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. High-quality atmospheric mercury (Hg) data are rare for South America,especially for its tropical region. As a consequence, mercury dynamics are stillhighly uncertain in this region. This is a significant deficiency, as South Americaappears 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) measurementsat the Chacaltaya (CHC) mountain site in the Bolivian Andes, which issubject to a diverse mix of air masses coming predominantly from theAltiplano and the Amazon rainforest. For the first 11 months ofmeasurements, 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 fromthe continent. For the remaining 9 months, we obtained a significantlyhigher TGM concentration of 1.34±0.01 ng m−3, a difference whichwe 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 thatlower mean TGM concentrations were linked to either westerly Altiplanic airmasses or those originating from the lowlands to the southeast of CHC.Elevated TGM concentrations were related to northerly air masses ofAmazonian or southerly air masses of Altiplanic origin, with the former possiblylinked to artisanal and small-scale gold mining (ASGM), whereas the lattermight be explained by volcanic activity. We observed a marked seasonalpattern, with low TGM concentrations in the dry season (austral winter),rising concentrations during the biomass burning (BB) season, and the highestconcentrations at the beginning of the wet season (austral summer). With thehelp of simultaneously sampled equivalent black carbon (eBC) and carbonmonoxide (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 toconstrain South American BB emissions. Through the link with CO2 measured in situ and remotely sensed solar-induced fluorescence (SIF) asproxies for vegetation activity, we detect signs of a vegetation sink effectin Amazonian air masses and derive a “best guess” TGM / CO2 uptake ratio of 0.058 ±0.017 (ng m−3)TGM ppmCO2-1. Finally, significantlyhigher Hg concentrations in western Altiplanic air masses during the wetseason compared with the dry season point towards the modulation ofatmospheric Hg by the eastern Pacific Ocean.

atmospheric chemistry and physics

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