Journal of Geochemical Exploration 139 (2014) 115–121

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Journal of Geochemical Exploration

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Characteristics and influencing factors ofmercury exchangeflux betweensoil and air in Guangzhou City

Fei Liu a,b,⁎, Hangxin Cheng b, Ke Yang a,b, Chuandong Zhao b, Yinghan Liu b, Min Peng b, Kuo Li b

a School of the Earth Sciences and Resources, China University of Geosciences, Beijing 100083, Chinab Key Laboratory of Geochemical Cycling of Carbon and Mercury in the Earth's Critical Zone, Institute of Geophysical and Geochemical Exploration, Chinese Academy of Geological Sciences,Langfang 065000, China

⁎ Corresponding author at: Key Laboratory of GeochMercury in the Earth's Critical Zone, Institute of GExploration, Chinese Academy of Geological Sciences, Lan316 2267635.

E-mail address: liufei@igge.cn (F. Liu).

0375-6742/$ – see front matter © 2013 Published by Elsehttp://dx.doi.org/10.1016/j.gexplo.2013.09.005

a b s t r a c t

a r t i c l e i n f o

Article history:Received 30 November 2012Accepted 30 September 2013Available online 9 October 2013

Keywords:TGMMercury exchange fluxMeteorology parametersGuangzhou City

This study aimed to characterize atmospheric mercury (Hg) as well as the Hg exchange flux between soil and airsurfaces in the urban area of Guangzhou. Total gaseousHg (TGM) concentration andHgexchange fluxweremea-sured in situ using a dynamic flux chamber coupled with a Mercury Vapor Analyzer. The TGM averaged 6.3±2.2 ng·m−3 at five sites, and average Hg exchange flux was 7.8 ± 7.1 ng·m−2·h−1. Both Hg content and soilpHwere significantly correlatedwithHgfluxes, suggesting that soil properties affectedHg exchanges. The Hg ex-change fluxes showed significantly positive correlations with solar radiation and soil temperature. Comparisonsdemonstrated that vegetation significantly interfered with the Hg emission flux. The annual Hg emission fromsoil in Guangzhou urban area was 51.46 g·km−2·yr−1 or 25.73 kg·yr−1.

© 2013 Published by Elsevier B.V.

1. Introduction

Mercury (Hg) is one of the 189 toxic air pollutants included in theClean Air Act Amendments (Wang et al., 2003). High volatility allowsHg to transport through atmospheric circulation, deposit into waterand soil, and emit again into the atmosphere. To establish the modelof the Hg geochemical cycle, atmospheric Hg sources need to be esti-mated well in different ecosystems. Recent estimates of the global an-thropogenic Hg emission range from 1400 to 2200 Mg·year−1

(Lohman et al., 2008; Pacyna and Pacyna, 2002; Pacyna et al., 2006),and East Asia is one of the regions that release large amount of Hg intothe atmosphere (Pacyna et al., 2006). Numerous studies have docu-mented the characteristics of Hg in the environment. Abundant datahave been obtained after long-term monitoring and characterizationof the temporal distribution of total gaseous Hg (TGM) and activitiesin many regions in Europe and North America (Cobbett and VanHeyst, 2007; During et al., 2009; Eckley et al., 2011; Laurier et al.,2008; Rinklebe et al., 2010). In contrast, fewmeasurements of mercury,to our knowledge, have been conducted in Chinese cities. Asia accountsfor 54% of global anthropogenic Hg emissions (Pacyna et al., 2006), andHg outflow from East Asia, particularly China, has become a serious con-cern (Jaffe et al., 2005; Streets et al., 2005; Z.W.Wang et al., 2007). Mer-cury exchange fluxes between soil and air in large cities were found tosignificantly differ from background areas because of the chemically

emical Cycling of Carbon andeophysical and Geochemicalgfang 065000, China. Tel.: +86

vier B.V.

and physically diverse nature of urban surface covers and their high spa-tial variability (Gabriel et al., 2005; Rodrigues et al., 2006). Therefore,urban surfaces may significantly affect the Hg cycle (Carpi and Chen,2002).

Studies have shown extensive Hg pollution in cities and severalsources of atmospheric Hg in urban areas. Increased urban industrial ac-tivity contributes to Hg pollution, and other human activities such asmedical and dental operations, waste generation, and transportationalso produce urban air pollution (Gustin et al., 2006; Xin et al., 2006).Thus, we conducted in situ monitoring of airborne Hg at five studyareas in Guangzhou to understand the transport and destination of Hgin the urban environment.

In March and April 2009, the TGM concentrations in five differenttypes of land usewere surveyed, including residential area, park, school,road verges, and uncultivated land. A dynamicflux chamberwas used tomeasure the Hg exchange fluxes between soil and air in situ. Theinfluencing factors of Hg exchange fluxes, such as soil properties andmeteorological parameters, were synchronously determined.

This study aims tomeasure the TGM concentrations in air in Guang-zhou, calculate the Hg exchange fluxes between soil and air, analyze theinfluencing factors, and estimate the annual Hg emission from the soil.

2. Materials and methods

2.1. Site descriptions

Guangzhou City (113.3°E, 23.2°N), the primary city of South China, islocated on the Pearl River Delta approximately 60 km from the PearlRiver Estuary (Fig. 1a). Covering an area of 500 km2, the urban regionof Guangzhou City has a population of over 5million.

Fig. 1. Sampling and monitoring sites in Guangzhou.

116 F. Liu et al. / Journal of Geochemical Exploration 139 (2014) 115–121

Continuous sampling of TGMwas conducted at 5min intervals for ap-proximately one day and one night at five study sites in Guangzhou City.Site F1 was located in Xintang town, outside the Guangzhou Ring Road,and it was the representative of rural–urban continuum. Site F2 was ar-ranged in anuncultivateddevious lawn inside theRingRoad, and it couldrepresent the original soil. Sites F3 and F4 were chosen at the grasslandsof Dongfeng Park and Guangzhou Academy of Fine Arts, and both ofthem were located near the center of Guangzhou; the soils were undis-turbed for years. Site F5 was situated in an old residential area calledHongmianyuan, which was built in the 1990s. Sites F3, F4 and F5 couldbe considered as the representatives of the main urban area. All siteswere situated away from garbage dumps, factories, and other potentialsources of Hg. The locations are shown in Fig. 1b. The description ofeach site, weather and time of monitoring are listed in Table 1.

2.2. Soil sample collection and analyses

The dynamic flux chamber was placed in the middle of the plot ineach monitor site, and surface soil samples were collected under the

Table 1Description of sampling and monitoring sites.

Site Site description

F1 Ke Tai Hotel Lawn, built in January 2009 with backfilledKetai Hotel adjacent to Daguan Middle Rd.,

F2 Chigang Uncultivated devious lawn, original soil, undbetween Ma'anshan Rd. and Guangdan Midd30 cm to 40 cm high and was cut before the

F3 Dongfeng Park Lawn and bush, outside the east gate of paradjacent to Guangzhou Middle Ave., with he

F4 The Guangzhou Academy of Fine Arts Lawn and backfilled soil, located at the eastnear Changgang East Rd.,with heavy traffic. The site was undisturbed

F5 Hongmianyuan Residential Area Lawn, located beside an artificial lake amongold residential area in Huadi North Ave., thefor three or four years.

dynamic flux chamber after monitoring. The soil samples were air-dried, milled, and sieved (b200mesh) using a nylon screen, sealed inamber glass sample containers, and stored at 4 °C until analysis.

Soil properties were determined as follows: S and Fe2O3 concentra-tions were quantified using an X-ray fluorescence spectrometer(PW4400/40, PANalytical Inc., Netherlands). Soil organic carbon wasdetermined using a total carbon and organic carbon monitor (YZYT-1,China). The soil pH value was measured with deionized water(1:2.5, w/w) using a pH meter (PHS-3CF, Chuangfa Inc., China). TotalHg concentration was analyzed by using a cold vapor spectrometer(CVAFS, XGY-1011A, Kaiyuan Inc., China) after digestion by using aquaregia (65% HNO3+37% HCl, 1:3 by volume). All tests were performedat the Analytical Center of the Institute of Geophysical and GeochemicalExploration (IGGE).

Standard reference materials and duplicate samples were used toensure analytical quality. Four standard reference materials of the GSSseries (GSS-1, GSS-2, GSS-3, and GSS-8), produced by the NationalResearch Center for Certified Reference Materials of China were treatedsynchronously with the samples. The logarithm deviation (ΔlgCSRM)

Weather Time

soil, near theXintang town.

Cloudy & rainy March 24 20:50–March 25 15:55

isturbed for several years,le Rd. The grass wasflux chamber was placed.

sunny March 25 18:20–March 26 18:05

k,avy traffic.

Rainy March 27 12:15–March 30 8:30

gate of the college,

for about 10 years.

sunny March 30 11:45–March 31 12:05

residential buildings in ansoil was undisturbed

Cloudy & rainy March 31 19:00–Apil 1 19:00

117F. Liu et al. / Journal of Geochemical Exploration 139 (2014) 115–121

between the analytical value and the standard value of each determina-tion was calculated to monitor the accuracy of the sample analyses asfollows:

ΔlgCSRM ¼ lgCd−lgCsj j ð1Þ

where Cd is the determined concentration, and Cs is the standard refer-ence concentration. The analyseswere considered acceptable ifΔlgCSRMwas b0.12 for samples with concentrations within three times the de-tection limit and b0.10 for samples with concentrations three timesthe detection limit.

Duplicate samples equal to 5% of the total number of samples wereinserted randomly to evaluate the precision of the analyses. The relativedeviation (RD) was calculated as follows:

RD %ð Þ ¼ C1−C2ð Þ= C1 þ C2ð Þ=2ð Þ½ � � 100 ð2Þ

whereC1 is thefirst determination, andC2 is the duplicate determination.Analyses were considered acceptable if the RDwas −40%≤RD≤40%.

The detection limits of Hg, S, Fe2O3 and organic C were 2 ng·g−1,50 μg·g−1, 0.1% and 1%, respectively. The logarithmic deviation of theGSS series and the average RD are listed in Table 2, indicating that theresults are acceptable.

2.3. Hg flux and TGM concentration in air

In situ Hg flux measurement was conducted using a dynamic fluxchamber (DFC) of quartz, whichwaswidely used tomeasure the Hg ex-change fluxes between air and soil/water surface (e.g., Eckley et al.,2011; Feng et al., 2004). A semi-cylindrical, open-bottom chamber oftransparent quartz (Ø20 × 30 cm) and a Model 2537B Mercury VaporAnalyzer (Tekran Inc., Canada) were employed in our study. The foot-print of the chamber was 0.06m2, and the internal volume was 4.71 L.To ensure the sampling airflow, the chamber had five Ø1.5 cm holeson one side of the chamber (the inlet) and air was pulled throughthree Ø0.6 cm holes on the opposite side of the chamber (the outlet).The Hg exchange flux between soil and air was calculated as follows(Poissant and Casimir, 1998):

F ¼ Co−Cið Þ � Q=A ð3Þ

where F is the flux of the TGM (in ng·m−2·h−1), Co is the outlet air Hgconcentration (in ng·m−3), and Ci is the inlet air Hg concentration(in ng·m−3). The chamber was sampled using an Automated DualSampling Unit (Tekran Inc., Canada) in 10-min intervals (two 5-minsamples) and analyzed respectively by Model 2537B Mercury VaporAnalyzer. A is the chamber-covered soil surface area (in m2), and Q isthe air flushing flow rate through the chamber (in m3·h−1). The Hgflux (F) values were calculated every 20min. The positive and the neg-ative results of F calculated from Eq. (3) represent the net Hg emissionflux and the net Hg deposition flux. The TGM concentrations in ambientair were calculated by averaging two Ci air Hg concentrations every20min.

The field blanks were obtained by placing the chamber over a cleanquartz class board before the field work. Prior to use, the chamber andthe quartz class board were cleaned by soaking in 10% nitric acid and

Table 2Parameters of quality assurance and quality control for sample analysis.

ΔlgCSRM

CSS1 CSS2

pH 0.002 (n=2) 0.002 (n=2)Hg 0.003 (n=2) 0.064 (n=2)S 0.013 (n=3) 0.015 (n=2)Org.C 0.007 (n=3) 0.010 (n=2)Fe2O3 0.005 (n=13) 0.010 (n=13)

ultrapure water. The flux blank was measured for 2 h, and the blankflux levels averaged at 0.22±0.71ng·m−2·h−1,whichwas significantlylower than the average Hg fluxesmeasured in Guangzhou, and we con-cluded that blank measurements would not significantly influence thesoil fluxmeasurements. Instrument calibrations were conducted beforesampling by using an internal Hg permeation source.

The meteorological parameters (soil temperature, UV irradiation,and relative humidity) were monitored on-site by using a portableweather station (GL300B Weather Station, China) with a time resolu-tion of 5min,whichmatched theModel 2537BMercury Vapor Analyzer.

The correlations among the parameters were statistically analyzedusingMicrosoft Excel 2003 (Microsoft Inc., USA) and SPSS 13.0 forWin-dows (SPSS Inc., USA).

3. Results and discussion

3.1. TGM concentration in air and Hg fluxes

The average TGM concentration in air in Guangzhou ranged from3.9 ± 0.7 to 7.4 ± 2.0 ng·m−3 and averaged 5.9 ± 1.3 ng·m−3

(Table 3). These values were obviously higher than the global back-ground value ranging from 1.5 to 1.8 ng·m−3, which was reportedusing similar instrumentation in the Northern Hemisphere (Valenteet al., 2007). The values were also higher than those in Mt. Changbaiat 3.58± 1.78 ng·m−3 (Wan et al., 2009) and those in Mt. Gongga inSichuan Province, southwestern China at 3.90 ± 1.20 ng·m−3 (Fuet al., 2008). However, the current average TGM concentration waslower than 18.4 ng·m−3 in Changchun (Fang et al., 2004) and 18.8 ±7.6ng·m−3 in Chongqing (Wang et al., 2006). Z.W. Wang et al. (2007)reported that the concentration of Hg0 was 13.5 ± 7.1 ng·m−3 inGuangzhou in January, 2005. Considering the large consumption ofcoal in heating season, the result was higher than the annual averagelevel.

TheHg exchangefluxes between the soil and the atmosphere are pre-sented in Table 3. The Hg fluxes ranged from 1.9 to 18.6ng·m−2·h−1 atfive sites and averaged 7.8 ± 7.1 ng·m−2·h−1 (n = 5). The resultsindicated that the Hg exchange between soil and air in Guangzhouwas approximately one order of magnitude stronger than that in thebackground area (0.9 ± 0.2 ng·m−2·h−1) in the United States(Ericksen et al., 2006). Compared with the Hg flux of other cities, theHg exchange fluxes in Guangzhouwere comparable to the reported av-erage Hg flux in Guiyang (Feng et al., 2005) and Changchun (Fang et al.,2004). Despite the Hg deposition at night observed atmost sites, net Hgemissionswere found at all sampling sites. The box plots showed the di-urnal variation of Hg fluxes in Guangzhou (Fig. 2).

3.2. Factors controlling the Hg exchange flux

Studies demonstrate that several factors influence the emission ofHg from soil. These factors mostly include: Hg content of soil (Fenget al., 2005; Nacht et al., 2004), sunlight intensity (Moore and Carpi,2005; Wang et al., 2005), soil temperature (Moore and Carpi, 2005;Rinklebe et al., 2010), soil moisture (Rinklebe et al., 2010), atmosphericHg content (Nacht et al., 2004; Wang et al., 2005), rainfall (Carpi andLindberg, 1997; Lindberg et al., 1999), air velocity on soil surface

RD (%)

CSS3 CSS8

0.002 (n=1) 0.001 (n=1) 0.790 (n=7)0.034 (n=1) 0.000 (n=1) −10.017 (n=7)0.041 (n=2) 0.115 (n=2) −2.919 (n=9)0.005 (n=2) 0.022 (n=2) −3.804 (n=9)0.006 (n=13) 0.006 (n=13) −2.58 (n=52)

Table 3Soil Hg concentration, TGM and Hg fluxes between soil and air.

Site Soil Hg concentration(ng·g−1)

TGM (ng·m−3) Hg flux (ng·m−2·h−1)

Mean N Mean N

F1 13 3.9± 0.7 111 2.5± 5.5 54F2 3616 5.9± 1.2 142 18.6± 6.9 69F3 456 7.4± 2.0 321 4.6± 8.8 158F4 2961 5.9± 1.5 148 11.5± 8.6 74F5 304 6.5± 2.6 146 1.9± 6.7 73

Fig. 3. Correlation between soil Hg concentration and Hg fluxes.

118 F. Liu et al. / Journal of Geochemical Exploration 139 (2014) 115–121

(Rinklebe et al., 2009), and vegetation coverage (Ericksen et al., 2006).The Hg emission flux direction reportedly follows a diel pattern thatpeaks at midday when sunlight is most intense and lowest at night(Coolbaugh et al., 2002; Engle et al., 2001). This finding indicated thatHg flux measurements vary when taken at different times from thesame location.

3.2.1. Soil Hg concentrationTotal Hg concentration in soil is considered a key indicator of pollu-

tion (Gustin, 2003). Soil with a high total Hg concentration exhibits highcapacity for Hg emission to the atmosphere. S.F. Wang et al. (2007) re-ported that the flux of mercury from the soil to the air could reach8385 ng·m−2·h−1±6770 ng·m−2·h−1 for the soil with THg concen-tration at 743.5ng·g−1, which was significantly higher than the values(0.9 ng·m−2·h−1 ± 0.2 ng·m−2·h−1) for background soils (Hgconcentrations b 0.1 ng·g−1) in the United States. The similar resultcan be observed in Guangzhou. The measured Hg emission fluxes atHg-rich sites (F2 and F4), where the topsoil Hg concentrations reached3616 and 2961 ng·g−1, were significantly higher than those at othersites. Conversely, fluxes at F1 and F5, where the Hg concentrationswere only 13 and 304ng·g−1, were the lowest (Fig. 3). This observationindicated that the Hg concentration in the soil could influence the Hgsoil emissions.

It was observed that there was a log–log relationship between thetotal Hg concentration in soil and the daily average Hg emission flux(Coolbaugh et al., 2002; Feng et al., 2005; Wang et al., 2005). This rela-tionship can efficiently estimate the annual Hg emission flux from anentire district, but a similar log–log relationship was not obtainedusing our datameasured in Guangzhou. It indicates that some other fac-tors apart from the total Hg concentration in soil can evidently affect theHg exchange.

3.2.2. Soil pHXin and Gustin (2007) observed a positive correlation between Hg

flux and soil pH. In contrast, we found that the soil pH (ranged from

Fig. 2. Box plots of Hg fluxes in Guangzhou.

4.87 to 8.21) was negatively correlated with the daily average Hg emis-sion flux (Fig. 4).

This negative correlation between the soil pH and theHgfluxmaybeattributed to the direct influence of the pH change on the quantities ofOH− and the hydroxides of Hg (HgOH+, HgOHCl, and Hg(OH)2),which are more likely adsorbed by the soil particulate and then chlori-nated (HgCl, Hg2Cl) (Lin et al., 2008). The salt water intrusion in thePearl River's estuary caused by groundwater exploitation has resultedin the accumulation of Cl− in the soil (Lu et al., 2010). In addition,more Hg hydroxides are produced when the soil pH value increases.Thus, more Hg2+ ions are adsorbed by the soil particulate, and fewerHg2+ ions existing in the soil interstices transform to Hg0 in thiscase. Research showed that when the pH ranged from 2 to 10, Hgfirmly joined with mineral substances in the soil as the pH value in-creased (Zhao et al., 2005). The negative correlation we found could in-dicate that soil acidification would promote soil Hg exchange to theatmosphere.

3.2.3. UV radiation intensityOur study in Guangzhouwas conducted at the beginning of the rainy

season. Thus, the weather was overcast and rainy at F1, F3, and F5. Thecorrelation between UV radiation intensity and Hg flux was unclear.Fortunately, it was extremely sunny at F2, and a significantly positivecorrelation was observed (Fig. 5).

The Hg0 released from the soil to the atmosphere mainly originatesfrom the Hg0 adsorbed in soil and the photo reduction of Hg2+/Hg+

(Schroeder and Munthe, 1998). It was found that the photo reductionof oxidized Hg was an important source of Hg0 in urban soil (Lindberget al., 1995). Experiments have proven that the Hg emission flux be-tween soil and air was related to the illumination intensity, which wasconsidered the main external factor influencing Hg emission (Gustin

Fig. 4. Correlation between soil pH and Hg fluxes.

Fig. 5. Correlation between UV radiation intensity and Hg fluxes at F2.

119F. Liu et al. / Journal of Geochemical Exploration 139 (2014) 115–121

et al., 2002). In addition,Moore and Carpi (2005) reported that differentbands of sunlight illumination produced different effects on the soil Hgphoto reduction action.WhenUV lightwas removed from incident radi-ation (λ ranging from 400 to 700 nm), the fluxes were similar to darkfluxes, whereas soil fluxes below the range of 320 to 580nm radiationwere significantly elevated compared with dark fluxes. In the presentstudy, a strong positive correlation (R2 = 0.615, n=69) between theUV irradiation and Hg flux was obtained. Thus, UV light significantly af-fects Hg flux between soil and air.

3.2.4. Soil temperatureStudies have shown a strong positive correlation between the soil

temperature and Hg flux (Rinklebe et al., 2010). Carpi and Lindberg(1997) conducted a study on the factors affecting on the emission of el-emental Hg from soils and found positive correlation between the soiltemperature and Hg0 emissions. Liu et al. (2002) also indicated thatHg in the soil and dust could evaporate back to the atmosphere in sum-mer when the soil temperature was high, and the atmospheric Hg con-centration increased. Several researchers used the Arrhenius equationto quantify the general dependence of Hg emission process on temper-ature (Feng et al., 2004;Wang et al., 2005). Meanwhile, the effects of UVradiation intensity and soil temperature, whichmore significantly affectthe Hg flux, are still under debate (Gustin et al., 2002; Moore and Carpi,2005; Rinklebe et al., 2010). In the current study, although both UV ra-diation intensity and soil temperature positively correlated with the Hgflux (Figs. 5 and 6), UV radiation intensity and Hg flux showed a moreremarkable correlation (R2=0.615) than did soil temperature and Hgflux (R2= 0.445). Therefore, we concluded that the solar factor was akey process that controlled Hg flux, which is consistent with the resultsof most studies (García-Sánchez et al., 2006; Wang et al., 2005).

Fig. 6. Correlation between soil temperature and Hg fluxes at F2.

The significant positive correlation between soil temperature andHgflux may be related to the air diffusion action. For example, at a hightemperature, the spread of air volume in soil pores drives the diffusionof Hg0 to the atmosphere. Given that soil temperature is mainly con-trolled by solar radiation, the positive relationship between soil temper-ature and Hg flux can reflect the correlation between UV radiationintensity and Hg flux.

3.2.5. VegetationThe effect of vegetation on the biogeochemical cycle of atmospheric

Hg remains unclear. Research showed that vegetation could absorb and/or release Hg (Ericksen and Gustin, 2004). The influence of vegetationon the Hg exchange flux in F2, which had bare uncultivated land, wascompared with that on other sites. The results showed that the diurnalvariation in Hg fluxes in F2 resembled sinusoids and all flux values werepositive (Fig. 7). This finding indicated that vegetation could interferewith the Hg emission flux. Previous studies proposed three hypothesesexplaining the effect of vegetation: (1) the construction of a new inter-face of soil–vegetation–air increases the resistance of Hg emissionfrom soil; (2) vegetation can strongly adsorb or absorb Hg with a highconcentration in the field flux chamber (Lodenius et al., 2003); and(3) solar irradiation reaching the soil surface is weakened by the vege-tation cover, and the rate of Hg2+ photo reduction decreases (S.F.Wang et al., 2007). In the current study, theflux chamberwas placed di-rectly on the soil surface in F2, and vegetation had no influence on thegaseous mercury in the chamber. Therefore, the decrease in mercuryemission fluxes at four other sites could be attributed to the weakenedphoto reduction of Hg2+ shaded by grass.

3.3. Assessment for modeled Hg flux

Urban soil has been reported as amajor source of the atmosphereHg(Carpi and Chen, 2002; Cheng et al., 2008), but no generally acceptedmethod has been developed to estimate Hg emission. Given that thesite with the higher soil Hg concentration indicated higher Hg emissionflux in Guangzhou, the soils in the Guangzhou urban area could beroughly divided into three groups to calculate the Hg flux for differentsoils: group A, soil with Hg concentrations less than 100 ng·g−1;group B, soil with Hg concentrations ranging from 100 to 500 ng·g−1;and group C, soil with Hg concentrations higher than 500 ng·g−1. Thepercentages of the area of each category over the area of the totalstudy region are 36.8%, 50.4%, and 12.8%, respectively.

We assume that the in situ Hg flux measurements from differentsampling sites pertain to soil from groups A, B, and C, represented bysites F1, F3 and F5, as well as F2 and F4, respectively. As stated inSection 3.2, UV radiation and soil temperature both showed significantpositive correlation with the Hg fluxes. We calculated the modeled Hgflux for each Hg flux sampling site by using SPSS 13.0, and the correla-tions among the UV radiation intensity (U), soil temperature (S), andHg flux (F) were as follows:

Group A:

F F1ð Þ ¼ 0:017U þ 0:331S−4:439 n ¼ 54;R2 ¼ 0:005� �

ð4Þ

Group B:

F F3; F5ð Þ ¼ 0:032U þ 1:269S−22:600 n ¼ 231;R2 ¼ 0:047� �

ð5Þ

Group C:

F F2; F4ð Þ ¼ 0:141U−0:730Sþ 27:149 n ¼ 143;R2 ¼ 0:139� �

: ð6Þ

The results showed that the annual Hg emission from the soils inGuangzhou city was calculated as 51.46g·km−2·yr−1 or 25.73kg·yr−1.

Fig. 7. Correlation between vegetation and Hg fluxes.

120 F. Liu et al. / Journal of Geochemical Exploration 139 (2014) 115–121

This value per unit area is about an order of magnitude greater than theaverage global emission rate of 6 g·km−2·yr−1 (Lindqvist et al., 1991),and it is obviously higher than the value of 21.7 g·km−2·yr−1 obtainedin Chongqing (Wang et al., 2006), comparable to that of Changchun(57.59g·km−2·yr−1, Fang et al., 2004) and lower than that of minerali-zation belt of North America, Canada and China (Coolbaugh et al., 2002;Gustin et al., 2000; Wang et al., 2005). Given that the study on Guang-zhou has a limited scope, an accurate estimation of mercury releaseflux needs further investigation.

4. Conclusions

The TGM concentration in air in Guangzhou urban area based on fivesites monitoring was 5.9±1.3ng·m−3, and the Hg fluxes between soiland air ranged between 1.9 and 18.6ng·m−2·h−1, which were signifi-cantly elevated compared to the values obtained in the global back-ground area. In accordance with other reported data, the total Hgconcentration in soil was found to significantly affect the Hg flux, andthe UV radiation intensity and soil temperature were indicated positivecorrelations with the Hg fluxes. In contrast, organic C, S, and Fe2O3

showed no significant correlations with Hg fluxes. The present studydemonstrates that soils with high Hg content and without vegetationcoverage can be sources of Hg emission to the atmosphere. A prelimi-nary estimate of regional mercury emission from land surface was51.46g·km−2·yr−1 or 25.73kg·yr−1 in Guangzhou urban area.

Other studies also reached various conclusions, particularly regard-ing the effect of several factors on Hg fluxes. A large proportion of the

soil in cities is backfilled, which possesses distinct properties from nat-ural soil or homogeneous layer. Given differentmethods and conditions,contradictory results may be obtained. Long-term and systematic mer-cury cycling measurements in urban area in China are lacking, andmore work needs to be conducted to clarify the processes that drivethe air–soil exchange of Hg and the effects of various factors on urbanarea.

Acknowledgments

This study was financially supported by the public welfare researchfund of the Ministry of Land and Resources of China (Grant no.200811043, 200911020) and theNewRound Land and Resource SurveyProjects (Grant no. 1212011220054). The authors thankDr. Xinbin Fengand his team at the Institute of Geochemistry, Chinese Academy ofSciences for their field work and valuable suggestions. We also thankthree anonymous reviewers and Prof. Changjiang Li (guest editor) forassessing and critiquing this manuscript. Their efforts resulted in agreatly improved paper.

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