Trace Metals in Global Air: First Results from the GAPS and GAPS Megacities Networks.

Trace metals, as constituents of ambient air, can have impacts on human and environmental health. The Global Atmospheric Passive Sampling (GAPS) and GAPS Megacities (GAPS-MC) networks investigated trace metals in the air at 51 global locations by deploying polyurethane foam disk passive air samplers (PUF-PAS) for periods of 3-12 months. Aluminum and iron exhibited the highest concentrations in air (x̅ = 3400 and 4630 ng/m3, respectively), with notably elevated values at a rural site in Argentina thought to be impacted by resuspended soil. Urban sites had the highest levels of toxic Pb and Cd, with enrichment factors suggesting primarily anthropogenic influences. High levels of As at rural sites were also observed. Elevated trace metal concentrations in cities are associated with local emissions and higher PM2.5 and PM10 concentrations. Brake and tire wear-associated metals Sb, Cu, and Zn are significantly correlated and elevated at urban locations relative to those at background sites. These data demonstrate the versatility of PUF-PAS for measuring trace metals and other particle-associated pollutants in ambient air in a cost-effective and simple manner. The data presented here will serve as a global baseline for assessing future changes in ambient air associated with industrialization, urbanization, and population growth.

Contrasting biological potency of particulate matter collected at sites impacted by distinct industrial sources.

BACKGROUND: Industrial sources contribute a significant proportion of anthropogenic particulate matter (PM) emissions, producing particles of varying composition that may differentially impact health. This study investigated the in vitro toxicity of ambient PM collected near industrial sites in relation to particle size and composition. METHODS: Size-fractionated particles (ultrafine, PM0.1-2.5, PM2.5-10, PM>10) were collected in the vicinity of steel, copper, aluminium, and petrochemical industrial sites. Human lung epithelial-like A549 and murine macrophage-like J774A.1 cells were exposed for 24 h to particle suspensions (0, 30, 100, 300 µg/cm2). Particle potency was assessed using cytotoxic (resazurin reduction, lactate dehydrogenase (LDH) release) and inflammatory (cytokine release) assays, and regressed against composition (metals, polycyclic aromatic hydrocarbons (PAHs), endotoxin). RESULTS: Coarse (PM2.5-10, PM>10) particle fractions were composed primarily of iron and aluminium; in contrast, ultrafine and fine (PM0.1-2.5) fractions displayed considerable variability in metal composition (especially water-soluble metals) across collection sites consistent with source contributions. Semi-volatile and PM-associated PAHs were enriched in the fine and coarse fractions collected near metal industry. Cell responses to exposure at equivalent mass concentrations displayed striking differences among sites (SITE x SIZE and SITE x DOSE interactions, p < 0.05), suggesting that particle composition, in addition to size, impacted particle toxicity. While both J774A.1 and A549 cells exhibited clear particle size-dependent effects, site-dependent differences were more pronounced in J774A.1 cells, suggesting greater sensitivity to particle composition. Plotting particle potency according to cytotoxic and inflammatory response grouped particles by size and site, and showed that particles of similar composition tended to cluster together. Cytotoxic effects in J774A.1 cells correlated with metal and PAH content, while inflammatory responses were associated primarily with endotoxin content in coarse particles. CONCLUSIONS: Industrial sources produce particulate emissions with varying chemical composition that differ in their in vitro potency in relation to particle size and the levels of specific constituents.

Air toxics in Canada measured by the National Air Pollution Surveillance (NAPS) program and their relation to ambient air quality guidelines.

UNLABELLED: This study reports ambient concentrations of 63 air toxics that were measured in Canada by the National Air Pollution Surveillance (NAPS) program over the period 2009-2013. Measured concentrations are compared with ambient air quality guidelines from Canadian jurisdictions, and compounds that exceeded guidelines are identified and discussed. Although this study does not assess risk or cumulative effects, air toxics that approached guidelines are also identified so that their potential contribution to ambient air toxics pollution can be considered. Eleven air toxics exceeded at least one guideline, and an additional 16 approached guidelines during the study period. Four compounds were measured using methods whose detection limits exceeded a guideline value, three of which could not be compared with guidelines, since they were not detected in any samples. The assessment of several metal(loid) concentrations is tentative, since they were measured only in fine particulate matter (PM) but compared with guidelines based on coarse or total PM. Improvements to sampling and analysis techniques for the latter compounds as well as for those whose methods are subject to known uncertainties would improve confidence in reported concentrations and their relation to applicable guidelines. Analysis of sampling strategies for all compounds found to exceed or approach guidelines would contribute to ensuring that their spatiotemporal coverage is adequate. Examination of the air toxics not measured by NAPS but having guidelines in Canadian jurisdictions or being included in other programs such as the U.S. National-Scale Air Toxics Assessment (NATA) would contribute to ensuring that the full suite of pollutants relevant to ambient air quality in Canada is subject to adequate study. The results of this study can be applied to evaluating the effectiveness of toxic substances management in Canada. IMPLICATIONS: Recent measurements of 63 air toxics in Canada by the National Air Pollution Surveillance (NAPS) program showed that 11 compounds exceeded daily or annual ambient air quality guidelines and that an additional 16 compounds approached such guidelines within an order of magnitude. The results of this study can be applied to evaluating the effectiveness of toxic substances management in Canada and to identifying compounds that merit further investigation.

Chemical characterization of exhaust emissions from selected canadian marine vessels: the case of trace metals and lanthanoids.

This paper reports the chemical composition of exhaust emissions from the main engines of five ocean going cargo vessels, as they traveled in Canadian waters. The emission factors (EFs) of PM2.5 and SO2 for vessels tested on various intermediate fuel oils (IFO), ranged from 0.4 to 2.2 g kW(-1) hr(-1) and 4.7 to 10.3 g kW(-1) hr(-1), respectively, and were mainly dependent on the content of sulfur in the fuel. Average NOx, CO, and CO2 EFs for these tests were 12.7, 0.45, and 618 g kW(-1) hr(-1), respectively and were generally below benchmark values commonly used by regulatory agencies. The composition of PM2.5 was dominated by hydrated sulfates, organic carbon and trace metals which accounted for 80-97% of total PM2.5 mass. A substantial decrease of measured emission factors for PM2.5 and SO2 was observed when the fuel was changed from IFO to marine diesel oil (MDO), in one of the tested vessels. The main component of PM2.5 in this case was organic carbon accounting for 65% of PM2.5 mass. In addition to commonly reported pollutants, this study presents EFs of the lanthanoid elements and showed that their distribution patterns in ship-exhaust PM2.5 were very similar to the PM2.5 emitted by oil refining facilities. Hence, using La:Ce:V tertiary diagrams and La/V ratios is necessary to distinguish ship plumes from primary emissions related to accidental and/or routine operation of oil-refining industry.

Cytotoxic and inflammatory potential of size-fractionated particulate matter collected repeatedly within a small urban area.

BACKGROUND: Exposure to coarse, fine, and ultrafine particles is associated with adverse population health impacts. We investigated whether size-fractionated particles collected repeatedly in the vicinity of industrial (steel mills and associated coking operations, wastewater treatment), high traffic, and residential areas display systematic differences in biological potency. METHODS: Particulate matter (PM<0.1, PM0.1-0.5, PM0.5-2.5, PM2.5-10, PM>10) samples collected at sites within Windsor, Ontario, were screened for biological potency in human A549 lung epithelial and murine J774A.1 macrophage-like cells using cytotoxicity bioassays (cellular ATP, resazurin reduction, lactate dehydrogenase (LDH) release), cytokine production, and transcript profiles. Potency was determined from the slope of each dose-effect relationship. RESULTS: Cytotoxic potency varied across size fractions and within a fraction across sites and sampling periods, suggesting that particle composition, in addition to size and mass, affected particle toxicity. While ATP and LDH profiles showed some similarity, resazurin reduction (a measure of metabolic activity) exhibited a unique pattern of response, indicating that the cytotoxicity assays were sensitive to distinct particle characteristics. Chemical speciation varied in relation to prevailing winds, consistent with enrichment of source emissions (e.g. higher metal and polycyclic aromatic hydrocarbon content downwind of the industrial site). Notwithstanding this variability, site-dependent differences in particle toxicity were evident, including greater potency of coarse fractions at the industrial site and of ultrafine particles at the traffic site (Site × Size interactions, p < 0.05). Regression of potency against particle constituents revealed correlations between resazurin reduction, induction of metal-responsive genes, and metal content, which were particularly strong for the coarse fraction, and between cytokine release and endotoxin, suggesting that these factors were important drivers of biological effects that explain, at least in part, the contrasting potencies of particles compared on an equivalent mass basis. CONCLUSIONS: The data show that 1) particle potency and composition can exhibit significant temporal variation in relation to source contributions; 2) sources may differentially impact the potency of specific size fractions; and 3) particle constituents, notably metals and endotoxin, may elicit distinct biological responses. Together, the data are consistent with the notion that sources and composition, in addition to size and mass concentration, are relevant to particle toxicity.

Effect of industrialization on the differences in sources and composition of ambient PM2.5 in two Southern Ontario locations.

PM2.5 was sampled over a seven-year period (2013-2019) at two locations ∼50 km apart in Southern Ontario (concurrently for five years: 2015-2019). One is a heavily industrialized site (Hamilton), while the other was a rural site (Simcoe). To assess the impact of industrialization on the composition and sources of PM affecting air quality in these two locations, positive matrix factorization coupled with dispersion normalization (DN-PMF) was used to identify six and eight factors at Simcoe and Hamilton, respectively. The Simcoe factors in order of diminishing PM mass contribution were: particulate sulphate (pSO4), secondary organic aerosol (SOA), crustal matter, particulate nitrate (pNO3), biomass burning, and vehicular emissions. At Hamilton, the effects of industrialization were observed by the ∼36% higher average ambient PM2.5 concentration for the study period as well as the presence of factors unique to metallurgy, i.e., coking and steelmaking, compared to Simcoe. The coking and steelmaking factors contributed ∼15% to the PM mass at Hamilton. Seasonal variants of appropriate nonparametric trend tests with the associated slopes (Sen's) were used to assess statistically significant changes in the factor contributions to PM2.5 over time. Specifically at Hamilton, a significant decline in PM contributions was noted for coking (-0.03 µg/m³/yr or -4.1%/yr) while steelmaking showed no statistically significant decline over the study period. Other factors at Hamilton that showed statistically significant declines over the study period were: pSO4 (-0.27 µg/m³/yr or -12.6%/yr), biomass burning (-0.05 µg/m³/yr or -9.02%/yr), crustal matter (-0.03 µg/m³/yr or -5.28%/yr). These factors mainly accounted for the significant decline in PM2.5 over the study period (-0.35 µg/m³/yr or -4.24%/yr). This work shows the importance of long-term monitoring in assessing the unique contributions and temporal changes of industrialization on air quality in Ontario and similarly affected locations.

Insights into Elemental Composition and Sources of Fine and Coarse Particulate Matter in Dense Traffic Areas in Toronto and Vancouver, Canada.

Traffic is a significant pollution source in cities and has caused various health and environmental concerns worldwide. Therefore, an improved understanding of traffic impacts on particle concentrations and their components could help mitigate air pollution. In this study, the characteristics and sources of trace elements in PM2.5 (fine), and PM10-2.5 (coarse), were investigated in dense traffic areas in Toronto and Vancouver, Canada, from 2015-2017. At nearby urban background sites, 24-h integrated PM samples were also concurrently collected. The PM2.5 and PM10-2.5 masses, and a number of elements (i.e., Fe, Ba, Cu, Sb, Zn, Cr), showed clear increases at each near-road site, related to the traffic emissions resulting from resuspension and/or abrasion sources. The trace elements showed a clear partitioning trend between PM2.5 and PM10-2.5, thus reflecting the origin of some of these elements. The application of positive matrix factorization (PMF) to the combined fine and coarse metal data (86 total), with 24 observations at each site, was used to determine the contribution of different sources to the total metal concentrations in fine and coarse PM. Four major sources were identified by the PMF model, including two traffic non-exhaust (crustal/road dust, brake/tire wear) sources, along with regional and local industrial sources. Source apportionment indicated that the resuspended crustal/road dust factor was the dominant contributor to the total coarse-bound trace element (i.e., Fe, Ti, Ba, Cu, Zn, Sb, Cr) concentrations produced by vehicular exhaust and non-exhaust traffic-related processes that have been deposited onto the surface. The second non-exhaust factor related to brake/tire wear abrasion accounted for a considerable portion of the fine and coarse elemental (i.e., Ba, Fe, Cu, Zn, Sb) mass at both near-road sites. Regional and local industry contributed mostly to the fine elemental (i.e., S, As, Se, Cd, Pb) concentrations. Overall, the results show that non-exhaust traffic-related processes were major contributors to the various redox-active metal species (i.e., Fe, Cu) in both PM fractions. In addition, a substantial proportion of these metals in PM2.5 was water-soluble, which is an important contributor to the formation of reactive oxygen species and, thus, may lead to oxidative damage to cells in the human body. It appears that controlling traffic non-exhaust-related metals emissions, in the absence of significant point sources in the area, could have a pronounced effect on the redox activity of PM, with broad implications for the protection of public health.

Characterization and source apportionment of airborne particulate elements in the Athabasca oil sands region.

The oil sands industries in Alberta, Canada are potential sources of particulate-bound elements in the region. This study explored the ambient concentrations and size distributions, and conducted source apportionment of 48 particulate elements, based on samples collected in 2016-2017 at four air monitoring sites in the Athabasca oil sands region: Fort McKay (AMS1), Buffalo Viewpoint (AMS4), Wapasu Creek (AMS17), and Stoney Mountain (AMS18). Element concentrations in fine and coarse particulate matter (PM2.5 and PM2.5-10 respectively) at the four sites were generally lower than their typical concentrations at other urban and industrial sites in North America. Among all elements, S was the most abundant in PM2.5 with mean concentrations ranging from 189 ng/m3 (AMS18) to 284 ng/m3 (AMS1). Of the trace, toxic elements in PM2.5, Zn was the most abundant with mean concentrations ranging from 3.43 ng/m3 (AMS18) to 5.37 ng/m3 (AMS4). Positive Matrix Factorization (PMF) modeling of the element concentrations in PM2.5 was used for source apportionment for Zone1 (including AMS 1, 4, and 17, situated closer to industrial activities) and for Zone2 (including AMS18, a background site). The sources of elements for Zone1, included crustal dust, bitumen processing, haul road dust, and biomass burning that explained ~33%, ~43%, ~15%, and ~9% of the total resolved elemental mass, respectively. The sources of elements for Zone2, included Pb-rich source, biomass burning, fugitive oil sands, crustal dust, and bitumen processing explaining ~8%, ~7%, ~3%, ~22%, and ~60% of the total resolved elemental mass, respectively. Elemental mass concentrations of the bitumen processing source factor at Zone2 was two-thirds of that in Zone1. Overall, mass proportions of the bitumen processing source factor at all four sites were significant, suggesting that the oil sands industries played a key role in ambient element concentration levels in the region.

The influence of chemical composition, aerosol acidity, and metal dissolution on the oxidative potential of fine particulate matter and redox potential of the lung lining fluid.

Air pollution is a major environmental health risk and it contributes to respiratory and cardiovascular diseases and excess mortality worldwide. The adverse health effects have been associated with the inhalation of fine particulate matter (PM2.5) and induction of respiratory oxidative stress. In this work, we quantified the oxidative potential (OP) of PM2.5 from several Canadian cities (Toronto, Hamilton, Montreal, Vancouver) using a recently developed bioanalytical method which measures the oxidation of lung antioxidants, glutathione, cysteine, and ascorbic acid, the formation of glutathione disulfide and cystine, and the related redox potential (RP) in a simulated epithelial lining fluid (SELF). We evaluated the application of empirical SELF RP as a new metric for aerosol OP. We further investigated how PM2.5 chemical composition and OP are related across various emission source sectors and whether these features are linked to specific properties of aerosol aqueous phase, such as pH and metal-ligand complexation. The OP indicators including SELF RP were strongly correlated among each other, indicating that the empirical RP could be used as a reliable metric in future studies. OP based on ascorbic acid showed dependency on the emission source sectors, most likely due to variation in the solubility of Fe. Traffic emissions resulted in the highest OP, followed by industrial emissions and resuspended crustal matter. OP presented low correlation with PM2.5 concentrations, low-moderate correlation with the aerosol organic matter, and moderate-strong association with black carbon and transition metals across the sites. We did not find strong association between the concentration of biomass burning tracers and OP. Copper was the only metal that showed high association with OP across all sites, whereas the correlation with other metals, such as iron, manganese, and titanium, showed clear dependency on the source sectors. The aerosol pH correlated negatively with ambient temperature and positively with biomass burning tracers and the levels of nitrate, ammonium, and aerosol liquid water content. The solubility of Fe was associated with sulfate and aerosol pH at most sites, suggesting the involvement of proton-mediated dissolution pathway, while this was not visible at the site influenced by industrial emission, most likely due to the abundance of pyrogenic Fe. The effect of metal-ligand complexation on the solubility of transition metals, in particular Fe, was clearly observed at all sites, whereas a combined effect with aerosol pH, and a subsequent impact on OP, was only seen at the traffic site in Toronto. The enhanced solubility of Fe due to proton- and ligand-mediated dissolution pathways and subsequent formation of reactive oxygen species may in part explain the health effects of PM2.5 seen in previous epidemiological studies.

Accurate and precise determination of silver isotope fractionation in environmental samples by multicollector-ICPMS.

High precision silver isotope ratios in environmental samples were determined by multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). Purification of Ag from sample matrixes was performed by a two stage tandem column setup with use of anion and cation exchange resin, sequentially. It was found that 1% HNO(3) and 3% HCl was efficient to stabilize Ag in the final purified sample digests prior to MC-ICPMS determination. Pd at 2 mug g(-1) was added to both sample and Ag standard solution as a common doping matrix as well as an internal standard for mass bias correction. Mass discrimination and instrument drift were corrected by a combination of internal normalization with Pd and standard-sample-standard bracketing, without assuming identical mass bias for Pd and Ag. NIST SRM 978a (silver isotopic standard reference material) was used for method validation and subjected to column separation and sample preparation processes. A value of -0.003 +/- 0.010 per thousand for delta(107/109)Ag (mean and 2SD, n = 4) was obtained, confirming accurate results can be obtained using the proposed method. To the best of our knowledge, this is the first report on delta(107/109)Ag variations in environmental samples. Significant differences in Ag isotope ratios were found among NIST SRM 978a standard, sediment CRM PACS-2, domestic sludge SRM 2781, industrial sludge 2782, and the fish liver CRM DOLT-4. The sediment CRM PACS-2 has a very small negative delta(107/109)Ag value of -0.025 +/- 0.012 per thousand (2SD, n = 4). The domestic sludge SRM 2781 has a negative delta(107/109)Ag value of -0.061 +/- 0.010 per thousand (2SD, n = 4), whereas industrial sludge SRM 2782 has a positive delta(107/109)Ag value of +0.044 +/- 0.014 per thousand (2SD, n = 4), which may indicate the contribution of Ag from different anthropogenic inputs. DOLT-4 has a much larger negative value of -0.284 +/- 0.014 per thousand (2SD, n = 4), possibly caused by biological processes. These observations confirm that Ag isotope fractionation may provide a useful tool for fingerprinting sources of Ag in the environment and for studying a wide variety of chemical and biological processes in nature. High precision of better than +/-0.015 per thousand (2SD, n = 4) obtained in real sample matrixes makes the present method well suited for monitoring small Ag isotope fractionation in nature.

Measurement of low-molecular-weight carboxylic acids in ambient air and vehicle emission by capillary electrophoresis.

Within the last few years, capillary electrophoresis (CE), especially with indirect ultraviolet detection, has successfully been utilized for the analysis of low-molecular-weight (LMW) organic acids in a wide variety of matrices (e.g., food, pharmaceutical, environmental, industrial, clinical). The speed, resolution, and simplicity of CE, combined with low operating costs, make the technique an attractive option for the development of improved methods in this field. Hence, CE is becoming increasingly accepted for routine analytical work. In this chapter, the unique capability and applicability of the five selected CE methods used in the analysis of LMW carboxylic acids in ambient air and/or vehicle-emitted samples are described.

Abiotic methylation of mercury in the aquatic environment.

Methylation of inorganic mercury in the aquatic environment has been considered to be largely the result of biological processes, primarily involving sulfate-reducing bacteria. However, these processes cannot account for all of the methylmercury that is formed naturally. A growing body of evidence suggests that chemical reactions represent another possible pathway for mercury methylation in the aquatic environment. In order to assess the abiotic contribution to mercury methylation in the water column, and specifically the conditions under which this contribution may be significant, the current state of knowledge about environmentally significant methylation reactions is reviewed. Results of our laboratory-based investigations of aqueous mercury reactions with some potential methyl donors, including MeCo(dmg)(2)(H2O), a simple model for methylcobalamin, various methyltin compounds and methyl iodide, are presented. In each reaction, the yield of methylmercury and the rate of methylation depend strongly on environmental factors such as pH, temperature, and the presence of complexing agents, especially chloride.

Development of a hydrophilic interaction liquid chromatography-mass spectrometry method for detection and quantification of urea thermal decomposition by-products in emission from diesel engine employing selective catalytic reduction technology.

The use of urea based selective catalytic reduction (SCR) technology for the reduction of NOx from the exhaust of diesel-powered vehicles has the potential to emit at least six thermal decomposition by-products, ammonia, and unreacted urea from the tailpipe. These compounds may include: biuret, dicyandiamine, cyanuric acid, ammelide, ammeline and melamine. In the present study, a simple, sensitive and reliable hydrophilic interaction liquid chromatography (HILIC)-electrospray ionization (ESI)/mass spectrometry (MS) method without complex sample pre-treatment was developed for identification and determination of urea decomposition by-products in diesel exhaust. Gradient separation was performed on a SeQuant ZIC-HILIC column with a highly polar zwitterionic stationary phase, and using a mobile phase consisting of acetonitrile (eluent A) and 15 mM ammonium formate (pH 6; eluent B). Detection and quantification were performed using a quadrupole ESI/MS operated simultaneously in negative and positive mode. With 10 µL injection volume, LODs for all target analytes were in the range of 0.2-3 µg/L. The method showed a good inter-day precision of retention time (RSD<0.5%) and peak area (RSD<3%). Satisfactory extraction recoveries from spiked blanks ranged between 96 and 98%. Analyses of samples collected during transient chassis dynamometer tests of a bus engine equipped with a diesel particulate filter (DPF) and urea based SCR technology showed the presence of five target analytes with cyanuric acid and ammelide the most abundant compounds in the exhaust.

An improved method for determination of lanthanoids in environmental samples by inductively coupled plasma mass spectrometry with high matrix introduction system.

The capability of ICP-MS equipped with the high matrix introduction system (HMI) for accurate analysis of lanthanoids in environmental samples was investigated. Compared to the conventional operation, the amounts of oxide and hydroxide molecular species formed in the plasma were reduced by up to 5 times. The relative yields of oxides did not exceed 0.02% for BaO(+) species and were as low as 0.3% for lanthanoids with the highest oxide-formation rates (LaO(+), CeO(+), PrO(+) and NdO(+)). Hydroxide formation was less than 0.02% when HMI system was used. In addition, two digestion procedures were evaluated by the analysis of standard reference materials (SRMs) of different matrices. The digestion efficiency and limits of detection (LOD) were improved when samples were digested with a mixture of HNO(3)/H(2)O(2)/HCl/HF, and HF was removed by evaporation in presence of concentrated HCl. Using this procedure and HMI-ICP-MS analysis, LOD ranged from 0.1 µg kg(-1) to 6 µg kg(-1). Recoveries ranged from 85 to 115% for La to Ho and from 75 to 85% for the other lanthanoids. Relative standard deviations for replicate analysis of SRMs were less than 10%.

Recent advances in CE and CEC of pollutants.

Recent advances in CE and CEC separation, detection and sample preparation/preconcentration methodologies, for the determination of a variety of compounds having current or potential environmental relevance, have been overviewed. The reviewed literature has illustrated the wide range of CE applications available, indicating a continuing interest in CE and CEC in the environmental field. New developments in chip-based CE systems are also discussed.

Recent advances in capillary electrophoresis and capillary electrochromatography of pollutants.

Recent advances in the CE and CEC separation, detection, and sample preparation methodologies applied to the determination of a variety of compounds having current or potential environmental relevance have been overviewed. The reviewed literature has illustrated the wide range of CE applications, indicating the continuing interest in CE and CEC in the environmental field.

Kinetics and mechanism of the mercury(II)-assisted hydrolysis of methyl iodide.

The kinetics and mechanism of the reaction of aqueous Hg(II) with methyl iodide have been investigated. The overall reaction is best described as Hg(II)-assisted hydrolysis, resulting in quantitative formation of methanol and, in the presence of excess methyl iodide, ultimately, HgI2 via the intermediate HgI+. The kinetics are biexponential when methyl iodide is in excess. At 25 degrees C, the acceleration provided by Hg2+ is 7.5 times greater than that caused by HgI+, while assistance of hydrolysis was not observed for HgI2. Thus, the reactions are not catalytic in Hg(II). The kinetics are consistent with an SN2-M+ mechanism involving electrophilic attack at iodide. As expected, methylation of mercury is not a reaction pathway; traces of methylmercury(II) are artifacts of the extraction/preconcentration procedure used for methylmercury analysis.