Insights on Chemistry of Mercury Species in Clouds over Northern China

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Characterization of Natural and Affected Environments

Insights on Chemistry of Mercury Species in Clouds over Northern China: Complexation and Adsorption Tao Li, Yan Wang, Huiting Mao, Shuxiao Wang, Robert W. Talbot, Ying Zhou, Zhe Wang, Xiaoling Nie, and Guanghao Qie Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06669 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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Environmental Science & Technology

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Insights on Chemistry of Mercury Species in Clouds

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over Northern China: Complexation and Adsorption

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Tao Li1,5, Yan Wang*,1, Huiting Mao2, Shuxiao Wang3, Robert W. Talbot4, Ying Zhou2, Zhe

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Wang5, Xiaoling Nie1, Guanghao Qie1 1

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School of Environmental Science and Engineering, Shandong University, Jinan 250100, China

Department of Chemistry, State University of New York, College of Environmental Science and

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Forestry, Syracuse, NY 13210, USA

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3

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Environment, Tsinghua University, & State Environmental Protection Key Laboratory of

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Sources and Control of Air Pollution Complex, Beijing 100084, China 4

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State Key Joint Laboratory of Environment Simulation and Pollution Control, School of

Department of Earth and Atmospheric Science, University of Houston, Houston, TX 77204, USA

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Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong, China

15 16

ACS Paragon Plus Environment

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ABSTRACT: Cloud effects on heterogeneous reactions of atmospheric mercury (Hg) are poorly

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understood due to limited knowledge of cloudwater Hg chemistry. Here we quantified Hg

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species in cloudwater at the summit of Mt. Tai in northern China. Total mercury (THg) and

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methylmercury (MeHg) in cloudwater were on average 70.5 ng L-1 and 0.15 ng L-1, respectively,

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and particulate Hg (PHg) contributed two-thirds of THg. Chemical equilibrium modeling

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simulations suggested that Hg complexes by dissolved organic matter (DOM) dominated

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dissolved Hg (DHg) speciation, which was highly pH dependent. Hg concentrations and

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speciation were altered by cloud processing, during which significant positive correlations of

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PHg and MeHg with cloud droplet number concentration (Nd) were observed. Unlike direct

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contribution to PHg from cloud scavenging of aerosol particles, abiotic DHg methylation was the

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most likely source of MeHg. Hg adsorption coefficients Kad (5.9 – 362.7 L g-1) exhibited an

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inverse-power relationship with cloud residues content. Morphology analyses indicated that

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compared to mineral particles, fly ash particles could enhance Hg adsorption due to more

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abundant carbon binding sites on the surface. Severe particulate air pollution in northern China

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may bring substantial Hg into cloud droplets and impact atmospheric Hg geochemical cycling by

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aerosol-cloud interactions.

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1. INTRODUCTION

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Mercury (Hg) is a persistent toxic contaminant that ubiquitously exists in the atmosphere.

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Cloud processing can significantly affect atmospheric Hg transport and fate via scavenging,

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dissolution, conversion and deposition. Most importantly, clouds, covering ~70% of the earth

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surface,1 provide sufficient medium for aqueous or heterogeneous reactions2-3 (e.g., gas-liquid

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partitioning,4 adsorption,5-6 photoredox,7-9 methylation,10-13 etc.), which fundamentally determine

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atmospheric Hg speciation and transformation.14-15 However, the Hg species, sources and

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chemical reactions in cloudwater are poorly understood due to the lack of cloudwater Hg

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measurements.

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In contrast to extensive work on atmospheric gas- and particulate-phase Hg16-17 (references

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therein) as well as Hg in precipitations,18-22 only a handful of studies reported measurements23-29

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and simulations15 of Hg species in cloud/fog water. For example, the concentrations of Hg in

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cloudwater at Puy De Dôme, France23 and Mt. Mansfield, USA24 were 10 – 50 ng L-1 and 7.5 –

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71.8 ng L-1, respectively, which were generally higher than precipitation Hg in most North

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America and Europe regions (< 23 ng L-1)16, 20. A mean concentration of 9.6 ng L-1 was observed

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in cloud water at Mt. Bamboo, Taiwan, which was attributed largely to coal burning and

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industrial activities in northern China using trajectory analysis.26 Recently, high methylmercury

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(MeHg) concentrations were measured in California coastal fogwater (3.4 ± 3.8 ng L-1)27 and

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marine stratus cloudwater (0.87 ± 0.66 ng L-1)29, which derived from the decomposition of

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dimethylmercury released by ocean upwelling. MeHg in aquatic environments has two major

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pathways of formation from reactive Hg species: biotic methylation by anaerobic bacteria30-32

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and abiotic methylation with methyl donors transfer.13,

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deposition might directly contribute MeHg to forest food webs, implying the existence of abiotic

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Tsui et al. found that atmospheric


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Hg methylation in atmospheric waters (e.g., rainfall, clouds, fogs),34 but the mechanism is

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unascertained yet.

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The objective of this study was to fill knowledge gaps of cloudwater Hg species and chemistry.

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Cloudwater samples were collected at Mt. Tai in polluted northern China to quantify total Hg

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(THg), dissolved Hg (DHg), particulate Hg (PHg) and MeHg with simultaneous measurements

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of other chemicophysical parameters. The data were used to address Hg chemical speciation and

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potential sources of MeHg. Cloud processing effects on Hg behaviors were then investigated.

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Finally, we elucidated Hg adsorption on different cloud residue particles from micro-view and its

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implication on atmospheric Hg transfer and transformation in clouds.

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2. MATERIALS AND METHODS

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2.1. Field Sampling

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During 15 June to 11 August 2015, 85 cloudwater samples were collected during 21 non-

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precipitation cloud events at the summit of Mt. Tai (36°16′N, 117°06′E, 1545 m a.s.l.) in the

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North China Plain (Figure S1). A single-stage Caltech Active Strand Cloudwater Collector

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(CASCC) was employed to sample cloudwater. The sampling duration ranged from one to

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several hours as appropriate to obtain as much as cloudwater volume for analysis. The liquid

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water content (LWC) and cloud droplet number concentration (Nd) were continuously measured

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using a fog monitor (FM-120, DMT, USA) employed in previous studies.35-36 Hourly PM2.5 was

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measured using a real-time particulate monitor (Model 5030 SHARP, Thermo Scientific).

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In-situ measurements of pH and electrical conductivity were immediately conducted after

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sampling with a portable pH meter (Model 6350M, JENCO). Acid-cleaned borosilicate glass

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bottles were used for Hg sample storage. The bottles were soaked in order in HNO3, BrCl and

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HCl solution for 12 h each, followed by rinsing with Milli-Q water and drying at 500 oC.


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Cloudwater samples were filtered through 0.45 µm microfilters (ANPEL Laboratory

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Technologies (Shanghai) Inc.) on site right after collection in priority for analysis of DHg and

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the unfiltered samples were then preserved for analysis of THg and MeHg. The Hg samples were

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acidified to 0.5% v/v using high pure HCl. Filtered aliquots (~10 ml each) were also prepared to

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analyze dissolved organic carbon (DOC), water-soluble ions and carboxylic acids. All liquid

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samples and filtered particles (i.e., the cloud residues) were stored at 4 oC and refrigerated at –20

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o

C in the dark.

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Concurrent total gaseous mercury (TGM) was measured using a Tekran model 2537A cold-

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vapor atomic fluorescence spectrometer (CVAFS) that was used to quantify Hg0 after TGM pre-

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concentration onto gold traps and thermal desorption, with a 5-minute time resolution, a

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sampling flow rate of 1.5 L min-1 and a limit of detection of 5–10 fmol/mol (1 ng m-3 = 112

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fmol/mol). The sampling inlet was ~3 m above the ground and Teflon tube was heated at 50 oC

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with pre-filtration of aerosols. Measurement details can be found in Mao et al. (2008).37

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2.2. Laboratory Analyses

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The US EPA method 1631 was applied for determination of THg and DHg, performed by BrCl

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oxidation, SnCl2 reduction, purging and gold trapping, and quantification by CVAFS. PHg was

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calculated as the difference between THg and DHg. Based on the US EPA method 1630, MeHg

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samples were distilled, ethylated, purged and trapped, desorbed and detected by GC-CVAFS.

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The method detection limits for Hg and MeHg were 0.2 and 0.01 ng L-1, and the recoveries for

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matrix spikes were 91 – 97% and 94 – 112%, respectively. The concentrations of Hg and MeHg

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in blank samples, which were taken by spraying the Teflon strands of the sampler using

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deionized water, were measured to be 0.9 and 0.03 ng L-1, respectively.


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Water-soluble ions (Na+, NH4+, K+, Mg2+, Ca2+, F−, Cl−, NO2−, NO3− and SO42−) were

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measured using ion chromatography (Dionex, ICS-90). Carboxylic acids (formate, acetate,

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propionate, lactate, butyrate, mesylate and oxalate) were analyzed with ion chromatography

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(Dionex, ICS-2500). DOC was calculated from the difference between total carbon and inorganic

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carbon, which were quantified by NDIR detection of CO2 after thermocatalytic oxidation at 650

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o

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particles were reweighed after balance at 20 oC for 24 h, to determine cloud residues

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concentration with the recorded filtration volume.

C with a TOC analyzer (TOC-LCPH/CPN, SHIMADZU, Japan). Preweighed filters retaining

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The morphology and elemental composition of three typical cloud residues samples were semi-

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quantitatively analyzed by scanning electron microscopy-energy dispersive X-ray spectrometry

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(SEM-EDS, GeminiSEM, ZEISS; QUANTAX, Bruker). Elemental mapping was obtained to

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understand the elemental distributions on individual particles surface.

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2.3. Chemical Equilibrium Modeling

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Hg equilibrium speciation was calculated using Visual MINTEQ v3.1.38 Input data include pH

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values and concentrations of DHg and other components including DOC, water-soluble ions and

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carboxylic acids (Table S1). A Gaussian DOM model39 was applied to model Hg complexes by

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DOM. Ionic strength and temperature corrections were made using the Davies equation and van’t

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Hoff equation, respectively. Redox reactions were not considered. Adsorption of Hg by cloud

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residues was assumed not to influence the equilibrium speciation of DHg. Details of modeling

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(Text S1) and stability constants for Hg species (Table S2) are summarized in the Supporting

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Information.

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2.4. Adsorption Coefficient


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The adsorption coefficient (Kad) (L g-1) of Hg onto cloud residues can be calculated using the following equation,6 [PHg](ng L-1 )ൗ[Cloud residues](mg L-1 )

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Kad =

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Here, solid Hg species that may exist but cannot be distinguished are all regarded as adsorbed

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[DHg](ng L-1 )

×1000

Hg.

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2.5. Trajectory and Potential Source Contribution Function (PSCF) Analysis

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Three-day backward trajectories of air masses arriving at Mt. Tai at a height of 1545 m a.s.l.

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during cloud events were simulated using the NOAA Hybrid Single-Particle Lagrangian

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Integrated Trajectory (HYSPLIT) model.40 Potential source regions of cloudwater MeHg were

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identified by PSCF analysis.41 Details of PSCF can be found in Text S2.

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3. RESULTS AND DISCUSSION

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3.1. Chemical Characterization of Cloudwater and Hg

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Significantly decreased cloudwater acidity was observed at Mt. Tai with volume-weighted

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mean (VWM) pH value of 5.79 (4.82 – 6.95), compared to the value of 3.86 measured during

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2007/2008.42 Concentrations of two major anions, sulfate and nitrate, decreased by 42% and

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30%, respectively, attributed mainly to reduced emissions of SO2 and NOx in recent years

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(www.stats.gov.cn/english/Statisticaldata/AnnualData/),36 which together with large emissions of

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ammonia across the North China Plain43 likely caused the increased cloudwater pH values. LWC

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and Nd varied between 0.01 – 0.90 g m-3 and 67 – 2180 cm-3, respectively. More details are in

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Table S1 and Figure S2.

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Concentrations of THg in cloudwater ranged from 10.2 to 773.3 ng L-1 with an average of 70.5

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ng L-1 (VWM = 47.6 ng L-1), several times higher than those measured in fogs and clouds

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worldwide (mean of 9.6–24.8 ng L-1, Table S3).23-27 These values were also significantly higher


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than precipitation concentrations of Hg (3.0 – 32.3 ng L-1) at most remote and urban sites in

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China, North American, and Europe.16 DHg accounted for 34.3% of THg averaged at 24.1 ng L-1

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while PHg for two-thirds of THg. The higher PHg proportion in cloudwater was similar to that

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observed in urban precipitation18 and snow over Arctic land.44 The mean concentration of MeHg

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was 0.15 ng L-1, 0.3% of THg. It was on average about an order of magnitude lower than that in

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California coastal fogwater and marine cloudwater,27-29 and slightly lower than that in urban

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precipitation in Southwest China, but higher than that in precipitation at other mountain sites

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(0.03 – 0.12 ng L-1).16

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3.2. Simulated Hg Speciation

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PHg was the most abundant species (67.8% of THg) (Figure 1a). Without considering DOM

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complexation, chemical equilibrium modeling simulated major DHg speciation to be Hg(OH)2

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(10.6%), HgClOH(aq) (7.8%) and HgCl2(aq) (5.9%) (Figure S3), similar to those in Sacramento

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Valley fogwater.15 However, a statistically significant correlation between DOC and DHg (r =

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0.592, p < 0.001) suggested the importance of Hg binding to DOM, in agreement with previous

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water/sediment studies.2,

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considering Hg complexation with DOM. Hg-DOM complexes became the predominant DHg

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speciation comprising 21.2% of THg, followed by much less Hg(OH)2 (4.9%), HgClOH(aq)

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(2.1%) and Hg(NH3)22+ (2.0%). It’s commonly known that bromide has high binding constant

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with Hg (log K > 20). However, the Hg-bromo speciation made up only 0.4% of THg due likely

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to the scarcity of bromide ligands in mountaintop cloudwater. In addition, Hg-Oxalate(aq)

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comprised ~0.2% of THg, while the Hg-acetate complexes, which were studied in laboratory

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experiments,10 were not found because of much lower acetate concentrations in cloudwater.15

9, 45-46

Figure 1a shows the overall Hg speciation distribution


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Despite the complexed chemical composition of actual cloudwater, high pH dependence of

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DHg speciation profile was apparently observed (Figure 1b). In general, Hg-DOM complexes

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dominated DHg at acidic pH < 6.0, but its proportion declined significantly from ~90% at

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pH=6.0 to 10 – 20% at pH near 7.0. In contrast, Hg(OH)2 became one of the major speciation

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(increasing from < 5% at pH = 6.0 to > 50% of DHg at pH near 7.0) with increasing alkalinity,

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owing to OH– surpassing other ligands at higher pH. This variation pattern was similar to what

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was previously found for ferric speciation in cloudwater in southern China: decreased Fe(III)-

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oxalate but increased Fe(III)-hydroxyl speciation with rising pH values.38 Figure 1b also

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demonstrates the significance of chloride ligand in DHg speciation, as notable amounts of Hg-

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chloride complexes were calculated in samples with elevated chloride concentrations (> ~3 mg

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L-1). The binding of Hg by chloride mainly formed HgCl2(aq) at pH < 6.0 and HgClOH(aq) at

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pH > 6.0. Furthermore, Hg(NH3)22+ was found to mainly exist at pH > 6.0 and more abundant

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ammonium (over 25 mg L-1) increased Hg(NH3)22+ complexes more often as pH values

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approached 7.0 (Figure S4).

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Figure 1. (a) Hg speciation distribution, and (b) simulated DHg speciation by chemical

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equilibrium modeling versus pH values for each cloudwater sample. Only species exceeding


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0.1% (mole ratio) of THg are displayed. The x-axis ticks in (b) represent pH values of each

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sample and the pH increment gradient is not evenly spaced.

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3.3. Correlations of Measured Hg Species with Cloud Parameters

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Since cloudwater parameters can control dissolved trace elements,38,

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the relationships of

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measured Hg species with pH, LWC, conductivity and Nd were analyzed. Inorganic Hg species

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and MeHg were correlated differently with pH and LWC (Figure S5). Specifically, increased

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cloudwater acidity with pH from neutral (~7.0) to weakly acidic (4.8) did not significantly

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promote DHg and PHg, but did MeHg due probably to lowered pH stimulating Hg methylation.48

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LWC had dilution effects on DHg and PHg concentrations with logarithmic inverse

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relationships, while the effects were negligible on MeHg.

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Figure 2. Cloud droplet number concentrations (Nd) versus (a) inorganic Hg species and (b)

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MeHg, and conductivity versus (c) inorganic Hg species and (d) MeHg. Concentrations in (a)

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and (b) were normalized to be their air equivalent concentrations by multiplying LWC.

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Nd was not correlated with cloudwater concentrations of inorganic PHg and DHg as well as

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MeHg, despite its good relationship with LWC during most non-precipitation cloud events

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(Figure S6). Note that DHg air equivalent concentrations remained nearly constant over a two

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orders of magnitude varying range of Nd (Figure 2a). However, PHg (r = 0.61, p < 0.001) and

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MeHg (r = 0.73, p < 0.001) were found to be significantly correlated with Nd (Figure 2a,b). This

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indicates that more cloud droplets dissolved larger absolute amounts of PHg and MeHg, but not

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DHg. It is most likely a result of high atmospheric particulate-bound mercury (PBM)

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concentrations (83 pg m-3 on average we measured at Mt. Tai, unpublished data) and fractions

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(~3.7% of total atmospheric Hg) produced via gas-to-particle partitioning due to the combined

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effect of severe particulate air pollution and intense Hg emissions.16 Moreover, high

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concentrations (508.5 pg m-3) and content (6.6 µg g-1) of PBM in PM2.5 were measured in the

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adjacent Jinan city,49 ~50 kilometers north of Mt. Tai. Thus, cloud scavenging of PBM should be

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an important source of PHg in cloudwater leading to the positive correlation between Nd and

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PHg. Figure 2c displays a strong correlation between PHg and conductivity, further indicating

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substantial, direct contributions of air pollution to PHg in cloudwater.

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The sources and transformation of DHg are very complex and many uncertain factors (e.g.,

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gas-liquid partitioning, Hg complexation and photoredox)2-3, 50 can control DHg in cloudwater.

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The correlations of DHg with PHg (r = 0.68, p < 0.001) and conductivity (r = 0.55, p < 0.001,

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Figure 2c) suggested common factors controlling DHg and PHg, and PBM is one of those. The

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poor correlation between DHg and Nd (Figure 2a) suggested that atmospheric particles unlikely


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contributed to DHg directly, but PHg photolysis could be one pathway for DHg formation.51

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Adsorbing gaseous elemental and/or oxidized Hg into cloud droplets was probably another

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source of DHg, as implied by higher correlation coefficients of DHg than PHg (Table S4) with

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secondary ions (e.g., sulfate, nitrate) and DOC, which were mainly formed from their gas

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precursors.

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For MeHg, its good correlation with Nd (Figure 2b) could not sufficiently demonstrate its

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origin from aerosol particles, because MeHg was poorly correlated with conductivity (Figure

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2d), PHg and DHg as well as other chemical components in cloudwater (Table S4), which

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indicates different sources of MeHg rather than direct air pollution contribution. Abiotic Hg

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methylation was speculated to be the most possible source of MeHg in cloudwater as discussed

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in the next section.

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3.4. Potential Sources of MeHg

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The presence of MeHg in cloudwater suggested that there definitely existed MeHg sources

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compensating for its (photo)degradation52-54. Identifying specific MeHg source is difficult at

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present. As discussed above, MeHg in cloudwater did not seem to be contributed directly from

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dissolution of atmospheric gaseous and particulate pollutants. Then MeHg could come from

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marine environments, biotic or abiotic methylation of inorganic Hg.

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The PSCF analysis indicates that the middle and southwest China were the regions

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contributing the most and the Yellow Sea the least to cloudwater MeHg at Mt. Tai (Figure S7).

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Considering the distance of Mt. Tai over 200 km inland, it was highly unlikely that cloudwater

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MeHg at Mt. Tai had marine influence, which was also indicated by the poor correlations of

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MeHg with Cl-, Na+ and mesylate (Table S4).


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MeHg in aquatic ecosystems is predominantly produced by anaerobic Hg methylating bacteria

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such as sulfate-reducing bacteria, iron-reducing bacteria and methanogens.30-31, 55 These bacteria

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commonly inhabit in anaerobic environments and possess two essential methylation gens, hgcA

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and hgcB.32, 56 In cloudwater at Mt. Tai, diverse pathogens and functional bacteria had been

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identified.36, 57 However, they were almost all aerobic bacteria rather than anaerobe because the

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distinctively high oxic envrionment of cloud droplets was unsuitable for anaerobic bacteria to

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survive. So the biotic methylation of inorgnic Hg to MeHg in cloudwater was likely to be

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insignificant.

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Figure 3. Mass ratios of MeHg to DHg in cloudwater versus ionic strength with corresponding

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Nd. Elevated MeHg formation generally occurred at lower ionic strength with higher Nd. The

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colors indicate MeHg concentrations and the circle sizes represent cloud droplet numbers.

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Although MeHg was independent of inorganic Hg concentrations, the increased mass ratios of

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MeHg to DHg along with decreased ionic strength (Figure 3) could be an indicator for abiotic


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formation of MeHg from DHg in cloudwater. Despite the not very high overall ionic strength, it

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seemed that both MeHg/DHg mass ratios and absolute MeHg concentrations were elevated when

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ionic strength lowered, indicating inhibited Hg methylation by stronger ionic strength. Since Hg

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complexation with organic compounds (methyl donors, e.g., acetate,10 methylcobalamin,58 low-

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molecular-weight organics11,

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methylation, the decreased DHg activity and growing competition of inorganic ligands with

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organics for DHg, which were likely caused by increased ionic strength, might have reduced the

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availability of DHg complexation with methyl donors, thus leading to the decrease in MeHg

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formation. In this study, MeHg exhibited no correlations with DOC and acetate. Instead, it was

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correlated significantly to propionate (Table S4), indicating formation of MeHg via Hg

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alkylation by propionic acid as proposed by Yin et al.33 Some volatile organic compounds

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(VOCs) that potentially provide the methyl groups were detected concurrently in in-cloud air

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(e.g., carbonyls, aromatics, chloromethane and bromomethane)59 and in cloudwater (e.g.,

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iodomethane) at Mt. Tai. The wide existence of potential methyl donors considerably supported

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the feasibility of abiotic Hg methylation to MeHg in cloudwater, even though the specific methyl

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donor has not been ascertained at present. Elevated MeHg concentrations were also found to be

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accompanied by higher Nd (Figure 3). We hypothesized that more cloud droplets could enhance

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the uptake of gaseous VOCs potentially providing more methyl donors, and furthermore, the

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presence of cloud residue particles may acting as catalyst for Hg reactions6 favored MeHg

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formation as indicated by their linear correlation (r = 0.46, p = 0.004). Future work is needed to

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identify the methyl donors and methylation mechanisms.

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and methyl iodine13) plays an essential role in abiotic Hg

3.5. Hg Species Variation by Cloud Processing


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Trace elements properties such as solubility, speciation, reactivity and micromorphology can

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be modified by cloud processing. 38, 41, 47 Figure 4 presents the temporal variations of Hg species

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and Nd during a continuous cloud event (1-3 August 2015). Clearly, concentrations of PHg and

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MeHg varied in phase with Nd. MeHg even rose to peak values as Nd reached up to near or more

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than 2000 cm-3, while DHg appeared to be independent of Nd. Slightly higher averaged daytime

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DHg and PHg corresponded to higher daytime Nd. These indicate more contributions of PBM to

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inorganic Hg than Hg2+ photoreduction to Hg0 followed by outgassing from cloud droplets.2, 8

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Note that averaged daytime MeHg concentration (0.071 pg m-3) was somewhat lower than

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nighttime one (0.097 pg m-3) despite higher Nd due maybe to MeHg photodegradation, which

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was found to be facilitated mainly by complexation with DOM depending on ligands types.2, 52-54

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In view of the abundant DOM in cloudwater, MeHg photodegradation in cloud droplets might

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have happened during daytime leading to lower daytime MeHg.

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Figure 4. Temporal variation of air equivalent concentrations of Hg species during the cloud

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event on 1-3 August 2015. The daytime and nighttime concentrations of Nd and Hg species

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(mean ± sd) are listed. The 30 min averaged Nd and hourly PM2.5 are displayed. Pies charts show


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Hg speciation distributions for selected cloudwater samples. The Nd could be a rough indicator

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of aerosol particles incorporated into clouds.

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During cloud event (Figure 4), most (~70%) of PM2.5 was scavenged within the first few hours

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followed by a constant lower concentration. DHg showed higher concentrations than PHg at the

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initial stage of cloud event, whereas PHg rapidly surpassed DHg as cloud processes went on.

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Much dissolution of PBM in cloudwater should be responsible for the more DHg in the first two

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cloudwater samples. Moreover, Hg speciation was markedly changed by cloud processing in

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different cloud periods: 52.3% of THg were the dissolved Hg-DOM(aq) complexes in the early

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stage; PHg overtook Hg-DOM(aq) and became the dominant species (more than 80% of THg) in

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the middle stage; in the dissipating stage, PHg still dominated Hg speciation, followed by

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increased Hg-DOM(aq). The changed Hg speciation, determined mainly by pH with more Hg-

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DOM(aq) complexes formed in more acid cloudwater, should play a critical role in Hg

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dissolution during cloud processing.

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Simple theoretical calculations were conducted to assess the relative importance of

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contributions from possible sources to Hg in cloudwater. Assuming that gas-liquid partitioning

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of Hg0 reached an equilibrium, 1.48 pg m-3 of cloudwater Hg (corresponding to 6.45 ng L-1 at

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mean LWC of 0.23 g m-3) was obtained using Henry’s law constant for Hg0 (1.3×10-3 mol m-3

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Pa-1)60 and the averaged TGM concentration that we measured at Mt. Tai (2.18 ng m-3, Hg0 can

312

approximately equal to TGM as Hg0 accounts for > 99% of TGM at remote sites in China16). It

313

was evidently far less than the mean concentration of THg (13.9 pg m-3) and even DHg (4.7 pg

314

m-3) in cloudwater. This clearly suggests more important sources for Hg in cloudwater other than

315

gaseous Hg0. If the measured 83 pg m-3 of PBM during the campaign had the same cloud

316

scavenging efficiency as PM2.5, i.e. ~70%, ~58.1 pg m-3 of PBM should have entered cloudwater.


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317

The amount was over 4 times more than THg in cloudwater. Even if cloud scavenging efficiency

318

for PBM was lowered to 20% (the scavenging ratio for Fe),38 PBM could still have contributed

319

16.6 pg m-3 of THg. These results again allude to the potential importance of PBM contributions

320

to cloudwater Hg.

321

3.6. Hg Adsorption on Cloud Residues

322

Reversible adsorption of DHg on soot in cloudwater significantly influences Hg species

323

transformation, which has been incorporated into atmospheric mercury modeling.2, 6, 14 Cloud

324

residues content was measured to be 0.7 – 290 mg L-1 (mean 55 mg L-1) and moderately

325

correlated to Nd (r = 0.51, p < 0.01). Adsorption coefficients Kad (5.9 – 362.7 L g-1) were highly

326

dependent on cloud residues content, rather than Nd, with an inversely power-law relationship

327

(Figure 5a). Solids effect,61-62 which results from the competition of solute binding to organic

328

matter between aqueous and solid phase, could explain the negative effects of cloud residues

329

content on Kad. More cloud residues in cloudwater could likely increase particulate aggregation

330

and decrease available binding sites for organic matter to complex Hg. THg concentrations,

331

which were slightly correlated to cloud residues, seemed to have very limited influence on Hg

332

adsorption, indicating that Hg adsorption should be adsorbent rather than adsorbate controlled. In

333

addition, Kad was elevated as cloudwater pH increased from 5.01 to 6.95, attributed partially to

334

the reduced Hg-DOM complexation in aqueous phase caused by the higher pH.

335

During field sampling, we found three types of filtered cloud residues that differed greatly in

336

color as represented by sample I, II and III (Figure 5a). The dark sample I, grey sample II and

337

light brown sample III had a high (67.0 mg L-1), moderate (42.5 mg L-1) and low (29.7 mg L-1)

338

concentration of cloud residues, respectively. More importantly, notably higher Kad was

339

calculated in sample III (165.4 L g-1) than in the other two samples (14.8 and 8.1 L g-1). To


17


Page 18 of 32

340

understand what may have caused different Kad values in the three cloudwater samples other than

341

solids effect, their physicochemical characterization was investigated.

342

343 344

Figure 5. (a) Dependence of Hg adsorption coefficient Kad on cloud residues content. Photos of

345

typical cloud residues filters of three cloudwater samples I, II and III are displayed. (b) Water-

346

soluble ions composition, and (c) SEM images and corresponding EDS spectra of cloud residues

347

for the three cloudwater samples. Only elements over 0.1% of total mass can be determined by

348

EDS.

349


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350

Water-soluble ions composition in the three cloudwater samples was compared in Figure 5b.

351

Sample I had much more secondary ions than samples II and III. Moreover, the ions composition

352

in sample III differed from the other two with NH4+ and Ca2+ accounting for 47% and 18% of

353

total ions, respectively, indicating soil dust being a significant component of the cloudwater

354

sample III. However, there were comparable amounts of anthropogenic trace elements such as

355

Zn, Pb and Se in samples III and I (Figure S8, unpublished data). The samples composition

356

suggested that the cloudwater had contributions from soil dust and industrial sources alike.

357

Surface morphology and elemental composition of particles in cloud residues were

358

characterized using SEM-EDS (Figure 5c). Overall, cloud residues in the three samples were

359

comprised of highly mixed particles whose sizes ranged from nanometers to about ten

360

micrometers. These particles were primarily identified as irregularly shaped and lamellate

361

mineral materials, spherical and ellipsoidal fly ash, aggregates of nanoparticles (chain-like soot

362

particles), porous substances, etc. Cloud residues in samples I and III were similar and consisted

363

of more small fly ash and aggregates of finer particles, while particles in sample II were coarser

364

and more diverse. The predominant elements in cloud residues determined by EDS were C, O,

365

Si, Al, Fe and Ca with less Mg, K, Na and Mn, indicating mixtures of abundant carbonaceous

366

and crustal materials. Carbon content in our results was the highest (36.2 – 48.0%), fairly

367

consistent with Li et al.’s study in which elemental carbon (EC) particles accounted for 49.3% of

368

cloud residues,63 indicative of large contributions from abundant EC in northern China.64 The

369

abundant refractory Si, Al and Fe in our cloud residues could come from dust materials (e.g.,

370

silicate, hematite) and fly ash (e.g., SiO2, Al2O3). Figure 5c shows more spherical particles and

371

about 2–3 times higher Si and Al content in sample III than I and II, indicating more fly ash

372

particles in sample III. Fly ash can substantially capture Hg in flue gas65-66 and in aqueous


19


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373

solutions,67 due to its stronger physisorption resulting from porous structure, smaller diameter

374

and higher specific surface area. Hence, a much higher Kad value was obtained for Hg in sample

375

III. Indeed, backward trajectories analyses (Figure S9) corroborated that air masses of sample III

376

could be greatly impacted by the industrial flue gases/dusts containing abundant fly ash particles

377

emitted from two huge iron and steel plants (Laiwu and Rizhao).

378

Elemental maps of single particles in cloud residues (Figure 6) illustrate the spatial

379

distributions of major elements (C and O, Si, Al, K) on the surface of individual crystal mineral

380

and fly ash particles. The crustal mineral particle surface exhibited high intensity (abundance) of

381

Si, Al, O and K while low intensity of C (Figure 6a). In contrast, the fly ash particle surface

382

(Figure 6b) showed higher intensity of C than the other elements. It should be noted that some

383

small minerals attached to the fly ash surface displaying stronger intensity of Si, Al and O than

384

that of C (Figure 6b). Unburnt carbon in fly ash plays significant roles in Hg oxidation and

385

surface adsorption,65 probably via the formation of C-M bond to facilitate the catalytic oxidation

386

of elemental Hg68 and via Hg chemisorption by surface carbon-oxygen functional groups.66

387

Therefore, the disparities of carbon distribution between fly ash and mineral particles shown in

388

Figure 6 indicate that carbonaceous matters (e.g., humic acid) are very prone to be associated

389

with fly ash particles rather than mineral particles, and thus provide more carbon adsorption

390

points on fly ash to enhance greatly the Hg chemisorption.

391


20

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392

Figure 6. Elemental mapping images of individual (a) crystal minerals and (b) ellipsoidal fly ash

393

particles in cloud residues. Ch0 represents the analyzed microdomain. The density of colored

394

dots in each image indicates the intensity (abundance) of analyzed element. The C distributions

395

on two particles are distinctly inconsistent. The sizes of both particles are about 1 µm.

396

These results highlighted the significant importance of fly ash particles for facilitating Hg

397

adsorption on cloud residues in cloudwater. The adsorption coefficient Kad was not only affected

398

by the quantity of cloud residues, but also depended on the nature of residue particles (e.g.,

399

surface properties, elemental composition). Given the complexity of cloud residues, other factors

400

such as halogen content, unburnt carbon type and association of natural organic matter (NOM)

401

with iron oxyhydroxides may also affect Hg adsorption,65-66, 69-70 which warrant further study.

402

3.7. Summary and Environmental Perspective

403

The results provide valuable information on cloudwater Hg chemistry in severely air polluted

404

northern China, including Hg concentration, chemical speciation and potential sources, etc. We

405

find highly polluted Hg species in cloudwater, dominated by PHg and Hg-DOM complexes. PHg

406

should be mainly contributed from ambient aerosols while MeHg may be formed via abiotic Hg

407

methylation. Cloud processing likely plays an important role in Hg transport and transformation

408

by changing Hg concentration and speciation.

409

Our findings of Hg adsorption on cloud residues are critical to improving the understanding of

410

atmospheric Hg behaviors in clouds. High concentrations of aerosols in northern China could

411

generate more, smaller cloud droplets35,

412

adsorption on cloud residues, especially the fly ash particles with more binding sites. The

413

measured Kad for cloud residues (5.9 – 362.7 L g-1, mean of 69.8 L g-1) is significantly higher

414

than that obtained with NIST atmospheric particulate matter (3 – 91 L g-1)6 and the value used in

71

and contribute greatly to PHg in cloudwater via


21


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415

transport model (45 L g-1).72 The stronger Hg adsorption on realistic cloud residues could inhibit

416

aqueous reduction of Hg(II) and subsequent volatilization of Hg0, leading to enhanced Hg wet

417

deposition2 and higher ecological risks than estimation. Since photodissolution of PHg has been

418

observed in rainwater,51 the abundant PHg in cloudwater could contribute largely to DHg

419

accompanied by DOM complexation. Once PHg dissolves in cloudwater to be reactive Hg,

420

heterogeneous reactions (e.g., photoredox, methylation-demethylation) would take place. In the

421

context of climate change induced by aerosol-cloud interactions,71,

422

microstructure and lifetime induced by particulate air pollution are expected to greatly impact

423

atmospheric Hg cycling and fate.

424

ASSOCIATED CONTENT

425

Supporting Information

426

Supporting Information Available: Text S1–2, Figure S1–9, Table S1–4 and 14 References

427

(PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

428

AUTHOR INFORMATION

429

Corresponding Author

430

*Phone: +086-531-88361157; e-mail: [email protected]

431

Notes

432

The authors declare no competing financial interest.

433

ACKNOWLEDGEMENTS

73

alterations of cloud

434

We appreciate Dr. Jeffrey L. Collett Jr. and Tao Wang for providing the cloudwater sampling

435

method. We also thank Dr. Jianmin Chen for supplying important data and Chen Wu, Yaxin Li,


22

Page 23 of 32


436

Chao Zhu, Xianmang Xu, Jiarong Li and Fengchun Yang for helping the field experiments. This

437

work was financially supported by the National Natural Science Foundation of China

438

(41475115), Ministry of Science and Technology of China (2016YFE0112200) and H2020

439

Marie Skłodowska-Curie Actions (690958-MARSU-RISE-2015).

440

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Insights on Chemistry of Mercury Species in Clouds over Northern China

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