Article pubs.acs.org/est
Microscopic Evaluation of Trace Metals in Cloud Droplets in an Acid Precipitation Region Weijun Li,*,†,& Yan Wang,*,‡ Jeffrey L. Collett, Jr.,§ Jianmin Chen,†,‡ Xiaoye Zhang,£ Zifa Wang,∥ and Wenxing Wang† †
Environment Research Institute, Shandong University, Jinan, Shandong 250100, China School of Environment Science and Engineering, Shandong University, Jinan, Shandong 250100, China § Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado 80523, United States £ Centre for Atmospheric Watch and Services, Chinese Academy of Meteorological Sciences, China Meteorological Administration, Beijing, China ∥ State Key of Laboratory of Atmospheric Boundary Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China & State Key Laboratory of Coal Resources and Safe Mining, China University of Mining & Technology, Beijing 250100, China ‡
S Supporting Information *
ABSTRACT: Mass concentrations of soluble trace metals and size, number, and mixing properties of nanometal particles in clouds determine their toxicity to ecosystems. Cloud water was found to be acidic, with a pH of 3.52, at Mt. Lu (elevation 1,165 m) in an acid precipitation region in South China. A combination of Inductively Coupled Plasma Mass Spectrometry (ICPMS) and Transmission Electron Microscopy (TEM) for the first time demonstrates that the soluble metal concentrations and solid metal particle number are surprisingly high in acid clouds at Mt. Lu, where daily concentrations of SO2, NO2, and PM10 are 18 μg m−3, 7 μg m−3, and 22 μg m−3. The soluble metals in cloudwater with the highest concentrations were zinc (Zn, 200 μg L−1), iron (Fe, 88 μg L−1), and lead (Pb, 77 μg L−1). TEM reveals that 76% of cloud residues include metal particles that range from 50 nm to 1 μm diameter with a median diameter of 250 nm. Four major metal-associated particle types are Pb-rich (35%), fly ash (27%), Fe-rich (23%), and Zn-rich (15%). Elemental mapping shows that minor soluble metals are distributed within sulfates of cloud residues. Emissions of fine metal particles from large, nonferrous industries and coal-fired power plants with tall stacks were transported upward to this high elevation. Our results suggest that the abundant trace metals in clouds aggravate the impacts of acid clouds or associated precipitation on the ecosystem and human health.
1. INTRODUCTION
Since toxic trace metals, especially those that bioaccumulate in the environment, cause adverse impacts on ecosystems and human health, increased attention has been paid to characterize them in precipitation and clouds/fogs in recent years.2,5−9 Information about the concentrations and oxidation states of bulk soluble trace metals are also essential for improving our understanding of their roles in atmospheric chemistry. Metal solubility depends on factors that control pH, which also influences metal-catalyzed sulfur oxidation in the cloud droplets. In addition, insoluble metal particles occur as nanoparticles in cloud droplets.10−12 Recent studies show not only that plants exhibit a hydrophilic pathway for soluble
Cloud/fog processing and precipitation are important removal mechanisms for aerosol particles and anthropogenic acidic gases. Clouds formed in moderate to heavily polluted areas can damage ecosystems.1 Questions have been raised about the chemical composition of clouds with reference to their potential role in depositing chemicals such as nutrients, acids, and trace metals to ecosystems.2 Once trace metals are transported into various terrestrial and aquatic ecosystems through wet or dry deposition, they can cause deleterious health effects when the products from these ecosystems are consumed by humans. De Temmerman et al.3 showed that As, Cd, and Pb concentrations in the leaves and the major stem correlated well with atmospheric metal deposition. Hu et al.4 indicated that airborne Pb is the most important source for the Pb accumulation in leaves of a wild plant (Aster subulatus). © 2013 American Chemical Society
Received: Revised: Accepted: Published: 4172
November 24, 2012 April 12, 2013 April 15, 2013 April 15, 2013 dx.doi.org/10.1021/es304779t | Environ. Sci. Technol. 2013, 47, 4172−4180
Environmental Science & Technology
Article
Figure 1. Sites of Mt. Lu and Mt. Tai and acid precipitation zone of China covering Zhejiang, Jiangxi, Fujian, Hunan, most parts of Chongqing, part of the Yangtze River delta (YRD), Pearl River delta (PRD), west of Hubei, southeast of Sichuan, north of Guangxi. (The data of acid precipitation area were obtained from the Annual Environment Report of China in 2011.33)
averaged growth rates at ∼5% from 1980 to 2007 of national total atmospheric metal emissions from coal combustion. Nonferrous smelting in Gansu and Jiangxi provinces ranks as the leading source of these emissions in China.25 Jiangxi province lies in the Asian humid continental and tropical monsoon climate zone, so cloud/fog and rain events are frequent from spring through autumn. These clouds/fogs contact the plants at elevated mountain areas for long periods. Once clouds form rain, pollutants in clouds also reach the ground ecosystem through washout. Therefore, this province was chosen to investigate insoluble and soluble toxic metals in cloud droplets. The first objective of the present study was to evaluate the pH of cloudwater collected in different cloud events. Second, the work aimed to determine the soluble trace metal composition with Inductively Coupled Plasma Mass Spectrometry (ICPMS) and to investigate metal concentrations in cloudwater. In addition, transmission electron microscopy with energy-dispersive X-ray spectrometry (TEM/EDS) was used to determine the size, mixing state, and elemental composition of individual metal particles in cloud drop residues and interstitial aerosol particles. Scanning TEM (STEM) was applied to obtain metal mapping in individual cloud drop residues. To our knowledge, this is the first study to fully characterize trace metals occurring in cloud droplets using a combination of bulk and individual particle techniques.
materials through leaves but also that a solid-state pathway for nanoparticles exists.13,14 Size, morphology, composition, and mixing state of metal particles in cloud droplets influence uptake rates by the leaves of the plants. Recently, several field studies have investigated compositions of individual cloud/fog residues collected in cloud/fog episodes in the world. Chemical speciation of individual residual particles from cloud droplets and interstitial aerosol particles collected during a marine stratus experiment was performed using a combination of complementary microanalysis techniques including time-of-flight secondary ionization mass spectrometry (TOF-SIMS), scanning transmission X-ray microscopy with near edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS), and scanning electron microscopy (SEM).15 Size, shape, and compositions of dry insoluble particles extracted from fogwater at Mt. Milesovka, Czech Repulic, were observed by SEM.16 Cloud interstitial particles were successfully collected using an impactor at Mt. Tateyama, Japan, and the mixing state of insoluble and soluble aerosol particles was characterized using transmission electron microscopy (TEM). 17 The replication technique using collodion film was used to collect individual fog droplets and microparticle-induced X-ray emission were used to obtain their compositions.18 The counter-flow virtual impactor was used to collect individual cloud droplets on nuclepore filters or copper grids, and their residual particles were analyzed by TEM and SEM.10,19,20 These studies investigated compositions of individual cloud droplets and interstitial aerosol particles in cloud/fog episodes over continental, coastal, and marine areas. The major chemical components of cloud/fog residues were sulfates, nitrates, organics, and element carbon and some metal particles in polluted areas. Mancinelli et al.21 pointed out that zinc (Zn), iron (Fe), and aluminum (Al) were the main dissolved trace metals in fog droplets in the Po Valley during two polluted fog events. Chérif et al.22 also discussed solubility of metals in fog might be associated with the sizes of fog droplets in urban Strasbourg, East of France. In addition, Ebert and Baechmann23 observed a relative solubility of lead (Pb) higher than 45% for all drop sizes. Few studies of trace metals in cloudwater have been performed in China,5 however, although many studies have evaluated metals in aerosol particles.24−26 Rapidly expanding industries significantly elevate the number and mass concentrations of anthropogenic trace metal particles in the atmosphere in East China. Tian et al.27 suggested annually
2. EXPERIMENTAL SECTION 2.1. Sampling Site. Mt. Lu, covering an area of 300 km2 (115°59′E, 29°35′N, 1,165 m), is surrounded by Jiujiang city and is located in northern Jiangxi Province, China, between the Yangtze River and Boyang Lake (Figures 1 and S1). Mt. Lu resides in the Asia humid continental and tropical monsoon climate zone, so cloud/fog and rain events are common from spring to autumn. About 200 cloud/fog days and 100 high-wind days (average wind speed of 4.5 m/s and maximum wind speed of 29.3 m/s averaged for 10 min) annually occur at Mt. Lu.28 Mt. Lu is referred to as “fog mountain” by local residents. Although there is no large agricultural field on top of Mt. Lu, the Chinese cloud-fog tea and over 500 medicinal plants naturally grow in the area. These tea and medicinal plants are used in China and exported to overseas countries. The town of Guling on top of Mt. Lu has a population of about 10,000; most residents are working in tourism or related services, so relatively little local pollution is produced. 4173
dx.doi.org/10.1021/es304779t | Environ. Sci. Technol. 2013, 47, 4172−4180
Environmental Science & Technology
Article
Figure 2. TEM images showing dry cloud droplet residues (a-b) and interstitial aerosols (c-d) collected in acid clouds.
and cloud drops are distributed throughout the grid. If the carbon film and aerosol distribution on the TEM grid were suitable for TEM analysis, the grid was placed in a sealed, dry plastic tube and stored in a desiccators at 25 °C and 20 ± 3% RH to minimize exposure to ambient air and preserve it for analysis. In cases where the film on the TEM grids was totally destroyed or the collected aerosol particles overlapped each other on the TEM grids, the samples were discarded. Figure 2 depicts example images of cloud drop residues and interstitial aerosol particles. 2.4. Sample Analysis. Cloud water samples were weighed after collection, and their pH was measured immediately before the sample was filtered. The pH was measured using a pH meter and combination electrode (Mettler Toledo Delta 320, precision ±0.01 pH). The samples were then filtered through a 0.45 μm cellulose acetate filter to remove suspended matter. Nitric acid (superpure 1% v/v) was added to the samples for trace element determination to adjust the pH to less than 2.0, in order to prevent any adsorption of trace elements to container walls. All collected samples were stored at 4 °C until laboratory analysis. Trace elements including Fe, Zn, manganese (Mn), Pb, chromium (Cr), arsenic (As), titanium (Ti), cobalt (Co), barium (Ba), nickel (Ni), antimony (Sb), and vanadium (V) were determined by Inductively Coupled Plasma Mass Spectrometry (ICPMS, Agilent 7500a). Sampling glass ware and plastic ware were stored in 2% nitric acid for at least 24 and then washed at least three times with deionized water. They were then dried corked and packed in two clean plastic bags and zipped until used in field sampling (EMEP/ CCC-Report 1/95, Norwegian Institute for Air Research, 2011).30 Field blanks and quality control standard were taken during the sample collection and ICPMS analysis. Differences between measured and quality control-standard concentrations were less than 5%. Internal standards of Sc, Ge, In, and Bi from the China National Standard Research Center were used to correct instrumental drift or matrix effects, according to US EPA Method 200.8.31 The same procedure and more detailed
Important nonferrous mines and coal-fired power plants are in Jiujiang city and other nearby cities. There are some large industries associated with nonferrous mining, smelting, and refining of pure metals. During the sampling periods, most air masses arrived from the south, southeast, or northeast direction and did not cross Guling town (Figure S1). 2.2. Sampling Approach. Sampling instruments were deployed at the Mt. Lu meteorological station which was free from disturbance by tourists and also free from local obstacles such as buildings or trees. The sampling period from August 27 to September 10, 2011,was chosen to investigate cloud chemistry in summer. In five major cloud events, bulk cloudwater samples were collected by a Caltech Active Strand Cloud water Collector (CASCC2).29 Droplets are collected on 6 rows of 508 μm diameter Teflon strands. The theoretical lower 50% size cut for the sampler corresponds to a cloud drop diameter of 3.5 μm. Collected droplets coalesce and run down the strands into a Teflon collection trough and then are routed by a Teflon tube to a 500 mL high-density polyethylene bottle. The sampler was thoroughly cleaned with deionized water at the beginning of the study and after each cloud event. In the field study, we successfully collected 11 cloudwater samples from August 27-September 9, 2011 (Table S1). Daily concentrations of NO2, SO2, and PM10 (mass of particles with aerodynamic diameter ≤10 μm) during the sampling period were obtained from an on-site environmental monitoring station (Table S2). One individual-particlesampling-system was built to collect cloud droplets and interstitial aerosols (unactivated aerosols in a certain supersaturation in the cloud) onto copper TEM grids coated with carbon film (carbon type-B, 300-mesh copper, Tianld Co., China) using a single-stage cascade impactor with a 0.5-mmdiameter jet nozzle and a flow rate of 0.5 l min−1.The low flow rate can reduce cloud droplet shattering on impact. More detailed information about this sampler can be found in Li et al.11 After each collection we used optical microscopy with magnification from ×500 to ×1200 to check how the aerosols 4174
dx.doi.org/10.1021/es304779t | Environ. Sci. Technol. 2013, 47, 4172−4180
Environmental Science & Technology
Article
Figure 3. Morphology and composition of individual cloud drop residues internally mixed with metal particles. EDS analyses typically examine compositions of metal particles. (a) S-rich cloud residue including two Fe-rich and one soot particle. (b) S-rich cloud residue including one Fe−Mn and one soot particle. (c) S-rich cloud residue including Zn−S, fly ash, and organic particles. (d) Ammonium sulfate ((NH4)2SO4) cloud drop residue containing one zinc iron oxide (ZnFe2O4) particle. (e) S-rich cloud drop residue including mineral and three Fe−Zn particles. (f) ((NH4)0.3K0.7)2SO4 cloud drop residue containing several metal particles. (g) S-rich cloud drop residue including Pb−S, Zn−Pb, and mineral particles. (h) S-rich particle containing anglesite (PbSO4) and fly ash particles. SAED and EDS data together confirmed some particle phases. These particles were collected from different acid cloud events at Mt. Lu. Elements in square brackets are dominant. C and O occur in all particles and are not shown in the figure.
Table 1. Average pH and Soluble Trace Metal and Ionic Concentrations in Clouds and Fogs Collected at Different Locations (Element Unit, μg L−1) date Mt. Lu, China Mt. Tai, Chinaa Mt. Brocken, Germanyb Mt. Elden, USAc Po Valley, Italyd Tuscan Appennines, Italye Changlagali, Pakistanf
V
Cr
Mn
Fe
Ni
Cu
Zn
As
Se
Cd
Ba
Pb
reference
3.52 3.95
pH
9.18 -
19.56 0.93
18.63 42.84
88.48 105.83
8.50 9.26
11.27 9.24
200.36 249.05
19.70 13.69
9.48 -
1.60 3.08
19.08 -
77.25 30.01
4.3
1.8
