Gaseous elemental mercury (GEM) fluxes over canopy of two typical subtropical forests in south China

Qian Yu,Lei Duan,Shuxiao Wang,Jiming Hao,Zhiqi Wang,Yao Luo

doi:10.5194/acp-18-495-2018

Abstract

Abstract. Mercury (Hg) exchange between forests and the atmosphere plays an important role in global Hg cycling. The present estimate of global emission of Hg from natural source has large uncertainty, partly due to the lack of chronical and valid field data, particularly for terrestrial surfaces in China, the most important contributor to global atmospheric Hg. In this study, the micrometeorological method (MM) was used to continuously observe gaseous elemental mercury (GEM) fluxes over forest canopy at a mildly polluted site (Qianyanzhou, QYZ) and a moderately polluted site (Huitong, HT, near a large Hg mine) in subtropical south China for a full year from January to December in 2014. The GEM flux measurements over forest canopy in QYZ and HT showed net emission with annual average values of 6.67 and 0.30 ngm-2h-1, respectively. Daily variations of GEM fluxes showed an increasing emission with the increasing air temperature and solar radiation in the daytime to a peak at 13:00, and decreasing emission thereafter, even as a GEM sink or balance at night. High temperature and low air Hg concentration resulted in the high Hg emission in summer. Low temperature in winter and Hg absorption by plant in spring resulted in low Hg emission, or even adsorption in the two seasons. GEM fluxes were positively correlated with air temperature, soil temperature, wind speed, and solar radiation, while it is negatively correlated with air humidity and atmospheric GEM concentration. The lower emission fluxes of GEM at the moderately polluted site (HT) when compared with that in the mildly polluted site (QYZ) may result from a much higher adsorption fluxes at night in spite of a similar or higher emission fluxes during daytime. This shows that the higher atmospheric GEM concentration at HT restricted the forest GEM emission. Great attention should be paid to forests as a crucial increasing Hg emission source with the decreasing atmospheric GEM concentration in polluted areas because of Hg emission abatement in the future.

Highlights

Mercury (Hg) is a globally concerning environmental contaminant due to its cyclic transport between air, water, soil, and the biosphere, and its tendency to bioaccumulate in the environment as neurotoxic methylated compounds (MeHg) (Driscoll et al, 2013), which can cause damage to the environment and human health (Lindqvist et al, 1991)
The forest ecosystem at site HT had a net gaseous elemental mercury (GEM) emission with a magnitude of 0.30 ng m−2 h−1 for a whole year. These results suggest that the subtropical forests in our study region should be the substantial GEM source, and the differences among seasons emphasized the importance of capturing GEM flux seasonality when determining total Hg budgets
From continuously quantitative micrometeorological method (MM)-flux measurements covering wide temporal scales at sites QYZ and HT in subtropical south China, it is inferred that forest ecosystems can represent a net GEM source, with the average magnitudes of 6.67 and 1.21 ng m−2 h−1 for a full year at a mildly polluted site (QYZ) and a moderately polluted site (HT), respectively

Summary

Introduction

Mercury (Hg) is a globally concerning environmental contaminant due to its cyclic transport between air, water, soil, and the biosphere, and its tendency to bioaccumulate in the environment as neurotoxic methylated compounds (MeHg) (Driscoll et al, 2013), which can cause damage to the environment and human health (Lindqvist et al, 1991). Hg emission flux from anthropogenic sources has been quantified with reasonable consistency from 1900 to 2500 t yr−1 (Streets et al, 2009, 2011; Zhang et al, 2015, 2016). Q. Yu et al.: GEM fluxes over canopy of two typical subtropical forests in south China anthropogenic emission (e.g., 2000 t yr−1: Lindqvist et al, 1991; 5207 t yr−1: Pirrone et al, 2010; 4080–6950 t yr−1: UNEP, 2013; 4380–6630 t yr−1: Zhu et al, 2016). The reliable quantification of natural Hg source, GEM exchange between terrestrial ecosystem and the atmosphere would contribute to the understanding of global and regional Hg cycling budgets (Pirrone et al, 2010; Wang et al, 2014b; Song et al, 2015)

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