Characteristics of atmospheric mercury in East China: implication on sources

Xiaofei Qin1, Xiaohao Wang2, Yijie Shi1, Guangyuan Yu1, Yanfen Lin2, Qingyan Fu2, Dongfang 3 Wang2, Zhouqing Xie3, Congrui Deng1,*, Kan Huang1,* 4 1Center for Atmospheric Chemistry Study, Shanghai Key Laboratory of Atmospheric Particle 5 Pollution and Prevention (LAP3), Department of Environmental Science and Engineering, Fudan 6 University, Shanghai, 200433 China 7 2Shanghai Environmental Monitoring Center, Shanghai, 200030 China 8 3School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui, 9 230026 China 10


Introduction 38
Mercury (Hg) is a global pollutant of great concerns for environment and human health. Based 39 on its physical and chemical properties, atmospheric mercury is operationally divided into three 40 forms, i.e. gaseous elemental mercury (GEM), particulate-bound mercury (PBM), and gaseous 41 oxidized mercury (GOM). GEM is the predominant form in the atmosphere (>90%), while PBM 42 consists of a small quantity of the total mercury as well as for GOM. Elemental mercury in the 43 atmosphere is relatively stable, which means that it has a long lifetime of 0.5-2 year and can 44 transport globally before they are oxidized and removed from the atmosphere via wet and dry 45 depositions (Yu and Luo, 2009). In contrast, GOM and PBM would be rapidly wiped out from the 46 atmosphere after emission due to their significantly greater reactivity, deposition velocities, and 47 water solubility (Yu, 2006;Zhu et al., 2015). 48 Both natural processes and anthropogenic activities release mercury into the atmosphere. 49 Natural sources of mercury include the ocean volatilization, volcanic eruption, evasion from soils 50 and vegetation, geothermal activities, and weathering minerals (Pirrone et al., 2010;Simoneit et al., 51 2004). Re-emissions of mercury that previously deposited onto the environmental surfaces are also 52 considered as natural source. As for the anthropogenic mercury, coal combustion, non-ferrous 53 smelters, cement production, waste incineration, and mining are considered to be the main sources. 54 After being emitted into the atmosphere, mercury will experience the chemical and physical 55 speciation and its forms were essential to understand its biogeochemical cycle. Previous studies 56 suggest that the oxidation of GEM in the terrestrial environments was generally initiated by O3 and 57 OH radicals (Zhang et al., 2013). Atomic bromine (Br) and bromine monoxide (BrO) are two 58 additional oxidation agents in the marine atmosphere (Xiao et al., 2018;Wang et al., 2016). 59 Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-1164 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 4 December 2018 c Author(s) 2018. CC BY 4.0 License.
Observational studies of GOM in the polar regions (Choi et al., 2013;Ye et al., 2016) and in the 60 subtropical marine boundary layer (Cheng et al., 2014;Zhu et al., 2014) as well as atmospheric 61 modeling studies about mercury cycling (Feng et al., 2004;Shon et al., 2005) have considered Br to 62 be an important oxidant of GEM.  even reported that Br is the primary oxidant 63 of GEM in tropical marine boundary layer (MBL). However, it still remain unknown and 64 controversial about the speciation and quantification of the GEM+O3 products, and the reaction of 65 GEM+OH is still under huge debate between theoretical and experimental studies due to the lacking 66 of mechanisms consistent with thermochemistry (Xiao et al., 2018). As the GEM converts into GOM, 67 a part of GOM will be adsorbed onto particulate matter since it has high water solubility and 68 relatively strong surface adhesion properties (Liu et al., 2010). GEM accounts for the vast majority 69 of total mercury in the atmosphere, and its concentration is an order of magnitude higher than that 70 of GOM and PBM. Hence, the part of GEM that is lost via the redox reactions might not cause a 71 huge disturbance to its concentration, while the GOM species from the GEM oxidation and 72 subsequent formation of PBM by adsorption on the particle matters can significantly affect their 73 ambient concentrations, especially in regions with high GEM levels. 74 Many efforts have been made by governments to reduce mercury emissions. In October 2013, 75 128 countries signed a global treaty "Minamata Convention for mercury" in order to reduce mercury 76 emissions from anthropogenic activities . However, the situation of mercury 77 pollution is still grim, especially in Asia, which contribute about half of the global mercury 78 emissions (Wu et al., 2006). Mainland China plays an important role in the biogeochemical cycling 79 of mercury, since about 27% of the global total atmospheric mercury exhausts are from this area 80 (Hui et al., 2017). The Yangtze River Delta (YRD) is one of the most industrialized and urbanized 81 regions in China. Some previous studies showed that the atmospheric mercury pollution in Shanghai 82 is very serious (Friedli et al., 2011;Duan et al., 2017). However, studies with respect to the sources 83 of Hg in East China and the associated formation and transformation processes among Hg species 84 in the atmosphere are still lacking. 85 In this study, one-year comprehensive measurements of GEM, GOM, and PBM were 86 conducted at Dianshan Lake Station (DSL), a suburban site in Shanghai. DSL is located in the 87 junction of Shanghai, Zhejiang, and Jiangsu provinces and is close to the East China Sea (ECS). 88 Few local sources and multiple surroundings make DSL a unique location for studying the main 89

Potential source contribution function (PSCF) 154
PSCF is a useful tool to diagnose the possible source areas with regard to the levels of air 155 pollutants when setting a contamination concentration threshold at the receptor site. Back trajectory 156 models are used to simulate the airflows. The principle of PSCF is to calculate the ratio of the total 157 number of back trajectory segment endpoints in a grid cell (i, j) which exceed the threshold 158 concentration (mij) to the total number of back trajectory segment endpoints in this grid cell (i, j) 159 during the whole sampling period (nij) as expressed by Equation 1 (Hopke, 2003;Cheng et al., 2015). 160 When a particular cell is associated with a small number of endpoint, weighting function (wij) 162 is applied to reduce this uncertainty and the value of wij is set as below . 163 In this study, we set the threshold concentration as the mean value of the whole sampling period. 166 Backward trajectories were calculated every two hours and the cell size was set as 0.5° 0.5°. 167 168

Positive matrix factorization (PMF) 169
The PMF model (Paatero and Tapper, 1994) is widely used to quantitatively determine the 170 source contributions of specific air pollutants. The essential principle behind PMF is that every 171 concentration is determined by source profiles and source contributions to every sample. The 172 equation of the PMF model is shown as Eq. (3): Xij is the concentration of the jth contamination at the receptor site in the ith sample. gik represent 175 the contribution of the kth factor on the ith sample, fkj is used to express the mass fraction of the jth 176 contamination in the kth factor, P is the number of factors, which represent pollution sources, eij is 177 Before the model determines the optimal non-negative factor contributions and factor profiles, 179 an objective function, which is the sum of the square difference between the measured and modeled 180 concentrations weighted by the concentration uncertainties, has to be minimized (Cheng et al., 2015). 181 The equation that determines the objective function is given by Eq. (4): 182 where Xij is the ambient concentration of the jth pollutant in the ith sample (m and n represent the 184 total number pollutants and samples, respectively). Aik is the contribution of the kth factor on the ith 185 sample and Fkj is the mass fraction of the jth pollutant in the kth factor. Sij is the uncertainty of the 186 jth pollutant on the ith measurement, P is the number of factors, which imply the pollutant sources. at DSL were 2.77 ng/m 3 , 60.80 pg/m 3 , and 82.13 pg/m 3 , respectively. As shown in Table 1, the levels 193 of GEM and PBM in this study were lower than some sites in China by a factor of 2-7, such as rural 194 Miyun, suburban Xiamen, and urban Guiyang (Zhang et al., 2013;Xu et al., 2015;Fu et al., 2011). 195 However, compared to the studies conducted in urban and rural areas abroad such as New York 196 (Choi et al., 2013), Chicago (Gratz et al., 2013), and Nova Scotia (Cheng et al., 2014), the 197 concentrations of GEM and PBM in the suburbs of Shanghai were much higher by a factor of 1-3 198 and 3-8, respectively. Different from GEM and PBM, the GOM concentrations at DSL were higher 199 than all the Chinese sites and other sites around the world listed in Table 1. The mean GOM 200 concentration in this study (82.13 pg/m 3 ) was even higher than that in Guiyang (35.7 pg/m 3 ), where 201 the emissions of GEM and GOM were quite intense due to the massive primary emission sources 202 such as coal-fired power plants and cement plants . The abnormally high GOM 203 concentrations observed in this study were likely attributed to both strong primary emissions and 204 secondary formation, which will be discussed further in Section 3.4. showed the occurrences of high GEM concentrations in both cold and warm seasons, which was 210 different from many urban and remote sites in China, such as Guiyang,Xiamen,and Mt. Changbai,211 where GEM showed significantly high concentrations in cold seasons than those in warm seasons 212 (Feng et al., 2004;Xu et al., 2015;Fu et al., 2012). The relatively high GEM concentrations during 213 the cold season in this study should be attributed to the increases of energy consumption. In contrast, was consistent with other sites in China such as Beijing and Nanjing (Zhang et al., 2013;Zhu et al., 220 2014). PBM concentrations at low altitude sites in the Northern Hemisphere were commonly 221 enhanced in winter, which was ascribed to intense emissions from residential heating, the reduction 222 of wet scavenging processes, enhanced gas-particle partitioning of atmospheric mercury under low 223 temperature, etc. (Rutter and Schauer, 2007). As for GOM, its seasonal mean concentrations were 224 the highest in winter (124.02 pg/m 3 ), followed by summer (77.27 pg/m 3 ), spring (68.14 pg/m 3 ), and 225 autumn (60.95 pg/m 3 ). The winter maximum of GOM suggested the significant influence of 226 anthropogenic emissions and unfavorable meteorological conditions. The relatively high GOM 227 concentrations in summer indicated that the formation of GOM from GEM oxidation was likely to 228 be crucial. 229  To ensure the time resolutions were consistent among all three mercury species, the temporal 231 resolution of measured GEM was converted from 5 minutes to a two-hour average. As shown in Fig.  232 4, GEM concentrations were higher during daytime with the maximum in the morning at around GOM were as similar as those in Nanjing (Zhu et al, 2012), but different from those in Guiyang,239 Xiamen, and Guangzhou (Feng et al., 2004;Chen et al., 2013). The elevated GEM concentrations at 240 DSL during daytime were likely related to the stronger emissions from both human activities and 241 natural releases. Apart from direct emissions, high GOM concentrations at daytime partially resulted 242 from in situ GEM oxidation. The high PBM concentrations at night were likely derived from the 243 adsorption of Hg species onto the preexisting particles and the subsequent accumulation in the 244 shallow nocturnal boundary layer. Fig. 4 shows that wind speed was relatively low while high for 245 relative humidity at night, which were conducive to the adsorption of GOM onto the particles. 246 247 concentrations from the north was expected. In this regard, the even higher atmospheric Hg 260 concentrations from the south and southeast than from the north cannot be simply explained by 261 anthropogenic emission sources, implying that there must be additional Hg emission sources. (Wang 262 et al., 2016) reported that the Hg concentrations in the surface soils of southern China were generally 263 higher than the northern China. It was possible that emissions from natural sources, such as soils, 264 vegetations and water, might play an important role. 265

Relationship between Hg species and meteorological factors 248
In order to confirm this conjecture, the relationship between temperature and Hg concentrations 266 PSCF was applied to identify the potential source regions of the three Hg species. As for GEM, 284 the major source areas were located in Anhui, Jiangxi, and Zhejiang provinces, and there were also 285 signals from Shandong province (Fig. 7a). As for the seasonal pattern ( Fig. S1), the potential source 286 regions of GEM in spring were mainly from Jiangsu and Zhejiang provinces. In summer, the PSCF 287 hotspots were identified in Anhui and Jiangxi provinces. Jiangsu province was likely to become the 288 main potential source region of GEM in autumn. In winter, Anhui and Zhejiang provinces showed 289 relatively high PSCF values. In addition, there were also signals from Henan and Shandong 290 provinces, suggesting the importance of long-range transport in wintertime. It was obvious from 291 southern provinces seemed unreasonable. In this regard, the re-emission of GEM from natural 297 surfaces in southern areas should be a crucial source, corroborating the discussion in Section 3.2. In 298 addition, it was found that vast areas of the East China Sea were also identified as potential source 299 regions of GEM in all four seasons (Fig. S1), indicating the non-negligible influence from shipping 300 activities. The detailed estimation of variable sources would be discussed in Section 3.3.3. 301 The PSCF pattern of PBM was quite different from that of GEM (Fig. 7b). The potential source 302 regions of all year round PBM were mainly from northeastern China, including Jiangsu, Anhui, 303 Shandong, and Hebei provinces. These provinces were regarded as the main Hg sources areas in 304 China and accounted for about 25.2% of the Chinese anthropogenic atmospheric Hg emissions (Wu 305 et al., 2016). As for the seasonal PSCF patterns of PBM (Fig. S2), its potential source regions in 306 spring, autumn, and winter shared certain commonalities that exhibited the consistent PSCF patterns 307 as the annual pattern. The exception was found for summer, which showed high PSCF values mainly 308 in the southern areas of Shanghai. This might be attributed to that the prevailing winds in summer 309 were from the south, southeast, and southwest where Zhejiang and Jiangxi provinces were important 310 mercury source regions. 311 The potential source regions of all year round GOM were mainly located in Anhui and Zhejiang 312 provinces and the coastal areas along Jiangsu province (Fig. 7c). Compared to the PSCF patterns of 313 GEM and PBM, the potential source regions of GOM were more from southern China rather than 314 from northern China, which might be due to the higher atmospheric oxidants levels in the southern 315 regions. This was most obvious in summer that the potential sources regions of GOM were mainly 316 from Zhejiang and Jiangxi provinces (Fig. S3). In the other seasons, there were somewhat different 317 PSCF patterns observed. In detail, while obvious PSCF signals from the inland areas were found, 318 moderate PSCF signals over the East China Sea and Yellow Sea also observed in spring. In autumn, 319 the high PSCF values mainly occurred in Zhejiang province and there were also moderate signals 320 over the Yellow Sea. In winter, the high PSCF values spread from the coastal areas of Jiangsu to a 321 vast ocean of the Yellow Sea. One previous study suggested that the marine boundary layer could 322 provide considerable amounts of oxidants such as chlorine and bromine, which were beneficial for 323 the production of GOM by oxidizing GEM (Auzmendi-Murua et al., 2014) and this may explain the 324 substantial PSCF signals over the ocean. It should be noted that the signals from the ocean in 325 summer were weaker than in the other seasons. This was likely due to the particularly high ozone 326 concentrations over land in summer (Lu et al., 2018), leading to the formation of GOM dominated 327 by mainland oxidants rather than the ocean oxidants. 328 The results of the PSCF analysis suggested the significant influences of adjacent areas of 329 Shanghai on contributing to all the atmospheric mercury species. It was also illustrated that the long-330 range and regional transport via both land transport and sea breeze were important. 331 332

Comparison between the impact of quasi-local sources and regional/long-range 333 transport on atmospheric mercury 334
According to the relationship between wind direction and Hg species as well as the PSCF 335 analysis discussed above, the elevated GEM, GOM, and PBM concentrations at the observation site 336 were generally related to the wind sectors from the southwest and north. In order to reveal the 337 relative importance of local sources and regional transport, the ratio of GOM/PBM was applied as 338 an indicator based on the fact that the residence time of GOM is generally considered to be shorter 339 than that of PBM. If regional/long-range transport was evident, the ratio of GOM/PBM should be 340 lower due to that GOM was more quickly scavenged than PBM during the transport, and vice versa 341 when local sources dominated. In this regard, the ratios of GOM/PBM during the whole study period 342 were grouped into four categories, i.e. 0-1, 1-2, 2-3, and higher than 3. The corresponding frequency 343 of wind direction in each category was compared in Fig. 8. It was clear that the higher GOM/PBM 344 ratios were associated with more frequent winds from the east and southeast. The frequency of these 345 two wind sectors increased significantly from 27% under the GOM/PBM ratios less than 1 to 52% 346 under the GOM/PBM ratios higher than 3. Winds from the east and southeast were typically 347 characterized of relatively clean air masses, suggesting the local sources around the observational 348 site should dominate. In contrast, the lower GOM/PBM ratios were associated with more frequent 349 winds clockwise from the west to the north and the frequency of these wind sectors decreased 350 significantly from 44% under the GOM/PBM ratios less than 1 to 21% under the GOM/PBM ratios 351 higher than 3. These winds were indicative of the long-range/regional transport from northern China 352 and were associated with the relatively low GOM/PBM ratios. According to the PSCF results above, south and southwest of the sampling site were complicated, and the phenomenon above can not be 357 simply explained by the impact of local sources or regional transport. 358 We further investigated the relationship among GEM, CO, secondary inorganic aerosols (SNA) 359 and GOM/PBM ratios. Fig. 9 displays the concentrations of GEM as a function of GOM/PBM ratios 360 colored by CO. The sizes of the circles represented the corresponding concentrations of SNA in 361 PM2.5. CO was commonly used as a tracer of fuel combustion and SNA were derived from secondary 362 formation via the gas-to-particle conversion. CO and SNA were collectively used as proxies of the 363 extent of anthropogenic air pollutants and especially for evaluating the extent of regional/long-range 364 transport. As shown in Fig. 9, GEM showed an overall increasing trend as the GOM/PBM ratios 365 increased. In addition, it could be clearly seen that the lower GOM/PBM ratios were associated with 366 higher CO and SNA concentrations and vice versa. This corroborated the discussion above that the 367 GOM/PBM ratio was a reliable tracer for assessing the relative importance of regional/long-range 368 transport vs. local atmospheric processing. 369 In the GOM/PBM ratio bins of less than 2.5, GEM fluctuated with the mean values less than 370 2.6 ng/m 3 . The mean GEM concentration increased from 2.61 ng/m 3 in the GOM/PBM ratio bin of 371 2.5-3.0 to 2.8 ng/m 3 in the bin of 3.0-3.5, and then remain relatively stable when the GOM/PBM 372 ratio bins higher than 3.0. Generally, GEM showed an increasing trend as the GOM/PBM ratios 373 increased while both SNA and CO decreased. The elevation of GEM concentrations tended to be 374 associated with the impact of quasi-local sources. In contrast, under the high SNA and CO 375 conditions when GOM/PBM ratios were lower, GEM was relatively low, suggesting its formation 376 was not favored via the regional/long-range transport. It has been recognized that the common 377 regional/long-range transport pathways on contributing to the particulate pollution events of 378 Shanghai were from the north and northwest originating mostly from the North China Plain. The 379 relatively lower GEM concentrations under the regional/long-range transport conditions 380 corroborated the PSCF analysis that only moderate probabilities of GEM source regions from 381 northern China were found (Fig. 7a). and contributions of major anthropogenic sources to GEM are shown in Fig. 10. It has to be noted 390 that since no tracers for the natural emissions (e.g. soils, vegetations, and ocean) were available in 391 this study, the identification of natural mercury sources was not possible. 392 Factor 1 had high loadings for Se, As, Pb, NO3 -, SO4 2-, and NH4 + . Se, As, and Pb, which were 393 typical tracers of coal combustion. SO4 2and NO3were also formed from the gaseous pollutants 394 emitted from coal burning. Hence, this factor was defined as coal combustion sources and accounted 395 for 12.3% of the annual mean GEM. 396 Factor 2 displayed particularly high loadings for Ni and V. The major sources of Ni in the 397 atmosphere can be derived from coal and oil combustions (Tian et al., 2012), and oil combustion 398 accounted for 85% of anthropogenic V emissions in the atmosphere (Duan and Tan, 2013). In 399 general, Ni and V have been considered as good tracers of heavy oil combustion, which has been 400 commonly used in marine vessels (Viana et al., 2009). Thus, this factor was identified as shipping 401 emissions. The sampling site is adjacent to the East China Sea and is located in Shanghai which has 402 the largest port in the world. It was reasonable that shipping emissions were contributable to the 403 atmospheric Hg, which shouldn't be ignored in the coastal regions. Shipping emissions accounted 404 for 19.6% of GEM and ranked as the second largest emission sector, highlighting the urgent need 405 of controlling the marine vessel emissions. 406 Factor 3 showed high loadings for Ca and moderate loadings for Ba and Fe. Ca and Fe are rich 407 elements in crust that can be used for cement production. As mercury could be released during 408 industrial processes of cement production, Factor 3 was assigned as cement production and 409 accounted for a minor fraction of 6.3 % of the GEM. 410

GEM. 417
Factor 6 was characterized by high loadings of Cd, Ag, K + , and Na + . The major sources of Cd 418 in China were iron and steel smelting industries (Duan and Tan, 2013). Ag was mainly used in 419 industrial applications, including electronic appliances and photographic materials. K + was a typical 420 tracer of biomass burning, which often stemmed from agriculture waste burning over the Yangtze 421 River Delta and the North China Plain. In this regard, Factor 6 was considered as a combined source 422 of the industrial and biomass burning emission sectors. It was estimated to contribute 47.8% of the 423 GEM. 424 425

The formation of GOM 427
A typical case from July 24 to July 27, 2015 was chosen to investigate the possible formation 428 process of GOM. As shown in Fig. 11, the shaded episodes represented nighttime from 18:00 to 429 6:00 the next day. It was obvious that both GEM and GOM exhibited rising trends during nighttime 430 (Fig. 11a). This was ascribed to nighttime accumulation effect due to the very shallow boundary 431 layer (Fig. 11c). Starting from 6:00 in the morning, GEM concentrations began to gradually decline 432 as the boundary layer developed. In contrast, the concentrations of GOM continued to rise from 433 6:00 until it reached the peak value at around 10:00. During this period, the levels of ozone and 434 temperature also kept rising until surpassed 200 μg/m 3 and 34℃, respectively. Accordingly, as an 435 anthropogenic emitting tracer, the concentration of carbon monoxide was basically stable and even 436 showing a downward trend, which suggested that anthropogenic activities were not the main driving 437 force for the increase of GOM. This phenomenon clearly revealed the acceleration of the conversion 438 process of GEM to GOM under favorable atmospheric conditions of higher O3 concentration and 439 ambient temperature. In the case of atmosphere dilution by the rise of PBL, the fact that GOM was 440 not falling but rising suggested the great influence of this process on ambient GOM concentrations. 441 Similar observation has been found at the high-altitude Pic du Midi observatory in southern France 442 (Fu et al., 2016), where was almost impervious to anthropogenic emission sources. The important 443 role of GEM oxidation in our sampling site, which located in one of the most developed industrial 444 areas in China, was most likely due to the presence of sufficient oxidants in this area. Severe ozone 445 pollution frequently occurred in the YRD due to strong anthropogenic emission intensities (Lu et 446 Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-1164 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 4 December 2018 c Author(s) 2018. CC BY 4.0 License. al., 2018). Previous studies suggested that the primary oxidants in the terrestrial environment were 447 O3 and OH radicals (Shon et al., 2005), while Br was an important oxidant in the subtropical marine 448 boundary layer (Obrist et al., 2011). It was possible that, in addition to O3 and OH radicals, Br might 449 also be an important inducing species to the oxidation of GEM as the DSL site is adjacent to the 450 East China Sea. 451 This further confirmed the case study above that the levels of oxidants under favorable 458 environmental conditions were crucial for the formation of GOM. Fig. 12 also demonstrated the 459 increases of GOM along with the ratios of GOM/PBM. The lower ratios of GOM/PBM were 460 associated with lower temperature and O3 concentrations, indicating the more probable long-461 range/regional events during the cold seasons with relatively weak photochemistry. On the contrary, 462 the higher ratios of GOM/PBM were associated with higher temperature and O3 concentrations, 463 indicating the more probable local events during the warm seasons with relatively strong 464 photochemistry. This suggested that the formation of GOM was more favored by local atmospheric 465 processing rather than the transport. This study demonstrated the abnormally high GOM 466 concentrations observed at DSL were largely ascribed to local oxidation reactions. However, the 467 explicit formation mechanism of GOM need to be investigated by measuring more detailed 468 components of GOM and atmospheric oxidants. 469 470

The formation of PBM 471
Gas-particle partitioning was considered to be an important pathway for the formation of PBM 472 (Amos et al., 2012). Since most of the areas in the YRD belong to non-attainment areas in regard of 473 particulate pollution and the concentrations of GOM were particularly high at DSL as discussed 474 above, the role of gas-particle partitioning in the formation of PBM should be investigated. Fig. 13  climbing, GOM began to show somewhat negative correlation with PM2.5, but not obvious. The 488 reason might be that the relatively high temperature and low humidity during this period were not 489 conducive to the transfer of GOM to particle matters. In Stage 3, GOM decreased as PM2.5 continued 490 to increase, showing a clear anti-correlation. During this period, PBM showed a consistent trend 491 with PM2.5 and CO. Temperature was relatively low but with relatively high humidity. This 492 phenomenon clearly demonstrated the process of gas-particle partitioning of PBM formation. In 493 stage 4, GOM and PBM showed similar decreasing trend with PM2.5 and CO. The low GOM 494 concentrations, low humidity, and high temperature resulted in no significant signs of GOM 495 adsorption to PM2.5 in this stage. In general, high PM2.5 and GOM concentration in our sampling 496 site made the process of gas-particle partitioning obvious, especially under high humidity and low 497 temperature conditions. 498 499

Conclusions 500
In this study, a year-long observation of three atmospheric Hg species was conducted at the 501 Dianshan Lake (DSL) Observatory, located on the typical transport routes from mainland China to 502 the East China Sea. During the whole measurement period, the mean GEM, PBM, and GOM 503 concentrations were 2.77 ng/m 3 , 60.8 pg/m 3 , and 82.13 pg/m 3 , respectively. concentrations were higher during nighttime, which was ascribed to the accumulation effect within 514 the shallow nocturnal boundary layer. 515 The relationship between meteorological factors and atmospheric Hg species showed that the 516 high Hg concentrations were generally related to the winds from the south, southwest, and north 517 and positively correlated with temperature. Both anthropogenic sources and natural sources 518 contributed to the atmospheric mercury pollution at DSL. Higher GOM/PBM ratios corresponded 519 to lower CO and SNA concentrations and vice versa. The ratio of GOM/PBM can be used as a tracer 520 for distinguishing local sources and regional/long-range transport based on the fact that the 521 residence time of GOM was shorter than that of PBM. GEM as a function of the GOM/PBM ratios 522 indicated that when the quasi-local sources dominated, GEM concentrations were relatively higher 523 than those events under the regional/long-range transport conditions. According to the PMF source 524 apportionment results, six sources of GEM and their contributions were identified, i.e. industrial 525 and biomass burning (47.8%), shipping emission (19.6%), coal combustion (12.3%), iron and steel 526 production (7.6%), incineration (6.4%), and cement production (6.3%). The significant contribution 527 of shipping emission suggested that in coastal areas mercury emitted from marine vessels can be 528