The pattern of air–surface gaseous mercury (mainly Hg(0)) exchange in the
Qinghai–Tibet Plateau (QTP) may be unique because this region is
characterized by low temperature, great temperature variation, intensive
solar radiation, and pronounced freeze–thaw process of permafrost soils.
However, the air–surface Hg(0) flux in the QTP is poorly investigated. In this
study, we performed field measurements and controlled field experiments with
dynamic flux chambers technique to examine the flux, temporal variation and
influencing factors of air–surface Hg(0) exchange at a high-altitude
(4700 m a.s.l.) and remote site in the central QTP. The results of field
measurements showed that surface soils were the net emission source of Hg(0) in
the entire study (2.86 ng m
Soils represent the largest Hg reservoirs in ecosystems and play a major role in the global Hg cycle (Selin, 2009; Agnan et al., 2016). Background soils receive Hg input from atmospheric deposition, which is mainly retained in organic-rich layers of upper soils (Schuster, 1991; Khwaja et al., 2006). Hg in soils can be reduced to Hg(0) and then emitted to the overlaying air because of its high volatility (Schlüter, 2006). Therefore, soils can serve as both sources and sinks of atmospheric Hg (Pirrone and Mason, 2009; Amos et al., 2013; Agnan et al., 2016).
In the past several decades, efforts have been made to improve the understanding of soil Hg biogeochemistry (Zhang and Lindberg, 1999; Lin et al., 2010; Schlüter, 2006; Jiskra et al., 2015). Measurements across various types of soils and climates show that air–soil Hg(0) exchange has highly spatial and temporal variation and bidirectional exchange behavior (Agnan et al., 2016, and references therein). Field measurements and laboratory experiments highlight that various factors and processes influence air–surface Hg(0) exchange, including concentrations and species of soil Hg (Gustin et al., 1999, 2002; Hintelmann et al., 2002; Bahlmann et al., 2006; Kocman and Horvat, 2010; Eckley et al., 2011; Edwards and Howard, 2013; Mazur et al., 2015), solar radiation (Gustin et al., 2002, 2006; Moore and Carpi, 2005; Xin et al., 2007; Fu et al., 2008a; Kocman and Horvat, 2010; Park et al., 2014), precipitation (Lindberg et al., 1999; Gustin and Stamenkovic, 2005; Gabriel et al., 2011), soil temperature and moisture (Gustin et al., 1997; Gustin and Stamenkovic, 2005; Ericksen et al., 2006; Xin et al., 2007; Briggs and Gustin, 2013; Park et al., 2014; Mazur et al., 2015), soil organic matter and pH (Yang et al., 2007; Xin and Gustin, 2007; Mauclair et al., 2008), land cover (Dommergue et al., 2003; Ericksen et al., 2005; Cobbett et al., 2007; Gabriel and Williamson, 2008; Zhu et al., 2011; Durnford et al., 2012a, b; Toyota et al., 2014a, b), atmospheric Hg(0) concentrations and other chemical compositions (Engle et al., 2004; Xin and Gustin, 2007; Fu et al., 2008a), biological activity (Choi and Holsen, 2009), and atmospheric turbulence (Gustin et al., 1997; Poissant et al. 1999). Existing studies on Hg(0) dynamics at the air–surface interface are mainly performed in temperate regions (Agnan et al., 2016, and references therein). The seasonal frozen soils and permafrost widely distribute, accounting for almost 70 % of terrestrial area of Earth (NSIDC, 2016). However, the knowledge of air–surface Hg(0) dynamics in cold regions is limited (Cobbett et al., 2007; Durnford and Dastoor, 2011). Most current parameters of air–soil Hg(0) exchange applied in Hg biogeochemical models are mainly derived from temperate regions of North America and Europe (Zhu et al., 2016).
The Qinghai–Tibet Plateau (QTP) is located in western China with an area
of 2.5 million km
It is noted that many studies addressed that precipitation greatly influences air–surface Hg(0) flux over different timescales (Lindberg et al., 1999). However, previous studies have mainly focused on the effect of rainfall/watering on air–soil Hg(0) flux (Lindberg et al., 1999; Johnson et al., 2003; Gustin and Stamenkovic, 2005; Song and Van Heyst, 2005; Corbett-Hains et al., 2012) or the fate and transport of Hg(0) at the air–snow interface (Lalonde et al., 2001, 2003; Ferrari et al., 2005; Dommergue et al., 2003, 2007; Faïn et al., 2007; Bartels-Rausch et al., 2008; Brooks et al., 2008; Steen et al., 2009; Durnford et al., 2012a, b; Mann et al., 2015). Field studies on the effect of snowmelt on Hg(0) flux are very limited (Cobbett et al., 2007).
In this study, we the applied dynamic flux chamber (DFC) technique to investigate the flux, temporal variation and influencing factors of air–surface Hg(0) exchange at a representative research station in the central QTP. At the same time, controlled field experiments were performed to explore the effect of rainfall and different wavebands of solar radiation on air–soil Hg(0) flux. Combining the results of this study and other knowledge, we discuss the effect of future climatic and environmental change on air–surface Hg(0) dynamics in the QTP.
The study was performed at the Beiluhe Permafrost Engineering and
Environmental Research Station affiliated with the Cold and Arid Regions
Environmental and Engineering Research Institute, Chinese Academy of Sciences
(CAREER–CAS). The elevation of the Beiluhe region is about 4700 to
4800 m a.s.l. The station (34
Locations of the Beiluhe station (4760 m a.s.l., this study), Mt. Waliguan (3816 m a.s.l., Fu et al., 2012), Mt. Gongga (1640 m a.s.l.; Fu et al., 2008b) and Nam Co (4730 m a.s.l.; Yin et al., 2015) where atmospheric Hg was determined.
Temporal variation in environmental variables, air Hg(0) concentrations inside and outside of the chamber, and air–surface Hg(0) flux at the Beiluhe station in the central QTP during four campaigns in 2014–2015.
The precipitation at the Beiluhe station mainly occurs during May to October under the influence of the southern Asian monsoon (Peng et al., 2015a). Due to the high-altitude location of the Beiluhe station, snow events commonly occur in May to June and late September to October. Because of intensive solar radiation and surface temperature, the snow melts or sublimates on a short timescale (1–100 h), i.e., little/no snow accumulation occurs for a long time (> 3 day). Therefore, the Beiluhe region provides an unique opportunity to investigate the different effects of rain, snow, and snowmelt on the air–surface Hg(0) flux over different timescales.
The dynamic flux chamber (DFC) technique was widely used to investigate
Hg(0) flux between the air and surface because it is inexpensive,
portable, easy to set up and operate (e.g., Kim and Lindberg, 1995; Carpi and
Lindberg, 1998; Gustin et al., 2006; Wang et al., 2006; Dommergue et al.,
2007; Fu et al., 2008a; Kocman and Horvat, 2010; Edwards and Howard, 2013).
Air–surface Hg(0) flux obtained by DFC technique was calculated using
Eq. (1),
In this study, quartz chambers were constructed for measuring Hg(0) flux and exploring the effect of different rainfall depths and radiation condition on the Hg(0) flux. Quartz glass has many advantages as a construction material used in chambers for determining Hg(0) flux in background soils. First, it has high transmittance of the full spectrum of solar radiation, especially the UV waveband (Fig. S1 in Supplement). Therefore, a quartz chamber is suitable for determining the more “actual” Hg(0) flux because the short wavelength of solar radiation has been found to have an important effect on Hg(0) dynamics at the air–soil interface (Moore and Carpi, 2005; Bahlmann et al., 2006). Second, it has low potential for Hg(0) adsorption and is easy to clean by heating to remove Hg bonding on the surface (Ci et al., 2016a). The low systematic blank of quartz chamber is critical for investigating Hg(0) flux over background soils (Carpi and Lindberg, 1998).
Our semi-cylindrical quartz chamber was 8 cm high and 24 cm long with a
footprint of 0.0384 m
Due to the harsh environmental conditions and the unstable power supply, use of the commercial automatic Hg analyzer (such as Tekran 2537) to
conduct field measurements of Hg(0) flux is challenging in the Beiluhe
station. Therefore, air Hg(0) concentrations in the both inlet and outlet of the
chamber were monitored manually by a gold trap simultaneously at 2–3 h
intervals (Ci et al., 2016b). The air was pumped through gold trap using air
pump (KNF Inc., Germany) with 0.50 L min
The turnover time obtained from this protocol was 0.68 min, which is similar
to previous studies (Eckley et al., 2010, and references therein). The flow
rates of air both the inlet and outlet of the chamber were adjusted by a needle
valve and controlled by a rotameter. Prior to the measurement, the rotameters
were calibrated by a mass flow meter and a volumetric gas meter. The accuracy
of flow rate was
In this study, a bare soil plot of 2 m
In this study, all materials in contact with Hg(0) were quartz, Teflon or
borosilicate glass. The chambers and tubing were rigorously acid-washed (Ci
et al., 2016a). The quartz chambers were heated to 650
To explore the effect of water addition in detail, we chose another similar
soil plot with homogeneous soil properties to conduct controlled field
experiments in order to investigate the effect of different rainfall depths on Hg(0)
flux over different timescales (from minutes to hours). The controlled
experiments were performed during the May–June 2015 campaign since this period
had high surface temperature and low precipitation (Fig. 2). In the Beiluhe
station, hourly rainfall depth rarely exceeds 15 mm (Peng et al., 2015a).
Therefore, we designed four different treatments of rainfall depth (0, 1, 5,
and 15 mm). The water addition to the dry soils commenced at night
(01:40 LT) on 30 May 2015 to exclude the
effect of photochemical process in the first hours of experiments. We added
the Milli-Q water (Hg concentration < 0.2 ng L
The QTP is characterized by high solar radiation with intense UV radiation. We performed the controlled experiment to quantify the role of different wavebands of solar radiation (UVB, UVA and visible light) in air–soil Hg(0) flux. Four chambers with different exposure treatments were used to measure Hg(0) flux simultaneously in the daytime. Chamber A was used to measure the Hg(0) flux in the natural light. Chamber B and chamber C were covered with UVB and UV filters to remove the corresponding wavebands from the natural light. Chamber D covered with foil was used to measure Hg(0) flux in the dark. The experiments were performed in 4 days without precipitation (21–22 December 2014 and 29–30 May 2015) to exclude the effect of precipitation. Hg(0) flux triggered by UVB, UVA and visible light was equal to difference of flux between chamber A and chamber B, between chamber B and chamber C, and between chamber C and chamber D, respectively. The transmittance of UVB filter and UV filter was shown in Fig. S1.
Surface soil samples (0–2 cm) were collected from four soil subplots during
the June 2014 campaign. Soil samples were freeze-dried and homogenized for total
Hg determination using a Milestone DMA direct Hg analyzer (detection limit:
0.01 ng Hg or 0.15
A meteorological station that located 60 m from the soil plot was used to
collect the following environmental variables: air temperature (
Soil Hg concentrations of four subplots varied from 13.11
Figure 2 shows the temporal variation in air Hg(0) concentrations inside and
outside of the chamber, air–surface Hg(0) flux and environmental variables
during four campaigns in 2014–2015. Hg(0) concentrations of ambient air
ranged from 0.93 to 1.78 ng m
Seasonal variation in Hg(0) concentration in ambient air and air–surface Hg(0) flux during four campaigns at the Beiluhe station in the central QTP in 2014–2015.
Temporal variation in Hg(0) flux over four soil plots with different treatment of water addition (T0: 0 mm treatment; T1: 1 mm treatment; T5: 5 mm treatment; and T15: 15 mm treatment) and the environmental variables.
The mean of air–surface Hg(0) flux in the entire study period were
2.86 ng m
Figure 2 shows that the Hg(0) flux was highly variable. The highest Hg(0)
emission fluxes of 28.46 ng m
Hg(0) flux generally showed a diurnal pattern with high emission in the daytime and remarkable deposition in nighttime, especially on days without precipitation (Fig. 2). Many studies have confirmed that solar radiation is one of the most important drivers for soil Hg(0) emission (Xin and Gustin, 2007; Choi and Holsen, 2009; Kocman and Horvat, 2010); high surface temperature also facilitates Hg(0) production and subsequent emission (Park et al., 2014). Therefore, the two environmental variables jointly regulate the diurnal pattern of Hg(0) flux. An in-depth discussion on synergistic effects of solar radiation and surface temperature on Hg(0) flux is provided below. Interestingly, the diurnal pattern of Hg(0) flux of each day during the December 2014 campaign was almost identical, which may be associated with very similar weather conditions throughout the entire campaign.
Hg(0) flux showed pronounced seasonality with high emission in three warm
campaigns (June 2014: 4.95 ng m
Firstly, the effect of rain events on Hg(0) flux was investigated. We found that the Hg(0) emission flux increased immediately following the rainfall (Fig. 2), which is consistent with many other studies (Gustin and Stamenkovic, 2005; Johnson et al., 2003; Lindberg et al., 1999; Song and Van Heyst, 2005). Previous studies have suggested that the dramatic increases in Hg(0) emission may be attributed to the physical displacement of Hg(0) present in soil air and desorption of loosely bound Hg(0) on soil particles by the infiltrating water (Johnson et al., 2003; Gustin and Stamenkovic, 2005). Notably, Fig. 2 shows that the pulse of Hg(0) emission after the rainfall was also observed at nighttime (such as 0:00 to 01:00 LT on 4 September 2014). A similar phenomenon was also documented by our controlled experiments (see below). This indicates that the immediate increase in Hg(0) emission might not be controlled by photochemical processes but by physical processes.
Soil moisture condition may also significantly regulate Hg(0) flux over relatively long timescales (from hours to several days). Therefore, many experiments studied the effect of water addition on the magnitude and pattern of air–soil Hg(0) flux over different timescales (Johnson et al., 2003; Gustin and Stamenkovic, 2005). However, most of studies were performed in controlled laboratory or mesocosm settings under certain well-defined but not necessarily environment relevant conditions (Johnson et al., 2003; Gustin et al., 2004; Gustin and Stamenkovic, 2005; Song and Van Heyst, 2005; Kocman and Horvat, 2010; Corbett-Hains et al., 2012; Park et al., 2014). In this study, it was also challenging to reveal the effect of rainfall on Hg(0) flux over relatively long timescales via field measurement since the intermittent rain events occurred irregularly during the June 2014 and September 2014 campaign (Fig. 2). Therefore, we performed the controlled field experiments (Sect. 2.3.1) to explore the effect of different rainfall depths on Hg(0) flux over different timescales (from minutes to hours).
The high-time-resolution measurements captured the immediate and dramatic
increases in Hg(0) emission flux after the watering of dry soils (Fig. 4).
The baseline Hg(0) flux of the 0 mm treatment was used as the benchmark for the
different rainfall depth treatments to be compared against. Obviously, the
higher amount of water addition resulted in longer duration and higher
accumulative flux of Hg(0) emission pulse. The duration of the Hg(0) emission
pulse for the 1 mm and 5 mm treatment was < 20 min (from 01:40 to
02:00 LT) and
As shown in Fig. 4, the cumulative flux of Hg(0) emission during the entire
study period mainly included two fractions: the pulse of Hg(0) emission after
the watering (i.e., emission flux by watering) and the Hg(0) emission during
the daytime (i.e., emission flux by radiation). Figure 5 shows that both
“emission flux by watering” and “emission flux by radiation” for the 15 mm
treatment were significantly higher than those of the 1 and 5 mm treatment.
As mentioned above, the dramatic increase in Hg(0) emission after the
simulated rain can be explained by physical displacement of interstitial soil
air by the infiltrating water. The long emission duration and large immediate
emission flux for soil plot with high water addition can be explained by more water needing a longer time to percolate the soil column and
displacing
more soil Hg(0). Many previous studies have suggested that the magnitude of Hg(0)
emission with a rainfall or stimulated rain depended on soil moisture
condition – i.e., if the amounts of water received by the soils were less than
needed to saturate, the soil surface showed an immediate increase in Hg(0)
emission, and after the soil became saturated, Hg(0) emission from surface soil
was suppressed (Klusman and Webster, 1981; Lindberg et al., 1999; Johnson et
al., 2003; Gustin and Stamenkovic, 2005). In this study, the pulse of Hg(0)
emission flux for the 15 mm treatment was significantly higher than that of the 5
and 1 mm treatment (Fig. 5). The field water capacity and bulk density of
soil in the Beiluhe region is about 28 % and 1 g cm
Increased Hg(0) emission for three different treatments (1, 5 and 15 mm addition of water) compared with the 0 mm treatment during the controlled experiment on 30 May 2015.
Daily Hg(0) flux and daily precipitation in the June 2014 campaign.
The water addition also increased the Hg(0) emission in the daytime, showing more water added, longer duration of Hg(0) emission, and more Hg(0) emitted (Figs. 4 and 5). After the surface soil was visibly dry, Hg(0) flux over soil plots with water addition had no significant difference from that of the soil plot without water addition (i.e., 0 mm treatment). This result is consistent with many other controlled studies. For example, Johnson et al. (2003) and Gustin and Stamenkovic (2005) found that once the soil water content became less than saturated, Hg(0) emission flux would be significantly enhanced, especially during the daytime, and once sufficient drying occurred, the magnitude of Hg(0) emission flux tended to gradually decrease. Investigators have suggested that as the water evaporates and soil dries, capillary action drives the upward movement of water and chemicals (including Hg components) and recharges the Hg pool in surface soils (i.e., the “wick effect”) and subsequently favors the Hg(0) production and emission via photochemical processes in the light (Gustin and Stamenkovic, 2005). In our study, even for the wettest soil plot (i.e., 15 mm treatment), the surface soils were visually unsaturated in the daytime because of the low water retention, high infiltration rate of local soils and intensive solar radiation. Therefore, the pattern of Hg(0) emission for soil plots with high water addition is comparable to those of the above-mentioned studies.
Secondly, the effect of snow events on Hg(0) flux were investigated. One of
the most significant differences between the rainfall and snowfall on the
effect of Hg(0) exchange was that the snowfall did not induce the remarkable
pulse of Hg(0) emission. For example, at 10:10 LT on 11 June 2014, a heavy
snowfall event occurred, which continued until 11:20 LT. The highest thickness of the snowpack
reached
We found that the snow melting led to the remarkable peak of Hg(0) emission.
For example, during the sunrise of 12 June 2014, a precipitation event with rain
and snow caused the snowpack (12 cm) to melt suddenly and completely (i.e., bare soil with no surface snow) and a pulse of Hg(0) emission
(
Finally, the effect of precipitation (including rain and snow) on daily Hg(0) flux was investigated. The above-mentioned results and discussion suggest that the precipitation has great potential to facilitate soil Hg(0) emission over different timescales via physical, chemical and biological processes. The main processes include the displacement of soil Hg(0) by water, the “wick effect” to increase the photo-reducible Hg(II) pool in surface soils, and the increased soil moisture to promote the biotic and abiotic reduction of Hg(II). Another well-documented process is that the atmospheric wet deposition of Hg will increase the Hg pool in surface soils and the newly deposited Hg is very active in reducing to Hg(0) (Hintelmann et al., 2002), although our study did not focus on this issue. During the June 2014 campaign, no precipitation occurred in the first 2 days (6–7 June 2014), but the rest of the days were rainy or snowy days (Fig. 2). We attempted to use the daily Hg(0) flux of the two sunny days as the benchmark to compare with those of rainy or snowy days in order to investigate the effect of precipitation on the Hg(0) flux over the timescale of one day. Figure 6 shows that the daily Hg(0) flux for rainy/snowy days was higher (ranging from 16 to 154 %) than the mean of the two sunny days. The result indicates that the precipitation increased soil Hg(0) emission on the timescale of 1 day, although the low solar radiation and temperature on rainy/snowy days would potentially decrease soil Hg(0) emission, as mentioned above.
Temporal variation in bulk Hg(0) flux in the light, Hg(0) flux in the dark, and net Hg(0) flux in the light (bulk Hg(0) flux in the light–Hg(0) flux in the dark) in six study days without precipitation during the December 2014 campaign and May–June 2015 campaign.
Almost all laboratory experiments and field measurements, including this study, show that the high solar radiation and elevated soil temperature synergistically facilitate the soil Hg(0) emission (Edwards and Howard, 2013; Park et al., 2014). The following hypotheses have been proposed to explain the role of solar radiation and temperature in promoting soil Hg(0) emission: (1) solar radiation promotes the photo-reduction of Hg(II) in surface soils to form Hg(0) on a short timescale, (2) solar radiation and high soil temperature reduce the apparent activation energy of Hg(0) desorption and increase Hg(0) emission from surface soils, and (3) the high soil temperature favors the Hg(0) production in soil column by biotic and abiotic processes (Carpi and Lindberg, 1998; Gustin et al., 2002).
Many studies used the Arrhenius equation (Eq. 2) to quantitatively
investigate the relationship between soil temperature and Hg(0) flux.
In this study, for determining the respective contributions of solar radiation and temperature to the Hg(0) flux, in addition to Hg(0) flux being measured in natural light, Hg(0) flux in the dark was also measured simultaneously with a foil-covered chamber. The temperature-corrected Hg(0) flux (i.e., bulk Hg(0) flux in the light minus Hg(0) flux in the dark) in the daytime (PAR > 0) was considered to be the contribution of the solar radiation. As mentioned above, the effect of precipitation should be excluded from the data set; therefore, we only collected Hg(0) flux data on days without precipitation during the December 2014 and May–June 2015 campaign.
Figure 7 displays the temporal variation in bulk Hg(0) flux in the light, Hg(0) flux in the dark, net Hg(0) flux in the light (i.e., bulk Hg(0) flux in the light – Hg(0) flux in the dark) and the environmental variables. Clearly, changes in solar radiation had a greater influence on soil Hg(0) flux than did changes in soil temperature. The data showed that the soil served as a Hg(0) sink during all study days in the December 2014 campaign in the dark with high deposition flux in low soil temperature and low deposition flux in high soil temperature. During study days of the May–June 2015 campaign, the soils served as a very low Hg(0) source at midday with relatively high soil temperature. This finding is consistent with many studies in background soils (Ericksen et al., 2006; Gustin et al., 2006; Fu et al., 2008a; Edwards and Howard, 2013). It indicates that the soil temperature plays an important role in Hg(0) dynamics at the air–soil interface, i.e., low soil temperature favors absorbing Hg(0) or reducing Hg(0) emission (e.g., Park et al., 2014, and references therein).
After the temperature was corrected, except for at midday on study days during
the May–June campaign, the net Hg(0) flux in the light was higher than the bulk
Hg(0) flux. A positive linear correlation was found between cumulative PAR
and cumulative Hg(0) flux in the daytime, although cumulative PAR only
explained
We used the Hg(0) emission data set in the dark to calculate the
Cumulative Hg(0) emission flux in the daytime triggered by UVB, UVA and visible light in four study days during the December 2014 campaign and May–June 2015 campaign.
Figure 8 shows that UV radiation was the dominant waveband of solar radiation for Hg(0) emission in the daytime, contributing > 80 % of Hg(0) emission in the light, and the contribution of UVB radiation accounted for > 50 % in all study days. This finding is consistent with previous laboratory studies (Moore and Carpi, 2005; Bahlmann et al., 2006; Xin et al., 2007).
In this study, we measured the Hg(0) flux between the air and surface permafrost soil in the QTP. We also performed controlled field experiments to explore the effect of precipitation and different wavebands of solar radiation on the air–soil Hg(0) exchange. The results showed that the environmental conditions, including solar radiation, soil temperature and precipitation, greatly influenced the Hg(0) exchange between air and surface.
This study and other field measurements and laboratory experiments have
clarified that the fate and transport of soil Hg is very sensitive to the
environmental variables (Krabbenhoft and Sunderland, 2013). Therefore, our
results have several important implications to the Hg biogeochemical cycle
in the soils of the QTP under the rapid climate warming and environmental
change. Firstly, the increased surface temperature in the QTP will
potentially promote the remobilization of soil Hg. Field measurements and a
modeling study have revealed that the surface temperature in the QTP is
increasing, and the warming trend exceeds those for the Northern Hemisphere
and the same latitudinal zone (Kang et al., 2010). Secondly, the increased
UV radiation in the QTP may enhance Hg(0) emission from surface soils. UV
radiation reaching the surface of the QTP is estimated to increase because
of the decrease of stratospheric O
The data are available on request from the first author.
The study was financially supported by the National Key Basic Research Program of China (no. 2013CB430002), the National Natural Science Foundation of China (no. 41573117, 41371461, 41203068), and the Young Scientists Fund of the Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences (no. RCEES-QN-20130048F). We thank the staff of the Beiluhe Permafrost Engineering and Environmental Research Station affiliated with the Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences (CAREER–CAS), for their assistance. Edited by: R. Ebinghaus Reviewed by: three anonymous referees