The marine boundary layer (MBL) is the largest transport place and
reaction vessel of atmospheric mercury (Hg). The transformations of
atmospheric Hg in the MBL are crucial for the global transport and deposition of
Hg. Herein, Hg isotopic compositions of total gaseous mercury (TGM) and
particle-bound Hg (PBM) collected during three cruises to Chinese seas in
summer and winter were measured to reveal the transformation processes of
atmospheric Hg in the MBL. Unlike the observation results at inland sites,
isotopic compositions of TGM in the MBL were affected not only by mixing
continental emissions but also largely by the oxidation of
The transport and deposition of atmospheric mercury (Hg) are largely
attributed to the transformations among three species, including gaseous
elemental Hg (GEM), gaseous oxidized Hg (GOM), and particle-bound Hg (PBM),
because of the different resident times and migration abilities of them in
the atmosphere (Schroeder and Munthe, 1998). Thus, the transformations of
atmospheric Hg are crucial to the global cycling of Hg. The marine boundary
layer (MBL) is the largest transport area and reaction vessel for
atmospheric Hg on Earth. It accepts 3400 Mg yr
Compared to other marine studies performed globally, elevated GEM concentrations in the MBL have been observed in areas of Chinese seas by both coastal and cruise-based observations (Fu et al., 2010; Wang et al., 2016a, b; Ci et al., 2011a, b, 2014, 2015). Such observations indicated that anthropogenic emissions from Chinese continental areas impact atmospheric Hg in the MBL. However, the transformations of atmospheric Hg in the MBL are rarely investigated in these studies.
The stable Hg isotopic method has been utilized to trace the sources and
environmental processes of atmospheric Hg. A ternary system employing mass-dependent fractionation (MDF, reported as
Several studies on atmospheric Hg isotopes were conducted at coastal areas, where as the receptors for mixing air plumes are from both continents and the MBL (Demers et al., 2015; Fu et al., 2018, 2019a; Rolison et al., 2013; Yu et al., 2016). These reported isotopic compositions in atmospheric Hg have been suggested as mixing results of continental anthropogenic emissions and the clean air from the MBL. However, the isotopic fractionations that occurred during transformations of atmospheric Hg in the MBL are diluted by the strong impacts of continental emissions. In order to track the transformations of atmospheric Hg in the MBL using the isotopic tracing method, in situ sampling is indispensable.
The objective of this study was to track atmospheric Hg transformations in the MBL using stable Hg isotopes. Both total gaseous Hg (TGM, composed of GEM and GOM) and PBM samples were collected during three cruises to Chinese seas in summer and winter. The isotopic signatures in TGM and PBM were compared with the observation results at continental sites to extract the fractionations outside of the influences from anthropogenic emissions and to reveal the potential mechanisms of the transformation processes of atmospheric Hg in the MBL.
The TGM and PBM samples were collected aboard the
The TGM collection system was constructed following previous publications (Fu et al., 2014; Yu et al., 2016) and using a chloride activated-carbon (ClC) trap (Fu et al., 2014) to capture the TGM in ambient air. A single TGM system was installed on the vessel during the 2016-summer and 2016-winter cruises, and two systems were deployed for the 2018-summer cruise. Two total suspended particle (TSP) collection systems equipped with quartz fiber filters were installed next to the TGM collection systems for the 2018-summer cruise. Sampling was interrupted during bouts of inclement weather occasionally experienced during the cruises and thus was not continuous (Supplement Table S1). Sampling durations were divided into daytime and nighttime periods during the 2016-summer and 2018-summer cruises, and 24h continuous sampling was conducted during the 2016-winter cruise. See the Supplement for more details.
A thermal decomposition method using double-stage tube furnaces was applied
for the preconcentrations (Sun et al., 2013; Yu et al., 2016). Acid-trapping
solution (40 %,
Isotopic compositions of the solution samples were measured by a Neptune
Plus multicollector inductively coupled plasma mass spectrometer using an
online vapor generation system (Yin et al., 2010). The instrument was tuned
according to a previous publication (Geng et al., 2018) to obtain high
sensitivity (
The
Data for QA/QC are listed in Table S2.
The performance of the ClC trap was evaluated twice by parallel sampling and
by performing breakthrough experiments. Before sampling,
BCR 482 (lichen, IRMM, Belgium) was used as the standard to evaluate the
recoveries of the preconcentration procedure. The measured isotopic
compositions in the two referenced standards, including BCR 482 and SRM 3177
(mercuric chloride standard solution, NIST), were comparable to reported data
(Sun et al., 2016; Estrade et al., 2010). Replicate measurements were
conducted during Hg concentration (
The meteorological data collected during the cruises were obtained from an
automatic weather station on the
One of the two parallel sampling filters collected during the 2018-summer cruise was
treated to measure
See the Supplement, Text S3 and Fig. S1, for more details on the calculation and illustration of 72 h
back-trajectories associated with higher and lower
Statistical summary table of isotopic data in this study. The comparison data from previous publications are characterized for TGM in remote areas (Demers et al., 2013, 2015; Fu et al., 2016, 2018; Yu et al., 2016), GEM in an island located in the East China Sea (Fu et al., 2018), PBM collected at inland sites (Yu et al., 2016; Das et al., 2016; Huang et al., 2015, 2016; Xu et al., 2017), and PBM collected in a coastal site in the USA (Rolison et al., 2013) and the same island in the East China Sea (Fu et al., 2019b).
A summary of measured isotopic values and concentrations is listed in Table 1.
According to a William–York bivariate linear regression (Cantrell, 2008)
applying
Scatter plot of
On the other hand, TGM collected in the two summer cruises were characterized by
significantly negative
The atmospheric processes of Hg in the MBL with inducing isotopic
fractionations (MDF and only odd-MIF directions). Process a: mixing with
continental emissions; b: oxidation by Br atoms; c: oxidation by Cl atoms;
d: oxidations by alternative oxidants; e: photoreductions of gaseous
Hg(II); f: adsorption and desorption of
The isotopic compositions in PBM collected from the MBL with negative
The isotopic compositions in PBM in this study and the similar isotopic
compositions in PBM collected at an island site in China (Fu et al., 2019a)
were distinguishable from those collected at inland urban and rural sites,
suggesting the dominant influences are from the marine environment rather than
continental anthropogenic emissions. The primary species of PBM examined in
this study was Hg(II), accounting for
Slightly negative
MIF induced by the magnetic isotope effect (MIE) mechanism produces a slope of
To date, few isotopic studies have been performed on isotopic fractionation
during GEM oxidation; the mechanism has been suggested to be NVE,
according to a study on
Alternative oxidizers other than Br and Cl atoms – including the derivatives
of
It should be noted that the primary process leading to the Hg(II) in PBM
could not be the primary oxidation processes of
On the other hand, all slopes obtained by performing a linear regression of
the
In addition, potential alternative factors might also contribute to the
transformations of TGM in this study, followed by these isotopic clues. A
negative correlation (
The correlation between
Scatter plot of
Emissions from surface sea water (Fig. 2 process h) are commonly considered
to be crucial to influencing atmospheric Hg in the MBL. Volatilization of
dissolved gaseous Hg should induce negative MDF to
Transformations of atmospheric Hg are complicated. The mechanisms and isotopic fractionations of transformation processes are poorly understood. For instance, the photoreduction of Hg(II) in the gaseous phase (Lin and Pehkonen, 1999; Horowitz et al., 2017) might also induce odd-MIF in the Hg(II) remaining on aerosol surfaces (Fig. 2 process e). On the other hand, some gaseous mercury, e.g., methylmercury (MeHg) and dimethylmercury (diMeHg) in plume, have been suggested as important components to atmospheric Hg in the MBL (Barkay et al., 2011; Baya et al., 2015), and the Hg isotopic compositions in those components remain unclear (Fig. 2 process k). Effects of these two factors on isotopic compositions in TGM and PBM in the MBL cannot be ruled out.
Odd-MIF occurrences are commonly associated with photochemical reactions
(Bergquist and Blum, 2007; Sun et al., 2016). However, isotopic compositions
in TGM or PBM collected in daytime and nighttime were insignificantly
different in this study (
Our measurements of TGM and PBM samples collected in the MBL in China suggest
that Br atoms could be the most probable oxidant to TGM, but alternative oxidants
other than Br or Cl atoms play a major role in the formation of Hg(II) in
PBM. These oxidation processes could largely shift the Hg isotopes in the
atmosphere, producing negative MDF, positive MIF, and elevated slopes in
linear regression results of
To our knowledge, isotopic fractionation that occurs during Hg environmental processes is diluted by isotopic signatures inherited from multiple emission sources, especially from anthropogenic emissions, and thus has been omitted in previous studies conducted at continental sites when a stable Hg isotopic tracking method was used. In this study, however, mixing with continental emissions could not entirely lead to the isotopic signatures in atmospheric Hg. The observed isotopic signatures indicated the importance of local Hg environmental behaviors caused by an abundance of highly reactive species. Therefore, isotopic fractionation occurring during environmental processes should be carefully considered when using stable Hg isotopes to trace sources.
In this study, isotopic compositions in atmospheric Hg collected from marine areas were different from those collected from most inland areas. Due to the low concentrations of TGM and PBM in the MBL, the time resolutions of isotopic signatures were low. This would dilute potential isotopic fractionations occurring within each sampling period, e.g., the isotopic fractionation following the GOM concentration increasing associated with air temperature and RH changes, or the potential isotopic diversities associated with the gradient PBM concentration from coastal areas to open seas (Wang et al., 2016a, b). In addition, many atmospheric Hg transformation processes, e.g., the reduction of Hg(II) in the gaseous phase, are still poorly understood. Moreover, isotopic fingerprints of many endmembers, e.g., re-emitted gaseous Hg from surface sea water, are unknown. More studies are therefore needed to constrain isotopic fractionation during these processes and isotopic compositions in these endmembers. When the sampling and isotopic measurement techniques improve, and the isotopic study of the oxidation of gaseous Hg is performed in the future, stable Hg isotopes could provide diagnostic information for clarifying the contributions of multiple environmental processes influencing atmospheric Hg chemistry, and they could serve as effective tools for tracking transformation processes of atmospheric Hg in the MBL and in other areas with a variety of atmospheric oxidants in the atmosphere.
A dataset for this paper can be accessed at
The supplement related to this article is available online at:
BY, LY, LW, and HL conducted sampling and measurements. CX and YL assisted the measurements. BY and JS designed research. BY, JS, QL, YY, LH, and GJ wrote the paper.
The authors declare that they have no conflict of interest.
The authors would like to thank the captain and crew of
This research has been supported by the CAS Interdisciplinary Innovation Team (grant no. JCTD-2018-04), the National Natural Science Foundation of China (grant nos. 41877367, 91843301, and 21707157), and the Sanming Project of Medicine in Shenzhen (grant no. SZSM201811070).
This paper was edited by Leiming Zhang and reviewed by two anonymous referees.