New evidence for atmospheric mercury transformations in the marine boundary layer

Marine boundary layer (MBL) is the largest transport place and reaction vessel of atmospheric mercury (Hg). The transformations of atmospheric Hg in MBL are crucial for the global transport and deposition of Hg. Herein, Hg isotopic signatures in total gaseous mercury (TGM) and particulate bound 15 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 in TGM from MBL were shaped not only by mixing continental emissions, but also largely by the oxidation of Hg primarily derived by Br atoms. Lower air temperature could promote the positive MIF in TGM in summer, while the relative processes might be weak in winter. In 20 contrast, the positive ΔHg and high ratios of ΔHg/ΔHg in PBM indicated that alternative oxidants other than Br or Cl atoms played a major role in the formation of Hg(II) in PBM, likely following the nuclear volume effect. Our results suggested the importance of local Hg environmental behaviours caused by an abundance of highly reactive species, and provided new evidence for understanding the complicated transformations of atmospheric Hg in the MBL. 25


Introduction
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 https://doi.org/10.5194/acp-2020-397 Preprint. Discussion started: 6 May 2020 c Author(s) 2020. CC BY 4.0 License.
(n = 3) and isotopic measurement (n = 2; except for parallel TGM samples collected in 2018-summer, n = 4). Method blanks were excluded when calculating the Hg concentrations and the pre-concentration recoveries. The mass bias during isotopic measurement was calibrated using sample-standard bracketing method, and using Tl aerosols as an internal spike (Yin et al., 2010).

Other supportive data 130
The meteorological data collected during the cruise were obtained from an automatic weather station on the Dongfanghong II research vessel.
One of the two parallel sampling filters collected in cruise 2018-summer was treated to measure Hg 0 , Hg(II), and Br speciation on airborne particles. The measurement of Hg 0 and Hg(II) in TSPs were conducted following previous publication (Feng et al., 2004). 1/4 sheet of sampling filter was rolled up 135 and settled in a heating tube installed in tube furnace. The furnace was maintained at 80 °C for 2h and then maintained at 500°C for 2h. Bubbler filled with 5 mL acid-trapping solution same as used in preconcentration stage were connected at outlet of the heating tube to capture the released Hg at 80 °C and 500°C, respectively. Purified nitrogen used as carrier gas was maintained at 25 mL min-1. The THg in acid-trapping solution was measured using CVAFS following USEPA Method 1631. The other 3/4 sheet 140 https://doi.org/10.5194/acp-2020-397 Preprint. Discussion started: 6 May 2020 c Author(s) 2020. CC BY 4.0 License. of sampling filter was treated following the National Environmental Protection Standards of the People's Republic of China HJ 799-2016 to obtain the concentration of Br atom, Br anion, and organic Br on TSPs, using ion chromatography.
See SI for more details on the calculation and illustration of 72 h Back-trajectories associated with higher and lower Δ 199 Hg in TGM using the Hybrid Single-Particle Lagrangian Integrated Trajectory 145 (HYSPLIT4) Model (Rolph et al., 2017;Stein et al., 2015) in Fig. S1.

Results and discussion
A summary of measured isotopic values and concentrations was listed in Table 1.

Isotopic composition in TGM
According to a William-York bivariate linear regression (Cantrell, 2008) applying δ 202 Hg and 150 Δ 199 Hg in all of the TGM samples, the observed fitted curve shaped a slope of -0.10 ± 0.01 (Fig. 1). This fitted curve always indicated a mixing of plumes from anthropogenic emissions characterized by negative δ 202 Hg and near-zero Δ 199 Hg values, and plumes from remote areas characterized by positive δ 202 Hg and negative Δ 199 Hg values in previous studies (Demers et al., 2015;Yu et al., 2016;Fu et al., 2018;Sun et al., 2014). The eastern region of China is dominated by subtropical monsoon climate, with winds moving 155 from the mainland to the ocean in winter and reversely in summer (Fig. S1). TGM collected during the 2016-winter cruise, that was supposed to be largely impacted by anthropogenic emissions from mainland China based on the monsoon (Fig. S1), but showed positive δ 202 Hg and negative Δ 199 Hg (δ 202 Hg: 0.19 ± 0.30‰; Δ 199 Hg: -0.13 ± 0.04‰, n = 14, 1σ) echoing the isotopic fingerprints of TGM at the remote sites (Demers et al., 2013;Demers et al., 2015;Fu et al., 2016;Fu et al., 2018;Yu et al., 2016) (Fig. 1). TGM 160 collected during this cruise also showed the highest concentrations (1.81±0.51 ng m -3 , n = 14, 1σ) among the three cruises, exceeding background value of northern hemisphere (~1.5 ng m -3 ) but falling below averaged GEM concentrations measured at both urban and remote sites in Chinese mainland (urban: 9.20±0.56 ng m -3 ; remote: 2.86±0.95 ng m -3 ) (Fu et al., 2015). Considering that the average wind speed of 6.9 m s -1 was measured during this cruise, the air mass leaving Chinese mainland could reach the 165 vessel within several hours. Therefore, isotopic compositions in TGM collected during 2016-winter cruise suggested limited influence from the anthropogenic emissions that diluted in the clean air in MBL. https://doi.org/10.5194/acp-2020-397 Preprint. Discussion started: 6 May 2020 c Author(s) 2020. CC BY 4.0 License.
On the other hand, TGM collected in two summer cruises with larger δ 202 Hg and Δ 199 Hg ranges (2016-summer: δ 202 Hg: -1.48 ± 0.56‰; Δ 199 Hg: 0.01 ± 0.05‰, n = 9, 1σ; 2018-summer: δ 202 Hg: -0.09± 0.48‰; Δ 199 Hg: -0.13 ± 0.06‰, n = 18, 1σ) indicated the mixing of continental emissions to marine 170 originated plumes ( Fig. 2 process a). TGM collected in 2016-summer cruise showed near-zero Δ 199 Hg values, most likely inherited from anthropogenic emissions (Demers et al., 2015;Yu et al., 2016;Fu et al., 2018;Sun et al., 2014), but also showed lower THg concentrations than TGM collected in the other two cruises (Table 1). This result was uncommon because higher TGM concentrations always associated with anthropogenic emissions in China (Fu et al., 2015). The positive correlation between TGM 175 concentrations and Δ 199 Hg values in TGM, commonly attribute to mixing of anthropogenic emissions and clean air, was also absent (P > 0.05) in this cruise (Fig. 3b). The back-trajectory analysis results for each cruise also showed the large overlaps of source areas corresponding to plumes with higher and lower Δ 199 Hg values in TGM (Fig. S1). These uncommon relationships and the large overlaps of source areas suggested alternative reasons rather than only mixing with continental emissions should contribute to 180 TGM in MBL in summer.

Isotopic composition in PBM
The isotopic compositions in PBM collected from the MBL with negative δ 202 Hg and positive Δ 199 Hg values (δ 202 Hg: -0.80 ± 0.58‰; Δ 199 Hg: 0.40 ± 0.21‰, n = 9, 1σ) were distinguishable from those in the TGM ( have been characterized by negative δ 202 Hg and near-zero Δ 199 Hg, due to anthropogenic emissions (Yu et al., 2016;Das et al., 2016;Huang et al., 2016;Huang et al., 2015;Xu et al., 2017). 190 The isotopic compositions in PBM in this study and the similar isotopic compositions in PBM collected at island site in China (Fu et al., 2019a) were distinguishable from those collected at inland urban/rural sites, suggesting the dominated influences from marine environment rather than continental anthropogenic emissions. The primary species of PBM examined in this study was Hg(II), accounting for 78.6±13.0% (1σ, n = 9, Table S3) of total PBM. Therefore, the isotopic fractionations between TGM 195 mostly composed by Hg 0 , and PBM in MBL, attributed to Hg(II) on the particle surfaces.

The oxidation processes in the MBL
MIF induced by the magnetic isotope effect (MIE) mechanism produces a ~1.0 slope in the linear regression of Δ 199 Hg and Δ 201 Hg in environmental samples, while a ~1.6 slope is created as a result of the nuclear volume effect (NVE) (Blum and Bergquist, 2007). Although the linear correlation between Δ 199 Hg and Δ 201 Hg in the PBM was insignificant (P > 0.05), ratios of Δ 199 Hg/Δ 201 Hg in the PBM samples 210 were significantly higher than 1.0 (Fig. 4), that was the common ratios observed in PBM from sites influenced by anthropogenic emissions (Yu et al., 2016;Das et al., 2016;Huang et al., 2016;Huang et al., 2015;Xu et al., 2017). The observed ratios of Δ 199 Hg/Δ 201 Hg in the PBM were also higher than those collected at an island site in China (Δ 199 Hg/Δ 201 Hg: ~1.14) (Fu et al., 2019a), and at a coastal site in USA (Δ 199 Hg/Δ 201 Hg: ~1.12) (Rolison et al., 2013). The insignificant correlation between Δ 199 Hg and Δ 201 Hg 215 in the PBM, and between the isotopic signatures and the percentages of oxidized Hg in the PBM (P > 0.05, Table S3), indicated multiple processes inducing different fractionation rather than single oxidation process occurred.
To date, few isotopic studies have been performed on isotopic fractionation during GEM oxidation, and the mechanism has been suggested to be NVE, according to a study on Hg 0 oxidation by Br and Cl 220 atoms (Sun et al., 2016). In this study, the Hg(II) in PBM should not be attributed directly to oxidation derived by Br or Cl atoms, because Br and Cl atoms would induce negative odd-MIF in the product Hg(II) during oxidation (Sun et al., 2016) ( Fig. 2 process b and c), that was inconsistent with the positive odd-MIF observed in the PBM with Hg(II) as the primary form. When the photo-reduction of aquatic Hg(II) involving dissolved organic matter occurs, especially in the MBL with high RH values, the direction of 225 https://doi.org/10.5194/acp-2020-397 Preprint. Discussion started: 6 May 2020 c Author(s) 2020. CC BY 4.0 License. odd-MIF might reverse because positive odd-MIF would be induced in Hg(II) in aquatic layer on particle surfaces (Zheng andHintelmann, 2009, 2010). The magnitude of photo-reduction should be much greater than oxidation derived by Br/Cl atoms to produce the observed positive odd-MIF in PBM. However, ratios of Δ 199 Hg/Δ 201 Hg in PBM measured in this study were much higher than 1.0, that value has been associated with the photo-reduction of aquatic Hg(II) (Zheng andHintelmann, 2009, 2010;Bergquist and 230 Blum, 2007). Therefore, Br/Cl atoms-derived photo-oxidation followed by the photo-reduction of aquatic Hg(II) should not be the primary routine leading to the isotopic compositions in PBM in this study. In addition, correlation between Hg isotopic compositions in PBM and speciated Br concentrations on the TSPs was also absent ( Table S3). All of these results suggested that Br or Cl atoms were not the direct contributor to the Hg(II) in PBM in the MBL in this study. 235 Alternative oxidizers other than Br and Cl atoms, including the derivatives of Br/Cl atoms (e.g., BrO, HOCl, OCl -), ozone, hydroxyl radicals, nitrate radicals, and iodine radicals, might play more important roles to the Hg(II) in PBM in this study. The limited isotopic study of Hg 0 oxidation prevented the specific oxidizers from being identified here. However, the following clues might be helpful to uncover the oxidizers and oxidation processes in the future. According to the isotopic signatures present 240 in TGM and PBM, the primary oxidation of Hg 0 by unidentified oxidizer should induce a positive odd-MIF in the Hg(II) (Fig. 2 process d). To date, no evidence suggests that odd-MIF could occur during the adsorption of Hg(II) on the surface of particles ( Fig.2 process g), and the high ratios of Δ 199 Hg/Δ 201 Hg observed in PBM in this study indicated the insignificance of continental impacts. Therefore, the high Δ 199 Hg and high ratios of Δ 199 Hg/Δ 201 Hg in the PBM should be primarily attributed to the oxidation of 245 Hg 0 , following the NVE mechanism. The MIF driven by NVE shares the same direction as the MDF induced during certain process (Schauble, 2007). Negative MDF must be subsequently induced in Hg(II) after oxidation, producing the negative δ 202 Hg and positive Δ 199 Hg values in the PBM examined here.
Lighter isotopes prefer to be bound on the surfaces of particles, that could be the most possible reason to negative MDF (Fig.2 processes g). 250 It should be noted that the primary process leading to the Hg(II) in PBM could not be the primary oxidation processes of Hg 0 in MBL in this study. The GOM attributed to multiple oxidizers would show a variety of occurrence forms, with different adsorption capacities on particle surfaces. The specific forms of Hg(II) on particle surfaces remained unclear in this study. Meanwhile, the oxidation processes leading to the Hg(II) in PBM might also occur in the interface reaction layer on particle surfaces. To date, the 255 https://doi.org/10.5194/acp-2020-397 Preprint. Discussion started: 6 May 2020 c Author(s) 2020. CC BY 4.0 License. mechanisms and isotopic fractionations on the formations of GOM were poorly understood, preventing the accurate cognitions to the Hg(II) in PBM.
On the other hand, all slopes obtained by performing a linear regression of the Δ 199 Hg and Δ 201 Hg in the TGM samples were higher than 1.0 (Fig. 4). A ~1.0 slope was commonly observed in TGM  (Fig. 2 process b), that was also consistent to the negative MDF in TGM 270 collected during two summer cruises, contrasting to the opposite direction of MDF in Hg 0 when oxidation was derived by Cl atoms (Fig. 2 process c). Therefore, Br atoms were suggested to be the primary oxidizer for Hg 0 in MBL, echoing the demonstration in previous publications (De Simone et al., 2013;Holmes et al., 2010;Holmes et al., 2009;Ye et al., 2016;Obrist et al., 2011).
In addition, potential alternative factors might also contribute to the transformations of TGM in this 275 study, followed by these isotopic clues. A negative correlation (P < 0.01) between Δ 199 Hg in the TGM and the atmospheric temperature (17.7 to 28.4°C) was observed during the 2018-summer cruise, indicating that process inducing positive odd-MIF in TGM could be more active at lower temperatures, enhancing the oxidation and scavenging of Hg 0 in the MBL (Hedgecock and Pirrone, 2004). However, the correlation was absent during the 2016-summer cruise, which is possibly due to the narrow 280 temperature range involved (22.5 to 24.7°C) (Fig. 3a), and also absent during the 2016-winter cruise with lower temperatures. Despite of the similar large temperature range (-1.4 to 12.0°C), and the similar positive correlation (P = 0.03) between Δ 199 Hg in TGM and TGM concentration with 2018-summer cruise (Fig. 3b), that correlation absence indicated the process might be weak during winter cruise. https://doi.org/10.5194/acp-2020-397 Preprint. Discussion started: 6 May 2020 c Author(s) 2020. CC BY 4.0 License.
Emissions from surface sea water (Fig. 2 process h) are commonly considered to be crucial to 285 influencing atmospheric Hg in MBL. However, in this study, this factor should be considered less important. Volatilization of dissolved gaseous Hg should induce negative MDF to Hg 0 in the MBL (Zheng et al., 2007), which partially contributed to the negative δ 202 Hg observed in the TGM. This process should not produce odd-MIF (Zheng et al., 2007). According to the negative correlation observed between air temperature and Δ 199 Hg in TGM in 2018-summer cruise, if the elevated temperature 290 accelerating Hg volatilization from surface sea water was an important factor shaping isotopic compositions in TGM, the similar correlation between Δ 199 Hg in TGM and air temperature should also be observed in winter, which was absent in this study.
Transformations of atmospheric Hg are complicated. The mechanisms and isotopic fractionations of transformation processes are poorly understood. For instance, the photo-reduction of Hg(II) in gaseous 295 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., MeHg and diMeHg in plume, have been suggested as important components to atmospheric Hg in 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 300 cannot be ruled out.
Odd-MIF occurrences are commonly associated with photo chemical reactions (Bergquist and Blum, 2007;Sun et al., 2016). However, isotopic compositions in TGM or PBM collected in daytime and nighttime were insignificant different in this study (T-test, P > 0.05). A possible reason is that the isotopic fractionations caused by photo chemical reactions were diluted due to the low time resolutions during 305 sampling. following NVE mechanism. Lower air temperature could promote relevant processes causing positive MIF in TGM in summer, while the relative processes might be weak in winter.
To our knowledge, isotopic fractionation that occurs during Hg environmental processes is diluted 315 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, the 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 behaviours caused by an abundance of highly reactive species. 320 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 325 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;Wang et al., 2016b). In addition, many atmospheric Hg transformation processes, e.g., the reduction of Hg(II) in the gaseous phase, are still poorly understood. More studies are therefore needed 330 to constrain isotopic fractionation during these processes. 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 could serve as effective tools for tracking transformation processes of atmospheric Hg in the MBL, and in other areas with a variety of 335 atmospheric oxidants in atmosphere.

Data availability
Dataset could be accessed in Supplementary Information