Abstract. China presently contributes the largest amount of anthropogenic mercury (Hg) emission into the atmosphere in the world. Over the past decade, numerous studies have been conducted to characterize the concentration and forms of atmospheric Hg in China, which provide insights into the spatial and temporal distributions of atmospheric Hg through ground-based measurements at widely diverse geographical locations and during cruise and flight campaigns. In this paper, we present a comprehensive review of the state of understanding in atmospheric Hg in China. Gaseous elemental mercury (GEM) and particulate-bound mercury (PBM) measured at the remote sites in China are substantially elevated compared to the background values in the Northern Hemisphere. In Chinese urban areas, the highly elevated GEM, PBM and gaseous oxidized mercury (GOM) were mainly derived from local anthropogenic Hg emissions, whereas regional anthropogenic emissions and long-range transport from domestic source regions are the primary causes of the elevated GEM and PBM concentrations at remote sites. Using 7-9 years of continuous observations at a remote site and an urban site, a slight increase in atmospheric GEM (2.4-2.5 % yr −1 ) was identified (paired samples test: p < 0.01), which is in agreement with the increasing domestic anthropogenic emissions. Anthropogenic GEM emission quantity in China estimated through the observed GEM / CO concentration ratios ranged from 632 to 1138 t annually over the past decade, 2-3 times larger than published values using emission activity data. Modeling results and filed measurements show dry deposition is the predominant process for removing Hg from the atmosphere, 2.5-9.0 times larger than wet deposition, due to the elevated atmospheric GEM and PBM concentrations that facilitate dry deposition to terrestrial landscapes. Further studies to reconcile the observed and simulated Hg concentrations, to understand the impact of domestic emission reduction on Hg concentration and deposition and to delineate the role of Hg emission and deposition of China in the global Hg biogeochemical cycle, are needed.

The mechanism of elemental mercury (Hg 0 ) re-emission from vegetation to the atmosphere is currently poorly understood. In this study, we investigated branch-level Hg 0 atmosphere-foliage exchange in a pristine evergreen forest by systematically combining Hg isotopic composition, air concentration and flux measurements to unravel process information. It is found that the foliage represents a diurnally changing sink for atmospheric Hg 0 and its Hg content increases with leaf age and mass. Atmospheric Hg 0 is the dominant source of foliar Hg and the involvement of Hg II is not supported by isotopic evidence. Upon Hg 0 uptake, maturing foliage becomes progressively enriched in lighter Hg isotopes and depleted in odd mass isotopes. The measured isotopic composition of foliage Hg and isotopic shift caused by Hg 0 evasion from foliage supports that Hg 0 emitted from foliage is derived from Hg previously metabolized and bound in the leaf interior then subsequently recycled after reduction, and not merely a retroflux of recently deposited Hg 0 on foliar surface. An isotopic differential mass balance model indicates that the proportion of foliar Hg 0 efflux to uptake gradually increase from emergence to senescence with an average flux ratio of 30%.