Mercury concentrations and pools in four adjacent coniferous and deciduous upland forests in Beijing, China

Abstract

Understanding of forest mercury (Hg) pools is important for quantifying the global atmospheric Hg removal. We studied gaseous elemental Hg (GEM) concentrations, litterfall Hg depositions, and pool sizes in four adjacent stands at Mount Dongling to assess Hg dynamics in the forested catchment and the potential of Hg release during wildfires. The average GEM concentration was 2.5 ± 0.5 ng m−3, about 1.5 times of the background levels in the Northern Hemisphere. In all four stands, Hg concentrations increase in the following order: bole wood < branch/twig < bark < mineral soil < needles/leaves < litterfall < Oi litter < Oe soil < Oa organic soil. The Hg pools of aboveground biomass were comparable in the forests of larch, oak, and Chinese pine, which were much greater than that of mixed broadleaf stands due to lower biomass. The total Hg pools in ecosystems were similar in the four stands, because of the comparable Hg pool in the soil horizons (0–40 cm), which accounted for over 97% of the total ecosystem Hg storage in the four stands. Although Hg pools of the forest ecosystem in north China were comparable to North America and North Europe, Hg storage in forests constituted a high threat for large Hg emission pulses to the atmosphere by wildfires. The potential Hg emissions from the combustion at the four stands were ranged from 0.675 to 1.696 mg m−2.

Deposition and Fate of Mercury in Litterfall, Litter, and Soil in Coniferous and Broad‐Leaved Forests

“…Paradoxically, the China Atmospheric Mercury Emission model estimated that total anthropogenic mercury emissions were increasing year by year from 2000 (356 t) to 2010 (538 t) and still increasing in recent years (Zhang et al, ). Thus, we estimated that interannual variations of litterfall components may influence THg concentrations in the litterfall of a forest (Zhou et al, , ; Zhou et al, ).…”

“…By reviewing the available TGM concentrations and litterfall mercury concentrations in 11 forests and 14 pairs of data in China, annual average atmospheric TGM concentrations were significantly correlated with the mercury concentrations in litterfall ( Figure S2). The significant correlation might confirm that tree leaf could effectively absorb TGM from the atmosphere by accumulating through stomatal uptake of GEM and adsorption of PBM and GOM (Fu, Feng, Zhu, et al, 2010;Fu, Feng, Dong, et al, 2010;Laacouri et al, 2013;Zhou et al, 2017). Paradoxically, the China Atmospheric Mercury Emission model estimated that…”

Atmospheric mercury deposition over the land surfaces and the associated uncertainties in observations and simulations: a critical review

“…Figure 1 summarizes the global distribution of the observed Hg wet deposition fluxes based on results from both these global or regional networks and individual studies. Overall, East Asia has the highest wet deposition flux (on average 16.1 µg m −2 yr −1 ), especially in the southern part of China where the RM concentration level is relatively high (Fu et al, 2008(Fu et al, , 2010a(Fu et al, , b, 2016aGuo et al, 2008;Wang et al, 2009;Ahn et al, 2011;Huang et al, 2012bHuang et al, , 2013aHuang et al, , 2015Seo et al, 2012;Sheu and Lin, 2013;Marumoto and Matsuyama, 2014;Xu et al, 2014;Zhu et al, 2014;Zhao et al, 2015;Han et al, 2016;Ma et al, 2016;Nguyen et al, Qin et al, 2016;Sommar et at., 2016;Travnikov et al, 2017;Chen et al, 2018;Lu and Liu, 2018). North America has an average Hg wet deposition flux of 9.1 µg m −2 yr −1 and exhibits a descending spatial profile from the southeastern part to the northwestern part, which is consistent with the distribution of the atmospheric Hg concentration (L. Gichuki and Mason, 2014;Lynam et al, 2017).…”

“…4). Results from size-resolved PBM analysis and PBM dry deposition models show that East Asia has a much higher average of PBM dry deposition flux (45.3 µg m −2 yr −1 ) than North America (1.1 µg m −2 yr −1 ) (Fang et al, 2012a, b;Zhu et al, 2014;Zhang et al, 2015;Huang et al, 2016;Guo et al, 2017). Zhu et al (2016) reviewed the air-surface exchange of GEM.…”

“…(1), the overall uncertainty in rainfall Hg wet deposition originates from both the analytical methods of Hg concentration in rainwater and the measurements of precipitation depth. Both manual and automatic precipitation sample collectors were used in previous studies (Fu et al, 2010a;Gratz and Keeler, 2011;Marumoto and Matsuyama, 2014;Zhu et al, 2014;Brunke et al, 2016;Chen et al, 2018). Automatic precipitation sample collectors cover the lid automatically when it is not raining to prevent potential contamination, while manual collectors require manually placing collectors before precipitation events and require them to be retrieved after events.…”