Analysis Methods, Occurrence, and Transformation of Reactive Gaseous Mercury in the Atmosphere

Yingying Fang; Ying Wang; Jianbo Shi; Yongguang Yin; Yong Cai; Guibin Jiang

doi:10.7536/PC200958

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (1) : 151-161 DOI: 10.7536/PC200958

Scan to read & Share

Analysis Methods, Occurrence, and Transformation of Reactive Gaseous Mercury in the Atmosphere

Yingying Fang ¹^,²
, Ying Wang ¹^,²
, Jianbo Shi ¹^,²^,⁴
, Yongguang Yin ¹^,²^,⁴^,^*
, Yong Cai ¹^,³
, Guibin Jiang ¹^,²^,⁴

Author information +

History +

PDF

Abstract

Reactive gaseous mercury(RGM), also known as gaseous oxidized mercury, dominates atmospheric mercury deposition and is critical to the global cycle of mercury. This review introduces various sampling and analysis methods of RGM in detail, and discusses the advantages and limitations of the current techniques. The environmental processes including the formation, occurrence, and depletion of RGM in the atmosphere and related mechanisms are reviewed, and the contribution of each process in the atmospheric mercury cycle is explored. In view of the current analytical difficulties of RGM(e.g., ultralow concentration and sampling problems) and key scientific issues(e.g., chemical form and transformation) in the RGM research, efforts should be made to develop the feasible methods for RGM collection and speciation determinatiton in the real environment, so as to further explore its environmental behavior. This is challenging but important for the research of atmospheric mercury and will be helpful for understanding the role of RGM in cycle processes of atmospheric mercury.

Contents

1 Introduction

2 Sampling and analysis of reactive gaseous mercury

2.1 Sampling of reactive gaseous mercury

2.2 Analysis of reactive gaseous mercury

3 Occurrence and transformation of reactive gaseous mercury in the atmosphere

3.1 Formation of reactive gaseous mercury in the atmosphere

3.2 Occurrence of reactive gaseous mercury in the atmosphere

3.3 Depletion of reactive gaseous mercury from the atmosphere

4 Conclusion and outlook

Graphical abstract

Keywords

reactive gaseous mercury / sampling / analysis / chemical species / occurrence / depletion

Cite this article

Download citation ▾

Yingying Fang, Ying Wang, Jianbo Shi, Yongguang Yin, Yong Cai, Guibin Jiang. Analysis Methods, Occurrence, and Transformation of Reactive Gaseous Mercury in the Atmosphere. Progress in Chemistry, 2021, 33(1): 151-161 DOI:10.7536/PC200958

登录浏览全文

4963

注册一个新账户忘记密码

1 Introduction

Mercury is the only toxic heavy metal pollutant that is transported globally over long distances primarily through the atmosphere. Compared with other environmental media, the atmosphere is an important place for mercury transport, transformation, deposition and re-emission in the earth's ecosystem. The mercury storage in the atmosphere is about 4400 tons^[1]. Mercury can be emitted into the atmosphere through natural and human activities. The global mercury emission from natural sources is 5207 t/a, and that from human activities is 2320 t/a^[2]. Natural sources mainly include natural mercury emission processes such as volcanic activities and forest fires, and re-emission processes of mercury after sinking to the surface. Anthropogenic sources mainly include combustion emission sources such as coal combustion, waste incineration and metal smelting and processing^[3]. After direct emission, mercury can enter terrestrial and aquatic ecosystems through atmospheric oxidative deposition and vegetation leaf absorption. Dry and wet deposition of mercury from the atmosphere is the main way of mercury input into terrestrial and aquatic ecosystems, and the deposited mercury can be further transformed into highly toxic methylmercury, which is enriched and magnified with the food chain^[4]^[5].

The main forms of mercury in the atmosphere are gaseous elemental mercury (GEM), reactive gaseous mercury (RGM) and particulate bound mercury (PBM). GEM is the main form of atmospheric mercury (concentration level is about 1~2 ng·m^-3), accounting for more than 95% of atmospheric mercury. It has stable chemical properties, low water solubility (3.03×10^-7M·atm^-1,25℃), and can stay in the atmosphere for about 0.5 to 2 years, so it has a strong ability of long-distance transport^[6]^[7]^[8,9]. RGM, also known as gaseous oxidized mercury (GOM), is chemically active and has a low concentration level in the atmosphere (gaseous oxidized mercury, accounting for about 3% of atmospheric mercury). It is easily dissolved in atmospheric droplets (such as Hg(OH)₂∶1.2×10⁴M·atm^-1;HgCl₂∶1.4×10⁶M·atm^-1;25℃) or adsorbed on the surface of particulate matter and converted into PBM^[10]^[7]. PBM accounts for less than 1% of atmospheric mercury^[11]. RGM and PBM can stay in the atmosphere for several days to several weeks, and are quickly removed from the atmosphere by dry and wet deposition after short-range transport^[5]^[8].

Atmospheric mercury will undergo complex homogeneous and heterogeneous chemical reactions in the air, and various forms will be transformed into each other^[12]. Under the action of solar radiation, atmospheric oxides (O₃, H₂O₂, etc.) And active radicals (OH ·, Br ·, Cl ·, BrO ·, ClO ·, I ·, etc.), GEM will be oxidized to RGM, thus reducing the residence time of atmospheric mercury^[13]. RGM can be redistributed between different phase interfaces (including gas-liquid partition and gas-particle partition) and transformed into water-soluble oxidized mercury or particulate mercury, and can also be reduced to GEM under the action of solar radiation or atmospheric reducing substances (SO₂, CO)^[14]^[15,16]. The mutual transformation between the forms directly determines the residence time and migration distance of mercury in the atmosphere. The types and rates of atmospheric mercury deposition are significantly different for different forms of mercury^[17,18].

As one of the main contributors of atmospheric mercury deposition, RGM is an important indicator to measure the impact of atmospheric mercury on terrestrial or marine ecosystems. However, there are still some deficiencies in the understanding of RGM at this stage, which are mainly reflected in the following aspects: (1) The sampling and analysis of RGM at present rely on the method of uncalibrated operation definition, and the uncertainty of field monitoring results is large; (2) The traditional RGM analysis methods are mostly based on the analysis of total mercury, and the understanding of the chemical forms of RGM is insufficient; (3) Compared with GEM, RGM is chemically active and unstable, and the species of mercury is easy to change during the collection process; (4) There is still a lack of systematic understanding of the transformation process and mechanism of RGM, and the uncertainty caused by the occurrence and transformation of RGM in atmospheric mercury models is large. Therefore, the study of the collection and analysis methods of RGM and its environmental behavior will help to understand its role in the oxidation and reduction of atmospheric mercury and improve the accuracy of atmospheric mercury models.

In this paper, various sampling and analysis methods of RGM are reviewed, and the advantages and limitations of current techniques are discussed and compared. Considering the importance of RGM morphology, the morphological analysis techniques of RGM were summarized. The environmental behaviors of RGM in the atmosphere, such as generation, occurrence and removal, and the related mechanisms were summarized, and the contribution of each process to the atmospheric mercury cycle was discussed.

2 Active gaseous mercury collection and analysis method

2.1 Method for collecting active gaseous mercury

According to the difference of sampling principles, there are three main RGM collection methods at present: KCl-coated diffusion tube method, multi-stage filter membrane method and reflux spray box method (as shown in Figure 1)^[19]. Details and comparisons of each acquisition method are shown in Table 1.

2.1.1 KCl plating diffusion tube method

The KCl-coated diffusion tube method selectively absorbs RGM by using the difference of diffusion coefficients between gas and particulate matter, while GEM and PBM can freely pass through the diffusion tube^{[20,23⇓ ~25]}^[15]. RGM molecules pass through the inner wall of the diffusion tube and are captured by the KCl coating layer on the surface, thus realizing the collection of RGM. There are mainly two types of diffusers: tubular and annular. Both of them have the same collection principle, but the annular diffuser has a larger effective collection area, a higher sampling flow rate (10 L·min^-1) than the tubular diffuser (1 L·min^-1), a shorter sampling time and a higher collection efficiency^[23]^[20]. The atmospheric active mercury sampler developed on the basis of the annular diffusion tube method has been applied to the commercial instrument Tekran ® 1130, which has a high degree of automation and high time resolution, and is widely used in global atmospheric mercury monitoring^[32].

2.1.2 Multistage filtration membrane method

The core of the multi-stage membrane filtration method is to filter out the particulate matter in the airflow by using the membrane first, and then intercept the RGM in the airflow by using the membrane with selective absorption capacity^{[21,28,33⇓⇓⇓⇓ ~38]}. The sampling flow rate of this method is low (1 L·min^-1) and the sampling time is long (1 ~ 2 w). Different materials of filter membrane combined with RGM in different ways, and the collection effect was significantly different. Cation-exchange membranes are widely used, and nylon membranes, polytetrafluoroethylene (PTFE) membranes and KCl-coated glass fiber membranes can also be used, but the latter three are not as effective as cation-exchange membranes, and are generally not used to quantify the concentration of RGM^{[28,34⇓ ~36]}^[39]^[40,41]. On the one hand, the advantages of the multi-stage filtration membrane method are that the cation exchange membrane can collect more RGM with more forms, and the quantitative results are 1.6 to 12 times higher than those of the KCl-coated diffusion tube method^[32,33]; On the other hand, nylon membrane sampling combined with temperature-programmed thermal desorption technique can be used to identify the composition of RGM, which shows a certain potential in the morphological analysis of RGM^[34]. Nylon membrane and cation exchange membrane are used together to form a Reactive Mercury Active analysis system (University of Nevada, Reno-Reactive Mercury Active Systems, UNR-RMAS) that can qualitatively and quantitatively measure RGM, which was first applied to the monitoring of RGM in the actual environment by Gustin, et al., University of Nevada, Reno^[37,38]. The system comprises six groups of sampling ports, each port is provided with a bracket consisting of two Teflon filters connected in series, and nylon membranes and cation exchange membranes are respectively arranged on the three brackets in pairs. The improved UNR-RMAS 2.0 system by Gustin et al has a shorter sampling time and has been applied in many places around the world in recent years^[35]^[42]. In addition, cation exchange membranes can be used not only as RGM sampling filters, but also as surrogate surfaces in passive samplers to simulate the deposition process of atmospheric mercury to the surface, so as to measure the dry deposition of atmospheric mercury^[43].

2.1.3 Reflux spray box method

The reflux spray box method uses dilute HCl or dilute HCl/NaCl mixture as the absorption solution to collect RGM^[10,22,29]. The single nozzle atomizer is usually used to make the absorption liquid form fine droplets in the box at high flow rate (15~20 L·min^-1) and fully react with the air. The RGM is accumulated in the absorption liquid after gas-liquid distribution in the droplets. The sampling time is 0.5 ~ 2 H, and the sampling efficiency is high. The collection efficiency of RGM is closely related to the composition and concentration of the absorption solution, and high concentration of chloride ion can maintain the stability of Hg (Ⅱ) in the solution^[29]. The operation of the backflow spray box method is relatively complex, and it is rarely used in the actual sampling of RGM.

2.1.4 Comparison of observation results of three sampling methods

Table 2 compares field observations from different sampling methods. It can be seen that 20 years ago, the sampling of RGM was mainly carried out by KCl-coated diffusion tube method and reflux spray box method. Subsequently, the KCl-coated diffusion tube method was commercially developed, and Tekran ® 1130 was widely used. In the past 10 years, with the increasing recognition of RGM forms, the multi-stage filter method has been gradually developed because it can simultaneously qualitatively and quantitatively analyze RGM in the atmosphere. The KCl-coated diffusion tube method is widely used and has high time resolution, so it is usually used as a control to compare with other methods. From the comparison of RGM observation values, it is found that the RGM concentration values measured by cation exchange membrane are generally 2 to 16 times higher than those measured by KCl-coated diffusion tube method, and 1.5 to 35 times higher than those measured by nylon membrane, but at the same time, the cation exchange membrane has the highest mercury blank, and the sampling time of UNR-RMAS is longer. At present, there is no standard method for monitoring RGM, which brings many uncertainties to the calibration of most monitoring work.

2.1.5 Influencing Factors of Sampling Methods for Active Gaseous Mercury

The RGM collection process is affected by the O₃, relative humidity and other gas components, and these factors have different effects on different sampling methods. The process of oxidation of Hg⁰ by O₃ to RGM is very slow, and its effect on the reflux spray box method with short sampling time (~ 1 H) is negligible^[29]. When sampling with the backflow spray box method, it may be affected by other gas components or aerosols (absorbed by the absorption liquid and reacted with RGM or GEM), resulting in inaccurate measurement results^[19]. The injection port temperature of the Tekran ® system is 50 ℃ (higher than the ambient temperature), which has a certain effect on the gas-particle partition of the RGM and may convert the PBM into RGM, resulting in higher RGM measurement results^[20]^[45]. In addition, it was found that O₃^[25][46] could promote the release of Hg⁰ from diffusion tubes. Water vapor can passivate the KCl coating on the surface of the diffusion tube and inhibit the adsorption of RGM, resulting in lower RGM measurement results^[33]. In the multistage filtration membrane method, the PBM intercepted by the front PTFE membrane may volatilize with the gas flow and be captured by the rear membrane, resulting in higher RGM measurement results^[19]. O₃ and water vapor may also compete with RGM for binding sites on nylon membranes, resulting in reduced harvesting efficiency^[33]. The O₃ has little effect on the cation exchange membrane, but due to the hydrophilicity of the cation exchange membrane, the higher the humidity, the more RGM collected by the membrane, which may lead to higher measurement results^[26]^[33,40]. RGM can be deposited on the inner wall surface of different samplers (especially passive samplers), resulting in different degrees of wall loss and low measurement results^[43,47]. Considering the different properties (solubility, diffusivity, affinity) of different RGM compounds, all current sampling methods cannot effectively collect all forms of RGM^[34]. These factors bring great uncertainty to the measurement of RGM. Therefore, it is still necessary to strengthen the research of RGM standard sampling method at this stage.

2.2 Method for analysis of active gaseous mercury

At present, the main analytical methods of RGM are wet acid hydrolysis -SnCl₂- cold vapor atomic fluorescence spectrometry (CVAFS) and thermal desorption-CVAFS. Among them, the wet acid hydrolysis mainly refers to EPA method 1631 E, and the thermal analysis method includes high temperature pyrolysis and temperature programmed thermal analysis^[48]^[34]. The samples collected by KCl-coated diffusion tube method and multi-stage filter membrane method can be used for quantitative analysis of RGM, while the reflow spray box method usually uses wet method for analysis. Generally speaking, the wet method is greatly interfered by the blank reagent mercury. For example, the absolute detection limit of the wet method is 10 pg, while the absolute detection limit of the improved pyrolysis method is reduced to 3 pg due to the influence of blank mercury when analyzing the samples collected by the KCl-coated diffusion tube method. Therefore, the latter has gradually become the main analysis method for the samples collected by the KCl-coated diffusion tube method^[23]^[24]. The material of cation exchange membrane is usually polysulfone polymer, and high temperature will destroy the membrane structure, so the analysis method based on wet method is generally used. In addition, the blank cation exchange membrane can not be heated at high temperature to remove mercury in advance, resulting in more mercury blank interference^[40]. The material of nylon film is polyamide polymer, which has certain heat resistance and can be analyzed by temperature programmed thermal analysis^[34].

The transport and transformation of mercury in the atmosphere depend largely on the speciation of mercury. However, at present, the morphology of RGM is not clear, which adds uncertainty to its sampling and analysis methods. In the past ten years, researchers have made many attempts on the morphological analysis of RGM. Temperature-programmed thermal desorption (TPD) has been widely applied to the analysis of mercury speciation in a variety of solid matrices, such as sediment, fluorescent lamp waste, and atmospheric particles^[49]^[50]^[51]. Gustin of the University of Nevada first applied the temperature-programmed thermal desorption technology to the speciation analysis of RGM collected by nylon membrane, and found that the sampling-temperature-programmed thermal desorption technology based on nylon membrane could be used to identify halogenated mercury (HgCl₂, HgBr₂) and oxidized mercury (HgO).The method was applied to the analysis of actual samples, which verified that the chemical composition of RGM had obvious spatial and temporal differences, and speculated that there might be more RGM compounds with unknown forms^{[28,34,36,37]}. Subsequently, they identified HgBr₂, HgCl₂, Hg(NO₃)₂, HgSO₄ and HgO that may exist in the actual samples by using nylon membrane sampling-temperature programmed thermal analysis technology. The results of thermal desorption of these mercury compounds are different from those in other matrices. The pyrolysis temperature of different mercury standard compounds is :HgO(T=100~105℃)<HgCl₂/HgBr₂(T=115~120℃)<HgN₂O₆·H₂O(T=140℃)<HgSO₄(T=150~160℃) in turn. Compared with the temperature programmed thermal analysis of mercury in actual samples, it is inferred that the chemical composition of RGM in different locations is significantly different^[28]^[28]. Figure 2 shows the temperature-programmed thermal desorption spectra of different mercury compounds. The peak convolution method can be used to obtain the specific concentration values of different mercury substances in the temperature-programmed thermal desorption, so as to realize the qualitative and quantitative analysis of RGM^[38]. In addition, the anions on nylon membranes were studied by ion chromatography, which can be used as an indirect means to understand the potential RGM compounds^[38]. By comparing the main elements (N, S, Br, Cl, etc.) In the compounds separated by temperature programmed thermal desorption with the anions (sulfate, nitrate, chloride, bromide, nitrite, phosphate and fluoride, etc.) And their respective proportions analyzed by ion chromatography, the RGM compounds on nylon membranes can be better inferred, thus realizing the possibility of their speciation analysis. In addition to TPD, mass spectrometry also has some potential in the study of RGM speciation, but it is still in the laboratory stage. The Hg(NO₃)₂ in the simulated samples were identified by gas chromatography-mass spectrometry (GC-MS)^[30]. Atmospheric pressure chemical ionization mass spectrometry (APCI-MS) was used to identify HgBr₂ and HgCl₂ in urban and indoor air^[31].

3 Occurrence and transformation of reactive gaseous mercury in the atmosphere

Compared with zero-valent mercury, the environmental behavior and related mechanisms of RGM are limited. Figure 3 shows the chemical transformation process of mercury in the atmosphere, in which the formation and transformation of RGM is an important part. It is helpful to understand the role of RGM in the biogeochemical cycle of mercury by clarifying the occurrence of RGM in the atmosphere and its participation in key processes such as mercury oxidation and reduction.

3.1 Formation mechanism of reactive gaseous mercury in the atmosphere

Industrial emissions are an important source of RGM in the atmosphere, and the global industrial emissions of RGM are 480 t/a, accounting for about 23% of the total mercury emissions^[52]. In addition to industrial emissions, the oxidation of Hg⁰ is also an important source of RGM in the atmosphere^[53]. The generation of RGM increases the atmospheric deposition, surface and ocean input of mercury in various regions of the world. With the discovery of Atmospheric Mercury Depletion Events (AMDEs), the importance of Atmospheric Mercury oxidation has been further recognized. After the polar sunrise, a sharp decrease in the concentration of Hg⁰ was observed, with a corresponding increase in the concentrations of RGM and PBM. Halogens (especially Br · and BrO ·) contribute significantly to the oxidation of polar Hg⁰^{[3,54⇓⇓⇓⇓ ~59]}.

At present, photochemical oxidation is considered to be the main mechanism of RGM formation. The main oxidants include O₃, OH ·, Br ·, Cl ·, BrO ·, ClO ·, I · and H₂O₂^[13]. The NO₃ radical is a nocturnal oxidant, which is generally considered to be not important in the oxidation of atmospheric mercury^[60]. Taking Br · as an example, the oxidation of Hg⁰ is initiated by a two-step reaction. First, a monovalent mercury intermediate (HgBr) is formed, which is unstable and prone to thermal dissociation^[57,61]. The monovalent mercury is then further oxidized to divalent mercury compounds BrHgX (X = Br, Cl, OH, I, NO₂, HO₂, BrO, IO, ClO, etc.)^[62]. The Br · produced by the photochemical reaction also reacts rapidly with the O₃ to form BrO ·, so the decrease of the concentration of the O₃ is often observed at the same time as the loss of mercury^[54]. The oxidation product of Br ·, BrO ·, can also be used as an oxidant to cause the change of mercury species. The oxidation mechanism of the Hg⁰ is shown in the following reaction formula:^{[61, 63~67]}

(1) $Br_{2}+h\nu \rightarrow Br \cdot$

(2) $Br \cdot+O_{3} \rightarrow BrO\cdot +O_{2}$ ^[63]

(3) $BrO\cdot+BrO\cdot \rightarrow Br_{2}+O_{2}$^[63]

(4) $Hg+Br\cdot \rightarrow HgBr$^{[61, 64]}

(5) $Hg+BrO\cdot \rightarrow HgBrO$ ^[65]

(6) $HgBr+X \rightarrow BrHgX(X=Br、Cl、OH、I、NO_{2}、HO_{2}、BrO、IO、ClO)$ ^{[66, 67]}

It is speculated that monovalent mercury compounds such as HgBr, HgCl, HgOH, HgBrO, Hg OBr, Hg IO and Hg OI may exist in the atmosphere^[62,65,68]. Theoretically, Hg (Ⅰ) is a key intermediate in the redox process of mercury, but it is difficult to analyze because of its unstable nature, so little attention has been paid to Hg (Ⅰ) in previous studies. The gas phase oxidation of Hg⁰ by BrO radical was simulated, and the existence of monovalent mercury compounds (HgBr, HgBrO, HgOBr, HgIO and HgOI) in the oxidation products was confirmed^[65,68]. However, the existing state of Hg (Ⅰ) in the atmosphere is still unclear due to the lack of field observation data.

3.2 Occurrence of reactive gaseous mercury in the atmosphere

RGM accounts for a high proportion of mercury emissions from coal combustion. Large mercury emissions also lead to increased mercury deposition^[69]. Generally, RGM concentrations (usually in the range of tens to thousands of pg·m^-3[70]) and mercury deposition are high in cities and near pollution sources; After direct emissions, a certain amount of RGM (below 50 pg·m^-3[71]) and mercury deposition can also be observed in remote areas due to long-distance transport of Hg⁰ and atmospheric oxidation^[72]. The rapid transformation of Hg⁰ to RGM was also observed in polar regions, where RGM accounted for more than 60% of the total gaseous mercury, and the concentration value could reach several hundred pg ·

m - 3 [73]

. The comparison results of RGM concentration values in different regions at home and abroad are shown in Table 3. The concentration of RGM in the northern hemisphere was significantly higher than that in the southern hemisphere. The concentration of RGM in urban and pollution source areas was significantly higher than that in rural and other remote areas. The concentrations of RGM in high altitude areas, remote coastal areas and oceans in European and American countries were significantly higher than those in Asian countries.

RGM has significant spatial and temporal differences, including regional differences, seasonal differences and diurnal variations^[70]. The concentration of RGM increased in spring and summer, while the concentration of Hg⁰ decreased and the concentration of RGM increased in the afternoon in most areas^[95]^[10]. The oxidation mechanism of the Hg⁰ is the driving factor for the seasonal and diurnal variations of RGM in the marine boundary layer. The concentration of RGM in the terrestrial boundary layer is mainly controlled by anthropogenic emissions, Hg⁰ oxidation, free tropospheric transport and dry and wet deposition^[70]. Using the UNR-RMAS sampling method, Gustin et al. Proved that the difference in chemical composition of RGM in different regions depended on different types of oxidants in the air: RGM at high altitudes was mainly affected by oxidation during long-distance transport of Hg⁰, and was mainly halogen-containing compounds (such as HgBr₂, HgCl₂, etc.)^[28]; Mobile pollution sources are the main sources of RGM near the highway in low altitude areas, and RGM are mostly nitrogen and sulfur compounds (such as HgSO₄, Hg(NO₃)₂, etc.).

3.3 Removal of reactive gaseous mercury from the atmosphere

RGM is chemically reactive and has a very short residence time in the atmosphere (days to weeks), so it is easily removed from the atmosphere. Environmental conditions and atmospheric physicochemical reactions are the key factors affecting the occurrence and removal of RGM in the atmosphere. There are four main ways to remove RGM in the atmosphere: dry deposition, transformation into water-soluble mercury oxide by gas-liquid partition, transformation into particulate mercury by gas-particle partition, and photoreduction. Table 4 lists the four removal pathways of RGM in the atmosphere and their influencing factors.

The speciation of mercury determines its deposition characteristics. RGM has the highest dry deposition rate (RGM:0.4~7.6 cm·s^-1>PBM:0.1~2.1 cm·s^-1>GEM:0.01~0.19 cm·s^-1) and is therefore prone to dry deposition to terrestrial or marine ecosystems^[96,97]. In addition to the different deposition rates of different forms of RGM, the dry deposition process of RGM showed the same spatial and temporal differences as the RGM concentration, which was mainly affected by environmental conditions and surface types. The dry deposition amount of RGM was different in different seasons, and it was significantly higher in spring and summer than in autumn and winter^[18]^[98]. Because plant leaves absorb RGM, the deposition rate of RGM in forest vegetation area is higher than that in bare ground environment^[96]. RGM will undergo gas-liquid partitioning in the atmosphere and then wet deposition to terrestrial or marine ecosystems^[8]. It is found that the gas-liquid partition of RGM mainly occurs within the cloud^[99]. The form of RGM and meteorological conditions such as temperature and rainfall type are the key factors affecting the wet deposition of RGM. Different forms of RGM have different gas-liquid partition coefficients, which affect their wet deposition process^[7]. In addition, the wet deposition of RGM had significant seasonal differences, and the concentration and wet deposition of Hg in summer rainfall were higher than those in winter^[100]. RGM can be transformed into PBM by gas-particle partitioning on the surface of atmospheric particles, and then deposited to the surface with dry and wet particles. The distribution of RGM was different in different particle sizes, and most of them were concentrated in the PM_2.5 fraction^[101]. The gas-particle partitioning of RGM is influenced by the chemical composition of the particles and environmental meteorological factors such as temperature^[79,102,103]. The results show that RGM has a higher gas-particle partition coefficient in the particles dominated by nitrate and chloride^[103]. Therefore, RGM in the marine boundary layer can be rapidly taken up by sea salt aerosols to form HgC

l 3 -

and HgC

l 4 2 -

, which can be removed by sea salt aerosols^[106]. The life cycle of RGM in the marine boundary layer is short and the diurnal variation range is large, so 80% ~ 95% of the reactive mercury in the atmosphere should be PBM rather than RGM^[104]. RGM can be photochemically reduced to form Hg⁰ in the gas phase. Considering that different RGM have different optical properties, it is speculated that the scavenging pathway may be related to environmental conditions such as solar radiation and the form of RGM^[105]. The model calculation results show that RGM can effectively absorb solar radiation, resulting in its rapid photoreduction in the atmosphere^[62,106]^[14]. At present, the mechanism of mercury reduction in gas phase is not clear, but such reactions have been found in field observations of mercury species in flue gas emissions from some power plants^[41]. Recently, the gas-phase reduction of RGM under light conditions was also found in the field observations on the Qinghai-Tibet Plateau^[107]. Due to the lack of direct evidence from field observations, the specific mechanism of RGM photoreduction remains to be further studied. The removal of RGM in the atmosphere directly or indirectly affects the deposition of atmospheric mercury, which plays an important role in the process of atmospheric mercury cycle, but the specific reaction mechanism needs to be further studied.

4 Conclusion and prospect

At present, the sampling of RGM still depends on the uncalibrated operational definition method, and the field monitoring is easily disturbed by many environmental factors. The analysis method of RGM is mainly traditional total mercury analysis method. Current studies have gradually realized the importance of RGM chemical species, and preliminary attempts have been made to identify different species of RGM by temperature programmed thermal desorption, ion chromatography and other methods. On this basis, the oxidation of Hg⁰ has been deeply understood at this stage, and the difference of chemical composition of RGM is mainly related to the type of oxidant in the air. However, many environmental behaviors of RGM, such as gas-particle partitioning, heterogeneous chemical reactions at the gas-particle interface, and photoreduction reactions in the gas phase, are still poorly understood. In a word, there are still many shortcomings in the collection and analysis methods of RGM, and the understanding of the occurrence and transformation of different forms of RGM still needs to be strengthened. Future RGM research can focus on the following points:

(1) To develop a standard method for stable, reliable and repeatable collection of various RGM compounds, which is easy to operate and convenient for field monitoring.

(2) On the basis of monitoring technology, develop analytical methods that can identify different forms of RGM compounds in the atmosphere (such as the analysis of Hg (I) by high performance liquid chromatography-inductively coupled plasma mass spectrometry (HPLC-ICP-MS)), and explore the possible existence of RGM compounds with different valences and anions^[108].

(3) To clarify the environmental behaviors of RGM in the atmosphere, such as generation, occurrence and removal, and reveal the important chemical processes and related mechanisms involved in RGM, such as gas-liquid partitioning, gas-particle partitioning and photoreduction, so as to improve the systematicness and accuracy of mercury atmospheric models and enhance the understanding of the role of RGM in the atmospheric mercury cycle.

References

Publishing order | Descend order by publishing year | Descend order by cited within

[1]	UNEP, Global Mercury Assessment 2018, Geneva, Switzerland, 2019.

[2]	Pirrone N, Cinnirella S, Feng X, Finkelman R B, Friedli H R, Leaner J, Mason R, Mukherjee A B, Stracher G B, Streets D G, Telmer K. Atmos. Chem. Phys., 2010, 10: 5951.

[3]	Steffen A, Douglas T, Amyot M, Ariya P, Aspmo K, Berg T, Bottenheim J, Brooks S, Cobbett F, Dastoor A, Dommergue A, Ebinghaus R, Ferrari C, Gardfeldt K, Goodsite M E, Lean D, Poulain A J, Scherz C, Skov H, Sommar J, Temme C. Atmos. Chem. Phys., 2008, 8: 1445.

[4]	Lindberg S, Bullock R, Ebinghaus R, Engstrom D, Feng X, Fitzgerald W, Pirrone N, Prestbo E, Seigneur C. Ambio., 2007, 36: 19.

[5]	Ariya P A, Amyot M, Dastoor A, Deeds D, Feinberg A, Kos G, Poulain A, Ryjkov A, Semeniuk K, Subir M, Toyota K. Chem. Rev., 2015, 115: 3760.

[6]	Schroeder W H, Munthe J. Atmos. Environ., 1998, 32: 809.

[7]	Lin C J, Pehkonen S O. Atmos. Environ., 1999, 33: 2067.

[8]	Lindqvist O, Rodhe H. Tellus , Ser. B : Chem. Phys. Meteorol., 1985, 37: 136.

[9]	Munthe J, Wangberg I, Pirrone N, Iverfeldt A, Ferrara R, Ebinghaus R, Feng X, Gardfeldt K, Keeler G, Lanzillotta E, Lindberg S E, Lu J, Mamane Y, Prestbo E, Schmolke S, Schroeder W H, Sommar J, Sprovieri F, Stevens R K, Stratton W, Tuncel G, Urba A. Atmos. Environ., 2001, 35: 3007.

[10]	Lindberg S E, Stratton W J. Environ. Sci. Technol., 1998, 32: 49.

[11]	Xiao Z F, Munthe J, Lindqvist O. Water , Air , Soil Pollut., 1991, 56: 141.

[12]	Tong Y D, Zhang W, Deng C Y, Chen L, Wang H H, Wang X J, Schauer J J. Acta Scientiae Circumstantiae , 2016, 36( 5): 1515.

[13]	童银栋, 张巍, 邓春燕 , 陈龙, 王欢欢, 王学军, Schauer J J. 环境科学学报, 2016, 36( 05): 1515.

[14]	Ye Z, Mao H, Driscoll C T, Wang Y, Zhang Y, Jaegle L. J. Adv. Model. Earth. Sy., 2018, 10: 668.

[15]	Frances-Monerris A, Carmona-Garcia J, Ulises Acuna A, Davalos J Z, Cuevas C A, Kinnison D E, Francisco J S, Saiz-Lopez A, Roca-Sanjuan D. Angew. Chem. Int. Edit., 2020, 59: 7605.

[16]	Landis M S, Ryan J V, ter Schure A F H, Laudal D. Environ. Sci. Technol., 2014, 48: 13540.

[17]	Pongprueksa P, Lin C J, Lindberg S E, Jang C, Braverman T, Bullock O R, Ho T C, Chu H W. Atmos. Environ., 2008, 42: 1828.

[18]	Amos H M, Jacob D J, Holmes C D, Fisher J A, Wang Q, Yantosca R M, Corbitt E S, Galarneau E, Rutter A P, Gustin M S, Steffen A, Schauer J J, Graydon J A, St Louis V L, Talbot R W, Edgerton E S, Zhang Y, Sunderland E M. Atmos. Chem. Phys., 2012, 12: 591.

[19]	Shia R L, Seigneur C, Pai P, Ko M, Sze N D. J. Geophys. Res. : Atmos., 1999, 104: 23747.

[20]	Li Z G, Feng X B, Zheng W, Shang L H, Fu X W. Environmental Monitoring in China , 2007,( 2): 19.

[21]	李仲根, 冯新斌, 郑伟, 商立海, 付学吾. 中国环境监测, 2007,( 2): 19.

[22]	Landis M S, Stevens R K, Schaedlich F, Prestbo E M. Environ. Sci. Technol., 2002, 36: 3000.

[23]	Miller M B, Dunham-Cheatham S M, Gustin M S, Edwards G C. Atmos. Meas. Tech., 2019, 12: 1207.

[24]	Stratton W J, Lindberg S E. Water , Air , Soil Pollut., 1995, 80: 1269.

[25]	Xiao Z, Sommar J, Wei S, Lindqvist O. Fresenius ’ J. Anal. Chem., 1997, 358: 386.

[26]	Feng X B, Sommar J, Gardfeldt K, Lindqvist O. Fresenius ’ J. Anal. Chem., 2000, 366: 423.

[27]	McClure C D, Jaffe D A, Edgerton E S. Environ. Sci. Technol., 2014, 48: 11437.

[28]	Huang J, Lyman S N, Hartman J S, Gustin M S. Environ. Sci. : Processes Impacts , 2014, 16: 374.

[29]	Lyman S N, Gustin M S, Prestbo E M. Atmos. Environ., 2010, 44: 246.

[30]	Gustin M S, Pierce A M, Huang J, Miller M B, Holmes H A, Loria-Salazar S M. Environ. Sci. Technol. , 2016, 50: 12225.

[31]	Stratton W J, Lindberg S E, Perry C J. Environ. Sci. Technol., 2001, 35: 170.

[32]	Olson E S, Sharma R K, Pavlish J H. Anal. Bioanal. Chem., 2002, 374: 1045.

[33]	Deeds D A, Ghoshdastidar A, Raofie F, Guerette E A, Tessier A, Ariya P A. Anal. Chem., 2015, 87: 5109.

[34]	Gustin M S, Amos H M, Huang J Y, Miller M B, Heidecorn K. Atmos. Chem. Phys., 2015, 15: 5697.

[35]	Huang J, Gustin M S. Environ. Sci. Technol. , 2015, 49: 6102.

[36]	Huang J, Miller M B, Weiss-Penzias P, Gustin M S. Environ. Sci. Technol. , 2013, 47: 7307.

[37]	Gustin M S, Dunham-Cheatham S M, Zhang L. Environ. Sci. Technol., 2019, 53: 14489.

[38]	Pierce A M, Gustin M S. Environ. Sci. Technol., 2017, 51: 436.

[39]	Huang J, Miller M B, Edgerton E, Gustin M S. Atmos. Chem. Phys. , 2017, 17: 1689.

[40]	Luippold A, Gustin M S, Dunham-Cheatham S M, Zhang L. Atmos. Environ., 2020, 224.

[41]	Lyman S N, Gustin M S, Prestbo E M, Kilner P I, Edgerton E, Hartsell B. Environ. Sci. Technol., 2009, 43: 6235.

[42]	Bu X, Zhang H, Lv G, Lin H, Chen L, Yin X, Shen G, Yuan W, Zhang W, Wang X, Tong Y. Environ. Sci. Technol. Lett. , 2018, 5: 600.

[43]	Tong Y, Eichhorst T, Olson M R, Rutter A P, Shafer M M, Wang X, Schauer J. J. Atmos. Res., 2014, 138: 324.

[44]	Luippold A, Gustin M S, Dunham-Cheatham S M, Castro M, Luke W, Lyman S, Zhang L. Environ. Sci. Technol. , 2020, 54: 7922.

[45]	Huang J, Gustin M S. Environ. Sci. Technol., 2015, 49: 432.

[46]	Sheu G R, Mason R P. Environ. Sci. Technol., 2001, 35: 1209.

[47]	Rutter A P, Hanford K L, Zwers J T, Perillo-Nicholas A L, Schauer J J, Olson M L. J. Air Waste Manage. Assoc., 2008, 58: 377.

[48]	Lyman S N, Jaffe D A, Gustin M S. Atmos. Chem. Phys., 2010, 10: 8197.

[49]	Gustin M S, Weiss-Penzias P S, Peterson C. Atmos. Chem. Phys. , 2012, 12: 9201.

[50]	U. S. EPA. EPA -821- R -02- 019, 2008.

[51]	Shuuaeua O V, Gustaytis M A, Anoshin G N. Anal. Chim. Acta , 2008, 621: 148.

[52]	Raposo C, Windmoller C C, Junior W A D. Waste Manage., 2003, 23: 879.

[53]	Feng X B, Lu J Y, Gregoire D C, Hao Y J, Banic C M, Schroeder W H. Anal. Bioanal. Chem., 2004, 380: 683.

[54]	Pacyna E G, Pacyna J M, Steenhuisen F, Wilson S. Atmos. Environ., 2006, 40: 4048.

[55]	Driscoll C T, Mason R P, Chan H M, Jacob D J, Pirrone N. Environ. Sci. Technol., 2013, 47: 4967.

[56]	Lindberg S E, Brooks S, Lin C J, Scott K J, Landis M S, Stevens R K, Goodsite M, Richter A. Environ. Sci. Technol., 2002, 36: 1245.

[57]	Sommar J, Gardfeldt K, Stromberg D, Feng X B. Atmos. Environ., 2001, 35: 3049.

[58]	Holmes C D, Jacob D J, Yang X. Geophys. Res. Lett., 2006, 33.

[59]	Holmes C D, Jacob D J, Corbitt E S, Mao J, Yang X, Talbot R, Slemr F. Atmos. Chem. Phys., 2010, 10: 12037.

[60]

Gratz L E, Ambrose J L, Jaffe D A, Shah V, Jaegle L, Stutz J, Festa J, Spolaor M, Tsai C, Selin N E, Song S, Zhou X, Weinheimer A J, Knapp D J, Montzka D D, Flocke F M, Campos T L, Apel E, Hornbrook R, Blake N J, Hall S, Tyndall G S, Reeves M, Stechman D, Stell M. Geophys. Res. Lett., 2015, 42: 10494.

[61]	Stephens C R, Shepson P B, Steffen A, Bottenheim J W, Liao J, Huey L G, Apel E, Weinheimer A, Hall S R, Cantrell C, Sive B C, Knapp D J, Montzka D D, Hornbrook R S. J. Geophys. Res. : Atmos. , 2012, 117.

[62]	Peleg M, Tas E, Obrist D, Matveev V, Moore C, Gabay M, Luria M. Environ. Sci. Technol., 2015, 49: 14008.

[63]	Goodsite M E, Plane J M C, Skov H. Environ. Sci. Technol. , 2004, 38: 1772.

[64]	Saiz-Lopez A, Ulises Acuna A, Trabelsi T, Carmona-Garcia J, Davalos J Z, Rivero D, Cuevas C A, Kinnison D E, Sitkiewicz S P, Roca-Sanjuan D, Francisco J S. J. Am. Chem. Soc., 2019, 141: 8698.

[65]	Sander S P, Finlayson-Pitts B, Friedl R R, Golden D M, Huie R, Keller-Rudek H. Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies (Evaluation No. 15), Pasadena, CA: Jet Propulsion Laboratory, 2006.

[66]	Goodsite M E, Plane J M C, Skov H. Environ. Sci. Technol., 2012, 46: 5262.

[67]	Raofie F, Ariya P A. Environ. Sci. Technol., 2004, 38: 4319.

[68]	Dibble T S, Zelie M J, Mao H. Atmos. Chem. Phys., 2012, 12: 10271.

[69]	Wang F, Saiz-Lopez A, Mahajan A S, Gomez Martin J C, Armstrong D, Lemes M, Hay T, Prados-Roman C. Atmos. Chem. Phys., 2014, 14: 1323.

[70]	Raofie F, Snider G, Ariya P A. Can. J. Chem., 2008, 86: 811.

[71]	Quan J N, Zhang X S, Zhang Q, Li H Y. Plateau Meteorology , 2009, 28( 1): 159.

[72]	权建农, 张晓山, 张蔷, 李宏宇. 高原气象, 2009, 28( 1): 159.

[73]	Mao H, Cheng I, Zhang L. Atmos. Chem. Phys., 2016, 16: 12897.

[74]	Valente R J, Shea C, Humes K L, Tanner R L. Atmos. Environ. , 2007, 41: 1861.

[75]	Wu X Y, Zheng Y F, Lin K S. China Environmental Science, 2009, 28( 1): 159.

[76]	吴晓云, 郑有飞, 林克思. 中国环境科学, 2009, 28( 1): 159.

[77]	Lindberg S, Landis M S, Stevens R K, Brooks S. Atmos. Environ. , 2001, 35: 5377.

[78]	Zhang H, Wang Z, Wang C, Zhang X. Environ. Pollut., 2009, 249: 13.

[79]	Zhang L, Wang S X, Wang L, Hao J M. Atmos. Chem. Phys., 2013, 13: 10505.

[80]	Hong Y, Chen J, Deng J, Tong L, Xu L, Niu Z, Yin L, Chen Y, Hong Z. Environ. Pollut., 2016, 218: 259.

[81]	Fu X, Feng X, Qiu G, Shang L, Zhang H. Atmos. Environ., 2011, 45: 4205.

[82]	Xu L, Chen J, Yang L, Niu Z, Tong L, Yin L, Chen Y. Chemosphere , 2015, 119: 530.

[83]	Zhang L, Wang L, Wang S, Dou H, Li J, Li S, Hao J. Aerosol Air Qual. Res. , 2017, 17: 2913.

[84]	Sheu G R, Ly Sy Phu N, Mirth Tri T, Lin D W. Atmos. Environ., 2019, 214.

[85]	Wang C, Ci Z, Wang Z, Zhang X, Guo J. Atmos. Environ., 2016, 131: 360.

[86]	Wang C, Wang Z, Hui F, Zhang X. Atmos. Chem. Phys. , 2019, 19: 10111.

[87]	Liu C, Fu X, Zhang H, Ming L, Xu H, Zhang L, Feng X. Sci. Total Environ., 2019, 663: 275.

[88]	Fu X W, Feng X, Liang P, Deliger, Zhang H, Ji J, Liu P. Atmos. Chem. Phys., 2012, 12: 1951.

[89]	Lyman S N, Gustin M S. Sci. Total Environ., 2009, 408: 431.

[90]	Liu B, Keeler G J, Dvonch J T, Barres J A, Lynam M M, Marsik F J, Morgan J T. Atmos. Environ., 2010, 44: 2013.

[91]	Song X, Cheng I, Lu J. J. Environ. Monit., 2009, 11: 660.

[92]	Rutter A P, Snyder D C, Stone E A, Schauer J J, Gonzalez-Abraham R, Molina L T, Marquez C, Cardenas B, de Foy B. Atmos. Chem. Phys., 2009, 9: 207.

[93]	Swartzendruber P C, Jaffe D A, Prestbo E M, Weiss-Penzias P, Selin N E, Park R, Jacob D J, Strode S, Jaegle L. J. Geophys. Res. : Atmos., 2006, 111.

[94]	Fain X, Obrist D, Hallar A G, McCubbin I, Rahn T. Atmos. Chem. Phys. , 2009, 9: 8049.

[95]	Steffen A, Bottenheim J, Cole A, Douglas T A, Ebinghaus R, Friess U, Netcheva S, Nghiem S, Sihler H, Staebler R. Atmos. Chem. Phys., 2013, 13: 7007.

[96]	Castagna J, Bencardino M, D’Amore F, Esposito G, Pirrone N, Sprovieri F. Atmos. Environ., 2018, 173: 108.

[97]	Angot H, Barret M, Magand O, Ramonet M, Dommergue A. Atmos. Chem. Phys., 2014, 14: 11461.

[98]	Soerensen A L, Skov H, Jacob D J, Soerensen B T, Johnson M S. Environ. Sci. Technol. , 2010, 44: 7425.

[99]	Fu X, Marusczak N, Heimbuerger L E, Sauvage B, Gheusi F, Prestbo E M, Sonke J E. Atmos. Chem. Phys., 2016, 16: 5623.

[100]

Poissant L, Pilote M, Xu X H, Zhang H, Beauvais C. J. Geophys. Res. : Atmos., 2004, 109.

[101]

Feng X B, Qiu G L, Fu X W, He T R, Li P, Wang S F. Progress in Chemistry , 2009, 21: 436.

[102]

冯新斌, 仇广乐, 付学吾, 何天容, 李平, 王少锋. 化学进展, 2009, 21: 436.

[103]

Sather M E, Mukerjee S, Smith L, Mathew J, Jackson C, Callison R, Scrapper L, Hathcoat A, Adam J, Keese D, Ketcher P, Brunette R, Karlstrom J, Van der Jagt G. Atmos. Pollut. Res., 2013, 4: 168.

[104]

Zhou H, Zhou C, Hopke P K, Holsen T M. Sci. Total Environ., 2018, 637: 943.

[105]

Selin N E, Jacob D J. Atmos. Environ., 2008, 42: 5193.

[106]

Zhang F W, Zhao J P, Chen J S, Xu Y. Environmental Science, 2010, 31( 10): 2273.

[107]

张福旺, 赵金平, 陈进生, 徐亚. 环境科学, 2010, 31( 10): 2273.

[108]

Rutter A P, Schauer J J. Atmos. Environ. , 2007, 41: 8647.

[109]

Rutter A P, Schauer J J. Environ. Sci. Technol., 2007, 41: 3934.

[110]

Holmes C D, Jacob D J, Mason R P, Jaffe D A. Atmos. Environ., 2009, 43: 2278.

[111]

Kunkely H, Horvath O, Vogler A. Coord. Chem. Rev. , 1997, 159: 85.

[112]

Saiz-Lopez A, Sitkiewicz S P, Roca-Sanjuan D, Oliva-Enrich J M, Davalos J Z, Notario R, Jiskra M, Xu Y, Wang F, Thackray C P, Sunderland E M, Jacob D J, Travnikov O, Cuevas C A, Acuna A U, Rivero D, Plane J M C, Kinnison D E, Sonke J E. Nat. Commun., 2018, 9: 4796.