Characteristics Of Atmospheric Mercury Pollution In Two Subtropical Forests In Southern China

Atmospheric mercury?Hg?has become a hot spot for global environmental issues due to its high toxicity,long-lasting,and capacity of long-distance transportation.In this study,two subtropical forests were selected,one in Qianyanzhou Ecological Station?QYZ?in Jiangxi Province typical for mildly polluted area,and the other in Huitong Ecological Station?HT?in Hunan Province as contaminated site by nearby Hg mine,for long-term monitoring of total gaseous mercury?TGM?over forest canopy for more than one year by Tekran 2537X Hg analyzer.The seasonal and daily variation patterns of TGM concentration and their correlation to metrological factors were analyzed.The potential sources of Hg pollution were then preliminarily traced using the backward trajectory and PSCF models.At the same time,bulk precipitation and canopy throughfall were collected and atmospheric Hg deposition,including wet deposition and dry deposition was calculated.The results were also compared with the sulfur?S?deposition and nitrogen?N?deposition to evaluate the contribution of natural sources of atmospheric Hg.Results showed that Hg pollution at HT station was significantly more serious than that at QYZ station.The average concentration of atmospheric Hg over forest canopy of the QYZ station from December 2014 to November 2015was 3.19?1.72 ng?m^-3.It decreased by 30%in 2015 than in 2014.The average annual atmospheric Hg concentration at HT station in 2014 was 6.67?5.07 ng?m^-3,which was 2.09 times that of QYZ Station.The atmospheric Hg concentrations at the two stations showed different seasonal patterns,probably due to the joint action of anthropogenic and natural sources.The TGM concentration at QYZ station showed decreasing order of winter,autumn,spring,and summer,while at HT station in the order of autumn,summer,winter,and spring.The daily variation of atmospheric temperature and solar radiation seemed the main factors which lead to higher Hg concentration in the day than at night.The diurnal variation of TGM firstly rose and then fell in the day,and rose slightly at night.This kind daily variation reflects the important contribution of local natural Hg sources such as soil emission.In 2014,atmospheric Hg deposition fluxes based on the throughfall at QYZ and HT stations were 30.6?g·?m²·a?^-1 and 37.3?g·?m²·a?^-1,respectively.The Hg deposition flux at QYZ station in 2017 decreased by 50%compared to 2014.Considering the litterfall deposition,the total deposition fluxes of Hg at the two stations were 52.1?g·?m²·a?^-1 and 70.8?g·?m²·a?^-1,respectively,of which the dry deposition was 33.7?g·?m²·a?^-1 and 55.2?g·?m²·a?^-1,respectively.The dry deposition dominated the total deposition,accounting for 64.7%and 78.0%at the QYZ and HT station,respectively.The correlation analysis showed that the Hg deposition was not related to the S deposition and N deposition,which were mainly from anthropogenic sources.It may be concluded that natural sources contributed more to the Hg deposition,especially at HT station.According to the simulation results of the backward trajectory and the PSCF model,combined with the wind direction frequency and TGM distribution in the atmosphere,the frequency of Hg pollution incidents in spring at HT and in spring and summer at QYZ was relatively low.Atmospheric Hg pollution at the two stations mainly occurred in autumn and winter,respectively.The atmospheric Hg mainly came from long range transportation via the large cities such as Wuhan,Changsha,and Nanchang,middle reaches of the Yangtze River.In general,the contribution of external natural and anthropogenic sources was equally important at the two stations.