Isotopic compositions of atmospheric total gaseous mercury in 10 Chinese cities and implications for land surface emissions

Land surface emissions are an important source of atmospheric total gaseous mercury (TGM); however, its role on the variations of TGM isotopic compositions and concentrations has not been properly evaluated. In this study, TGM isotope compositions, a powerful tracer for sources and transformation of Hg, were measured at 10 urban sites and one rural site in China. TGM concentrations were higher in summer than in winter in most cities except in Guiyang and Guangzhou in the low latitudes. The summertime high TGM concentrations coincided with prevailing low TGM δ202Hg and high TGM Δ199Hg signatures. These seasonal patterns were in contrast with those typically observed in rural areas in the Northern Hemisphere, suggesting that atmospheric oxidation chemistry, vegetation activity and residential coal combustion were likely not the dominant mechanisms contributing to the TGM concentration and isotopic composition seasonality in Chinese cities. The amplitudes of seasonal variations in TGM concentrations and Δ199Hg (or TGM δ202Hg) were significantly positively (or negatively) correlated with that of the simulated soil GEM emission flux. These results suggest that the seasonal variations in TGM isotopic compositions and concentrations in the 10 Chinese cities were likely controlled by land surface emissions that were observed or reported with highly negative δ202Hg signatures.

Seasonal patterns of atmospheric mercury in tropical South America as inferred by a continuous total gaseous mercury record at Chacaltaya station (5240 m) in Bolivia

High-quality atmospheric mercury (Hg) data are rare for South America, especially for its tropical region. As a consequence, mercury dynamics are still highly uncertain in this region. This is a significant deficiency, as South America appears to play a major role in the global budget of this toxic pollutant. To address this issue, we performed nearly 2 years (July 2014–February 2016) of continuous high-resolution total gaseous mercury (TGM) measurements at the Chacaltaya (CHC) mountain site in the Bolivian Andes, which is subject to a diverse mix of air masses coming predominantly from the Altiplano and the Amazon rainforest. For the first 11 months of measurements, we obtained a mean TGM concentration of 0.89±0.01 ng m−3, which is in good agreement with the sparse amount of data available from the continent. For the remaining 9 months, we obtained a significantly higher TGM concentration of 1.34±0.01 ng m−3, a difference which we tentatively attribute to the strong El Niño event of 2015–2016. Based on HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory) back trajectories and clustering techniques, we show that lower mean TGM concentrations were linked to either westerly Altiplanic air masses or those originating from the lowlands to the southeast of CHC. Elevated TGM concentrations were related to northerly air masses of Amazonian or southerly air masses of Altiplanic origin, with the former possibly linked to artisanal and small-scale gold mining (ASGM), whereas the latter might be explained by volcanic activity. We observed a marked seasonal pattern, with low TGM concentrations in the dry season (austral winter), rising concentrations during the biomass burning (BB) season, and the highest concentrations at the beginning of the wet season (austral summer). With the help of simultaneously sampled equivalent black carbon (eBC) and carbon monoxide (CO) data, we use the clearly BB-influenced signal during the BB season (August to October) to derive a mean TGM / CO emission ratio of (2.3±0.6)×10-7 ppbvTGM ppbvCO-1, which could be used to constrain South American BB emissions. Through the link with CO2 measured in situ and remotely sensed solar-induced fluorescence (SIF) as proxies for vegetation activity, we detect signs of a vegetation sink effect in Amazonian air masses and derive a “best guess” TGM / CO2 uptake ratio of 0.058 ±0.017 (ng m−3)TGM ppmCO2-1. Finally, significantly higher Hg concentrations in western Altiplanic air masses during the wet season compared with the dry season point towards the modulation of atmospheric Hg by the eastern Pacific Ocean.

Isotopic compositions of atmospheric total gaseous mercury in ten Chinese cities and implications for land surface emissions

Land surface emissions are an important source of atmospheric total gaseous mercury (TGM), however, its role on the variations of TGM isotopic compositions and concentrations has not been properly evaluated. In this study, TGM isotope compositions, a powerful tracer for sources and transformation of Hg, were measured at ten urban sites and one rural site in China. TGM concentrations were higher in summer than in winter in most cities except in Guiyang and Guangzhou in the low latitudes. The summertime high TGM concentrations were coincided with prevailing low TGM δ202Hg and high TGM Δ199Hg signatures. These seasonal patterns were in contrast with those typically observed in rural areas in the Northern Hemisphere, suggesting atmospheric oxidation chemistry, vegetation activity as well as residential coal combustion were not likely the dominant mechanisms contributing to the TGM concentration and isotopic composition seasonality in Chinese cities. The amplitudes of seasonal variations in TGM concentrations and Δ199Hg (or TGM δ202Hg) were significantly positively (or negatively) correlated with that of the simulated soil GEM emission flux. These results suggest that the seasonal variations in TGM isotopic compositions and concentrations in the ten Chinese cities were likely controlled by land surface emissions that were observed or reported with highly negative δ202Hg signatures.