the global biogeochemical cycling of Hg and its environmental impacts.
The link of the article is https://doi.org/10.5194/acp-19-315-2019.
By the Institute of Surface-Earth System Science
Editors: Eva Yin & Doris Harrington
