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Uncertainties of Gaseous Oxidized Mercury Measurements Using KCl-coated Denuders, Cation-Exchange Membranes, and Nylon Membranes: Humidity Influences Jiaoyan Huang, and Mae Sexauer Gustin Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b00098 • Publication Date (Web): 16 Apr 2015 Downloaded from http://pubs.acs.org on April 20, 2015
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Uncertainties of Gaseous Oxidized Mercury Measurements Using KCl-coated Denuders, Cation-Exchange Membranes, and Nylon Membranes: Humidity Influences
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Jiaoyan Huang *1, and Mae Sexauer Gustin*2
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Department of Natural Resources and Environmental Sciences, University of Nevada, Reno,1664 N. Virginia Street, Reno, NV, USA, 89557
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Jiaoyan Huang, +1-775-784-4722,
[email protected] Mae S. Gustin, +1-775-784-4203,
[email protected] ACS Paragon Plus Environment
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Abstract
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Quantifying the concentration of gaseous oxidized mercury (GOM) and identifying the chemical compounds in the atmosphere are important for developing accurate local, regional, and global biogeochemical cycles. The major hypothesis driving this work was that relative humidity affects collection of GOM on KCl-coated denuders and nylon membranes, both currently being applied to measure GOM. Using a laboratory manifold system and ambient air, GOM capture efficiency on 3 different collection surfaces, including KCl-coated denuders, nylon membranes, and cationexchange membranes, was investigated at relative humidity ranging from 25 to 75%. Recovery of permeated HgBr2 on KCl-coated denuders declined by 4-60 % during spikes of relative humidity (25 to 75%). When spikes were turned off GOM recoveries returned to 60±19% of permeated levels. In some cases, KCl-coated denuders were gradually passivated over time after additional humidity was applied. In this study, GOM recovery on nylon membranes decreased with high humidity and ozone concentrations. However, additional humidity enhanced GOM recovery on cation-exchange membranes. In addition, reduction and oxidation of elemental mercury during experiments was observed. The findings in this study can help to explain field observations in previous studies.
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Keywords:HgBr2, manifold experiment, ozone, water vapor
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Introduction
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Understanding the global mercury (Hg) cycle is important due to the adverse health effects for
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humans and wildlife [1]. In the atmosphere, Hg is measured as 3 operational defined forms
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gaseous elemental Hg (GEM), gaseous oxidized Hg (GOM), and particulate-bound Hg (PBM)
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[2-4]. Measurement of GEM is reliable; however, uncertainties associated with GOM
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measurements are high [5, 6]. GOM is the major Hg form involved in deposition processes that
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transfer Hg from the atmosphere to ecosystems [4]. The chemical composition of GOM is not
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understood, and varies with location and season [6-9]. Field investigations have identified
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potential GOM compounds including HgCl2, HgBr2, Hg(NO3)2, HgSO4, and HgO based on
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comparison with thermal desorption profiles of Hg evolved from high purity permeation tubes
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[7, 10]; however, the exact chemistry of the GOM compounds volatilized from these permeation
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tubes has not been identified [7, 9].
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Methods for measurement of atmospheric GOM include KCl-coated denuders (manual and
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automatic system), mist chambers, the University of Nevada-Reno (UNR) active system with
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nylon and cation-exchange membranes, membrane filter packs including Teflon, quartz, cation-
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exchange membranes, and a differential method that quantifies total gaseous Hg (TGM) and
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GEM (the Detector for Oxidized Hg system (DOGHS)), [3, 6, 7, 11-15]. After field
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intercomparisons [3, 12], the mist chamber was discounted due to a potential positive artifact
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related to PBM, and the membrane pack measurement is influenced by upstream filters [6, 9].
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KCl-coated annular denuders within the Tekran® automatic system have been the most popular
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method for ambient GOM measurements, and is applied in the North America –Atmospheric
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Mercury Network (AMNet), and the Global Mercury Observation System (GMOS) [16, 17].
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However, concerns regarding the use of the Terkan® system for GOM and PBM measurements
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have risen since 2009. In summary, a significant mass balance discrepancy between GOM and
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GEM was observed during daytime when atmospheric oxidation would occur in Nevada [18] led
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to uncertainties regarding the denuder; and a mismatch of GOM temporal trends using Tekran®
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and passive samplers in Florida was observed [19], again indicating potential uncertainties.
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Swartzendruber et al. [20] also pointed to disagreement of GOM measurements in the free
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troposphere using the KCl-coated denuders and a precursor to the current DOHGS instrument
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that determines the TGM and GEM, and then calculates reactive Hg (RM=GOM + PBM) by
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difference. The effect of ozone on GOM collection by the KCl-coated denuder was investigated
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by Lyman et al. [21]. Results indicated GOM loss from denuders loaded with HgCl2 and ambient
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air GOM collected in Nevada during ozone exposures. The UNR active system was compared
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with Tekran® system using a laboratory manifold with different potential GOM compounds and
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concentrations [7]; and GOM concentrations measured by cation-exchange membranes were 1.6-
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12 times higher than the numbers measured by Tekran® system. The Reno Atmospheric
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Mercury Intercomparison eXperiment (RAMIX) that was a field project using ambient air also
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pointed to low GOM recovery at known HgBr2 permeation rates [6].
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Recently, McClure et al. [22] concluded that KCl-coated denuder within the Tekran® system
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showed low capture efficiency of GOM under conditions of high humidity (6.6-15.7 g kg-1) and
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ozone (20-78 ppb) concentrations. The HgBr2 collection efficiency of KCl-coated denuder with
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Tekran® system was 95% in zero air; however, the Tekran® inlet was heated to 100ºC. Using
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AMNet protocols, the temperature of the inlet is kept at 50ºC [3], and this lower temperature
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explains the lower recovery (by ~30%) of HgCl2 on KCl-coated denuder due to the wall loss on
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the elutricator [23], the inlet temperature explains the different Tekran® performance between
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McClure et al. [22] and Huang et al. [7].
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Based on the results of RAMIX, it appeared that the KCl-coated denuder became passivated with
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time and was significantly influenced by relative humidity (RH). In addition, data collected in
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the marine boundary layers of California and in Florida [7, 10], RH was suggested to affect the
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capture of GOM on the nylon membrane. Thus, this work investigated the hypothesis that the
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KCl-coated denuder and nylon membrane capture of GOM is impacted by RH. We also wanted
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to determine whether the cation-exchange membrane was affected. Therefore, in this study the
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cation-exchange membrane, nylon membrane, and KCl-coated denuder within the Tekran®
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system were inter-compared using a laboratory manifold into which HgBr2 was permeated into
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filtered ambient air that had controlled RH exposures. Our goal was to determine the influence of
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RH on these three materials that have been used in previous studies [7, 19, 24].
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Methods
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Huang et al. (2013) used the similar manifold in zero air and reported: 1) HgBr2 permeation rate
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ranged from 61 to 107 pg min-1 depended on temperature using a pyrolyzer with a 2357A
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directly; 2) HgBr2 concentrations collected by cation exchange membranes were 70% higher
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than those measured by Tekran® denuder system.
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Manifold
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The 8-port manifold used in this study is described in Huang et al. [7] with a slight modification.
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For the tests of the denuder alone all the ports were sealed; for comparison of the cation-
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exchange membranes and the denuder 4 ports were sealed; and in order to collect three nylon and
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cation-exchange membranes at same time, three ports on each side were used and 2 were sealed
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(Graphical abstract, Figure 1 and 2). In all systems, glass filters were used up- and down-stream
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of the pump to remove aerosols in ambient air and generated from pump. These were replaced
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before every batch experiment. The flow rate generated by the pump was ~13 Lpm. There was
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an orifice metering valve at location 3 to adjust and generate constant flow (0.02-0.05 Lpm) for
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air carrying permeated GOM to the manifold. The flow through the water source was controlled
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by a metering valve (2-7 Lpm, from a zero-air tank or ambient air); this allowed for the target
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RH in the manifold to be controlled and confirmed by the humidity sensor in the end of the
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system. An impactor after the water source was used to remove liquid water. The manifold and
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all Teflon® tubing after the GOM source were heated to 100ºC to ensure that water was in the
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gas phase, and to reduce the potential for deposition of GOM to the walls. Previous work [21]
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indicated that heating the system to 100°C can increase the mobility of GOM in the line and
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reduce wall loss. However, the filter holders were maintained at room temperature (~20ºC). Only
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HgBr2 was used as permeation source (Tekran® measurement: 200-1000 pg m-3, except for one
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unusual event ~4000 pg m-3). Manifold GOM blanks in ambient air were usually below 20 pg m-
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manifold blanks sometimes reached 50 pg m-3 (the blanks were 35%), GOM not captured or reduced on the denuder wall
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was 10-60% (Wilcoxon Signed Rank, p-value < 0.001). Loss was correlated with humidity (RH,
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21.5-62.2%: r2=0.49, p-value < 0.01, absolute humidity, 5.2-13.6 g m-3: r2=0.45, p-value
