Switch of gaseous element mercury source/sink in a Hg-contaminated subtropical forest by variation in atmospheric Hg pollution

Abstract

The exchange of gaseous element mercury (GEM) between forests and atmosphere is an important part of the global mercury (Hg) cycle. However, there is ongoing debate about whether forests are as a net sink or source of GEM, especially in Hg-polluted regions. This study monitored the GEM fluxes over the canopy of an Hg-contaminated forest in subtropical Southwest China from August 2017 to December 2020 b y the aerodynamic gradient method. From August 2017 to July 2018, the atmospheric GEM concentration was 4.41 ± 1.49 ng m⁻³, with the forest serving as a source, exhibiting an average GEM flux of 101 ± 226 ng m⁻² h⁻¹, higher in summer (270 ng m⁻² h⁻¹) and lower in spring (13.3 ng m⁻² h⁻¹). By contrast, after 2019, the atmospheric concentration increased to 10.8 ± 36.3 ng m⁻³, due probably to adjacent city construction, resulting in the forest being switched to a net GEM sink (−48.5 ± 254 ng m⁻² h⁻¹). These findings indicated that atmospheric GEM concentration might play a major role in the GEM sink/source transition of forests. Future decreases in atmospheric Hg concentration may prompt the forest to release GEM again. Further studies are required to determine the time needed for the GEM exchange between the atmosphere and forest to reach a balance.